THE GRANITIC ROCKS OF THE YOSEMITE REGION
By FRANK C. CALKINS
DESCRIPTION OF THE ROCKS
EL CAPITAN GRANITE
The walls of the lower half of the Yosemite Valley are composed in large part of a light-colored biotite granite of moderately coarse and in part obscurely porphyritic texture.79 This rock is here named El Capitan granite,80 because it forms the greater part of El Capitan. It is the oldest and the most extensively developed of the biotite granites in the region.
Because of its prevailingly sparse jointing and its great resistance to weathering the El Capitan granite constitutes many of the outstanding cliffs and rock monuments of the valley. Besides El Capitan it forms the cliffs about the recess of the Ribbon Fall and most of the massif of the Three Brothers, including Eagle Peak. It also forms the bulk of the great promontory of the Cathedral Rocks and is the cause of the narrowing of the valley between that promontory and El Capitan. It forms a large part of the walls west of Dewey Point, as well as the sloping barrier that closes the valley at its lower end. The walls of the Merced Gorge, as far down as the Gateway, are composed almost wholly of this rock, and so are the adjoining uplands for considerable distances to the north and south.
In the upper half of the valley the El Capitan granite occurs in smaller massesin the cliffs west and east of the Yosemite Falls, in the cliffs under Union Point, on Sentinel Dome, and on Illilouette Ridge. Sentinel Dome is the only one of the many dome-shaped features of the Yosemite region that is made of this granite.
The El Capitan granite is cut by all the adjoining intrusive rocks excepting the granodiorite of the Gateway and some of the gabbro and diorite that occur near the lower end of the valley. It is therefore clearly among the oldest of the intrusive rocks of the Yosemite region.
In composition and appearance the El Capitan granite varies somewhat from place to place. Most typical and most abundant, probably, is the vaguely porphyritic variety which is readily seen in place at the foot of El Capitan. Its phenocrysts consist of unstriated alkali feldspar, which is faintly flesh-tinted and partly transparent. Most of them are about half an inch long. Their outlines are not sharply defined, so that the porphyritic texture is not conspicuous on freshly broken surfaces; but the phenocrysts are readily distinguished on weathered surfaces, from which they project slightly in relief. The groundmass in which the phenocrysts are embedded is a moderately coarse granular aggregate of whitish feldspar and brownish-gray quartz, interspersed with a little biotite in small and irregular flakes.
In the areas north of the Yosemite Valley and west of Ribbon Creek the El Capitan granite is more distinctly porphyritic than elsewhere, and fairly clean phenocrysts are somewhat common in the sand formed by the disintegration of this facies of the granite. Near the eastern margin of the main mass, on the other hand, the texture of the El Capitan granite passes gradually but rather rapidly from porphyritic to nonporphyritic. The nonporphyritic facies is to be seen on Illilouette Ridge, in the upland valley of Yosemite Creek, and on the ridge east of that stream. In this facies the grains of potash feldspar, distinguished by their pinkish hue, are but little larger than the grains of plagioclase and are irregular in form. The nonporphyritic El Capitan granite has some resemblance to the Taft granite, but it is coarser and more pinkish.
Within a zone a few feet wide next to the surface of contact with the older rocks, such as the granodiorite of the Gateway, the El Capitan granite is commonly nonporphyritic and richer in biotite than elsewhere; it even contains sporadic grains of hornblende.
Microscopic study of representative specimens of El Capitan granite shows that plagioclase is somewhat more abundant than quartz and that alkali feldspar is somewhat less abundant than quartz. The potash feldspar is chiefly orthoclase, intergrown with a little albite. The biotite is of the common brown sort. The accessory minerals, present in minute quantities, are titanite, magnetite, apatite, zircon, and allanite.
The Taft granite, named for Taft Point, which it composes, somewhat resembles the El Capitan granite, with which it is intimately associated, but it is not porphyritic. Its texture is even, medium coarse, and typically granitic. It consists chiefly of white feldspar and smoky-gray quartz in grains that are mostly about 5 millimeters in diameter, and it contains a little biotite in evenly disseminated small flakes. The plagioclase and alkali feldspar do not differ markedly in habit or color, though some of the alkali feldspar is faintly tinged with flesh color and may be seen to form larger grains than the snow-white plagioclase. On weathering the rock assumes the same yellowish and reddish tints as the El Capitan granite. The composition and texture of the Taft granite are remarkably uniform, though the texture is noticeably finer near the contacts with older rocks than elsewhere.
Microscopic examination shows that the alkali feldspar, which is perthitic orthoclase or microcline, is distinctly more abundant than the plagioclase. Titanite and other accessory minerals are constantly present in minute quantities.
The Taft granite is so similar to the El Capitan granite in general appearance that on superficial examination it might be regarded as a mere facies of that rock. The two rocks do not intergrade, however, and were not intruded at the same time. Their relations are readily observed in the outcrop a little south of Taft Point, and it is here quite evident that the Taft granite is the younger, for it incloses fragments of the El Capitan granite. But though unquestionably younger than the El Capitan granite, the Taft granite is probably not much younger. So far as could be observed, it is cut by all the rocks that cut the El Capitan granite, and it seems to have been the next to be intruded.
The Taft granite occurs in considerable abundance on both sides of the middle part of the Yosemite Valley. On the north side a large mass, which forms part of El Capitan, extends westward to the hanging valley of Fireplace Creek. It is bounded on the north, east, and west by the main body of El Capitan granite. South of the valley the Taft granite is rather irregularly distributed. It occurs in many places in the basins of Bridalveil Creek and near-by streams. Boulders of it occur in the moraines at Ostrander Lake. Its southern limit has not been definitely traced.
The name Bridalveil granite was given by Turner81 to a rock which occurs in the valley of Bridalveil Creek and forms the brink of the Bridalveil Fall. Fragments of the rock are abundant in the talus at the foot of the fall and may there be most conveniently studied, though care must be taken to avoid confusion with the Leaning Tower quartz monzonite, with which it is associated and which it superficially resembles.
The Bridalveil granite is a fine-grained pepper-and-salt mixture of feldspar, quartz, and a little biotite, whose general tint is a very pale bluish gray. The plagioclase, alkali feldspar, and quartz are present, as a rule, in nearly equal proportions. The alkali feldspar is usually orthoclase but sometimes microcline. The accessory minerals are the common ones, including titanite and, rarely, allanite. A little muscovite (white mica) occurs here and there.
The main bodies of Bridalveil granite occur on the south side of the Yosemite Valley, from the vicinity of Taft Point as far west as Crocker Point and at various places in the Bridalveil Basin. On the north side only narrow tongues of the rock invade the coarser granites at the base of El Capitan and the diorite and gabbro exposed on the Big Oak Flat Road.
The Bridalveil granite appears to be the youngest of the granitoid rocks of the Yosemite region, for it cuts all the rocks with which it is in contact, except some aplite and pegmatite which are closely related to it. Because of this fact its intrusive bodies are not interrupted or obscured by later intrusions, and they are therefore readily traced in the walls of the valley. The greater number of them are sheets, inclined to the horizon at angles less than 45° and ranging from a few inches to 100 feet or more in thickness. One of the most massive of these sheets forms the brink of the Bridalveil Fall. From the talus west of the Leaning Tower it ascends gradually toward the east and is readily traced to the cleft between the north and middle summits of the Cathedral Rocks, where it appears to split into a number of branches.
Another sheet caps the north summit of the Cathedral Rocks and cuts through the upper part of the middle summit. Still other sheets are visible on the faces of these rocks and on the sloping sides of the gulch of Bridalveil Creek. In the embayment east of the Cathedral Rocks the intrusions of Bridalveil granite are particularly large and irregularly shaped. Because of their superior resistance to weathering they form the summits of several of the pinnacles and crags situated in that embayment. Bodies of considerable size, finally, are exposed on the upland, south of the rim. Their form and extent can not be completely traced, owing to masking soil, but it is entirely probable that they, like the sheets visible in the cliffs of the valley, are in general lenticular or wedge-shaped bodies of no great thickness.
LEANING TOWER QUARTZ MONZONITE
The character of the Leaning Tower quartz monzoniteso named because it forms a considerable part of the slopes of the Leaning Toweris readily observed at the foot of the Bridalveil Fall. On weathered surfaces, especially those frequently wetted by the spray from the fall, the rock is deeply decayed and rusty; but on fresh fractures it has the dull-gray tone characteristic of granodiorite. Its texture is moderately fine, and at first glance it appears fairly even grained.
The orthoclase and plagioclase in this rock are difficult to distinguish from each other, and only by microscopic examination is it ascertained that the two are present in nearly equal quantity. Quartz is rather abundant. Biotite and hornblende are both present in moderate amount. In part they are evenly disseminated in particles about 1 millimeter or less in diameter; in part, however, they are assembled in roundish clusters about 5 millimeters in diameter, which give the rock a characteristic spotted appearance.
Viewed under the microscope the rock appears to be in reality porphyritic, although the phenocrysts, which are of plagioclase, are sparse and not much larger than the grains in the groundmass. Orthoclase is about as abundant as quartz and but little less so than plagioclase.
The typical Leaning Tower quartz monzonite appears to form but one small intrusive body restricted to the mouth of the gulch of Bridalveil Creek, though a more definitely porphyritic facies of the same rock occurs near the Big Oak Flat Road, near Taft Point, and at points directly opposite on the north side of the valley. The larger phenocrysts in this facies consist of orthoclase.
In the upland valley of Bridalveil Creek and in the massif of the Cathedral Rocks there occurs a granodiorite whose aggregate quantity is considerable but which it is impracticable to represent on the geologic map (pl. 51) because of the extreme irregularity and small size of its ramifying intrusions. Its upwelling must have followed a remarkably thorough shattering of the older rocks. The El Capitan granite, the gabbro, and the diorite are penetrated by a veritable web of this granodiorite, and much of the body of diorite that is shown on the map as lying in the upland valley of Bridalveil Creek is in reality a sort of agglomerate of fragments of diorite in a matrix of granodiorite.
The name Pohono granodiorite is applied because the rock occurs near the Pohono Trail, which is laid along the south rim of the Yosemite Valley.
This granodiorite is composed largely of a grayish-white mixture of quartz and feldspar in which elongated grains of hornblende and biotite, about 1 millimeter in average diameter, lie embedded. Some of the crystals of orthoclase are poikiliticthat is, they inclose small grains of other minerals. Some of the biotite is ragged and also full of small inclusions. The plagioclase, viewed under the microscope, appears variably zoned and mottled.
HORNBLENDE GABBRO AND HORNBLENDIC DIORITE
Contrasting with all the rocks above described and, indeed, with all the other granitic rocks of the Yosemite region by their darker color are the hornblende gabbro and associated diorite, which occur notably in the lower part of the Yosemite Valley and form the great Rock Slides over which the Big Oak Flat Road is laid. The granites, monzonites, and granodiorites present, on fresh surfaces, various tones of light gray or medium gray; the gabbro is very dark gray or nearly black. It is, however, an extremely variable rock, not only in color but in texture and composition, as may be readily observed in the Rock Slides.
Comparison of almost any two adjacent boulders in this great talus is likely to show some appreciable difference in color and texture. Some of the rock resembles basaltic lava, being greenish black and so fine grained that its constituent minerals can hardly be distinguished. In the coarser-grained boulders, which as a rule are not so dark in color, the principal minerals are readily seen to be feldspar and hornblende, which are generally accompanied by small amounts of quartz and biotite. Between rocks equally coarse in texture the most conspicuous difference is in the abundance and the form of the hornblende crystals. With all their thousand variations they approach one or the other of two types, of which one may be distinguished as the gabbroic, the other as the dioritic.
The most striking development of the gabbroic type occurs near the mouth of the upland valley of Bridalveil Creek. There the hornblende forms irregular but somewhat roundish grains most of which measure one-fourth of an inch to an inch in diameter and some of which attain a diameter of 2 inches. The luster of the cleavage faces is as a rule broken by numerous included grains of feldspar. Small interstices between these large individuals of hornblende are filled with a granular mixture of plagioclase, hornblende, and quartz. Hornblende makes up about half the volume of the rock. Biotite is present in very small amounts.
Besides the essential minerals mentioned, the microscope reveals the common accessories and in some sections a little orthoclase. Most specimens contain some augite, or amphibole that is clearly derived from augite. In the dioritic type hornblende is less abundant than feldspar and is distinctly prismatic in form. Biotite is invariably present in this type, its amount being sometimes considerable, though always subordinate to that of hornblende. As in the gabbro, orthoclase may be present but is always scarce.
The age of some of the gabbro and diorite intrusions relative to the other rocks is still obscure. Evidently the intrusions are not all of the same age, for some are cut by the El Capitan granite, which is the oldest granite in the valley proper, and others cut rocks that are themselves younger than the El Capitan granite.
The gabbro and diorite in the lower part of the valley are unmistakably pregranitic. The evidence of this relationship may be well observed on the Wawona Road north of Inspiration Point, near the south bank of the Merced River between Pulpit Rock and the Pohono Bridge, and near the base of the cliff under Old Inspiration Point. At these and other localities the diorite is penetrated by tongues of granite. An intimate commingling of granitic and dioritic material appears to have occurred at the contact, for the granite is there enriched in biotite, and the small fragments of diorite included in the granite have a porphyritic texture that must be secondary. The granite on Turtleback Dome and Fireplace Bluff contains swarms of dioritic inclusions, and though these lie at some distance from the nearest large visible mass of diorite, they are in all probability fragments of a large body of the older diorite rather than segregations of mineral from the granite magma.
On the great façade of El Capitan, on the other hand, may be seen intrusions of diorite that are clearly of later origin than the El Capitan and Taft granites. (See pl. 17.) The most conspicuous of these intrusions presents a remarkable resemblance to the map of North Americaa wall map about 2,000 feet in height, showing the continent all the way down to the Isthmus of Panama and a part of South America. Another large intrusion farther east has a very irregular shape that defies description.
As no consistent differences of composition between the older and younger gabbros and diorites appear to exist, it is evident that the age of any particular intrusion must be established from local evidence. This evidence is in many places singularly inconclusive, even where the contact of diorite and granite is fairly well exposed, and consequently the age of much of the diorite is still in doubt.
GRANITIC ROCKS OF LOWER YOSEMITE REGION
The lower part of the Merced Gorge, from the Gateway, at Elephant Rock, down to its mouth, near El Portal, is cut through three rock bodies of moderate extent that form part of the complex of intrusions characteristic of the Yosemite region. (See pl. 51.) Just below the Gateway is a small body of granodiorite; then follows a body of biotite granite extending 1-1/2 miles along the river; then another body of the granodiorite first mentioned, extending for about 3 miles to the lower end of the gorge.
The granodiorite is of peculiar interest, as it is even older than the El Capitan granite and therefore one of the very oldest intrusive rocks in the Yosemite region. Its relation to the El Capitan granite may be observed along the lower part of the Coulterville Road, where fragments of the granodiorite are inclosed in the granite. It is conspicuously finer grained and darker in tone than the El Capitan granite. Considerably more than half of it consists of feldspar and quartz in a confused grayish-white mixture, but the appearance of the rock is characterized chiefly by the form and distribution of the biotite and hornblende. The biotite, which is the more abundant of the two, forms irregular or rudely hexagonal tablets, mostly less than 2 millimeters in diameter. The hornblende occurs in slender rodlike crystals about 2 millimeters long.
The biotite granite is readily identifiable in Arch Rock, the huge fallen boulder between whose overhanging fragments the highway is laid. It is a medium-grained nonporphyritic rock, somewhat like the Taft granite, but it contains a larger proportion of biotite, and its quartz is less brownish and consequently less easily distinguished from the feldspar. The potash feldspar, moreover, unlike that of the Taft granite, is poikilitic, a characteristic which on a sunny day when the light flashes from the bright cleavage planes of the orthoclase, is readily observed in the fresh fragments that have fallen in the neighborhood of Arch Rock. The individual crystals of this mineral, which are nearly an inch in diameter, inclose small grains of quartz, plagioclase, and biotite. The biotite flakes measure 3 millimeters or less in diameter and are of rather irregular form. The rock, though somewhat lighter in color, is not strikingly different in appearance from the granodiorite by which it is flanked on both sides, but it contains no hornblende.
In the immediate vicinity of El Portal, where the batholith of the Sierra Nevada abuts upon the western belt of slates and other metamorphic rocks, there is, as is to be expected, a zone containing a considerable variety of intrusive rocks. There are several types of hornblende gabbro, diorite, granodiorite, and biotite granite.
Particularly noteworthy is the gabbro, which consists almost wholly of hornblende and calcic feldspar, mainly bytownite, and is remarkably varied in texture. In some places its hornblende crystals are slender prisms which lie rudely parallel to one another and determine a ready cleavage of the rock; in other places the hornblende forms roundish grains about half an inch in diameter, which inclose smaller grains of feldspar; elsewhere the gabbro is extremely coarse grained, containing ragged prisms of hornblende as much as a foot in length.
From the Three Brothers nearly as far east as the Royal Arches and from the Fissures eastward to Glacier Point the walls of the Yosemite Valley consist mainly of granodiorite, a light-gray granitoid rock, in which soda-lime feldspar predominates decidedly over potash feldspar, quartz is abundant, and hornblende and biotite are present in moderate and nearly equal quantities. The rock forms part of a large irregular intrusive body, but the portion of it here considered is a belt about 2 miles broad that crosses the valley from north to south. This granodiorite is named the Sentinel granodiorite, because it is the sole material of Sentinel Rock. It is the oldest member, so far as known, of the Tuolumne intrusive series.
At the west side of the belt crossing the valley the Sentinel granodiorite is in contact with the Taft and El Capitan biotite granites, which are older. The contact is a rather smooth, steep surface, but a few narrow tongues of granodiorite extend into the granite, which had become solidified and fissured before the granodiorite was injected. Along the east side, on the contrary, the granodiorite is undercut by the Half Dome quartz monzonite, which is slightly more recent and sends flat sheets into the granodiorite. The granodiorite does not extend continuously between these main eastern and western contacts, for along its medial portion, near the longitude of Sentinel Dome and the Yosemite Falls, it incloses a great swarm of fragments of El Capitan granite, some of them hundreds of feet across, others ranging downward in size to mere crumbs.
The Sentinel granodiorite is less uniform in composition than most of the other intrusive bodies. In the western part of the zone crossed by the Yosemite Valley this rock is relatively light-colored and homogeneous; the eastern part is much more streaky and uneven in composition and texture and on the whole is considerably darker than the western part. The eastern portion, again, may be roughly divided into two or three bands that differ mainly in texture.
The western type, as it may be called, is exposed on both walls of the Yosemite Valley from the foot of Eagle Peak and of Taft Point at the west to an indefinite zone, passing through old Yosemite Village, at the east, where it grades into the darker type. The rock of the western type forms the greater portion of this intrusive body. It is fairly uniform, and its general tint is rather light gray, but it is a little darker and more bluish than the El Capitan granite. Its most abundant constituent is white or gray soda-lime feldspar. Potash feldspar is present in rather small amount; it is not conspicuously different from the dominant soda-lime feldspar, though it may be distinguished by its greater transparency. Quartz, which is a moderately abundant constituent, is sugar-white and is also inconspicuous. Against the whitish background formed by these light minerals black hornblende and biotite stand out prominently. In large part the two are clustered together and not readily distinguished, but some separate crystals of hornblende show roughly prismatic forms and intersecting cleavages, and on the larger individuals of biotite one may see the well known micaceous cleavage. Neither mineral ever shows, in this rock, conspicuously perfect crystal form. Rock of this type may be observed most easily in the great aprons of talus ascended by the trail to the Yosemite Falls and the short trail to Glacier Point.
Near its contact on the west with the older biotite granites the granodiorite is a little streaky and darker than in the main body, but this variation here extends only a few feet or a few yards from the contact. In the zone where the inclusions of granite are abundant this streaky character becomes very marked. The granodiorite is darker on the whole than in the western facies, but this fact is due in large measure to the presence of abundant roundish or lens-shaped, nearly black masses, a few feet or a few inches in diameter, in which the black minerals, hornblende and biotite, are strongly concentrated. These dark inclusions tend to be alined, like the streakiness of the inclosing rock, which roughly follows the boundaries of the granitic inclusions in sinuous curves.
In the fairly broad clear space between the zone of inclusions and the contact with the Half Dome quartz monzonite, the granodiorite has not the light color that it has farther west. It is not so streaky as in the vicinity of the older granite but is uniformly rather dark. On the north side of the valley it has a character that may readily be observed in the great talus cone at the mouth of Indian Creek. This type of granodiorite has a faint schistosity along nearly north-south vertical planes, and it carries many of the small dark inclusions already described. The dark minerals have the same character as in the western facies of the granodiorite, though they are here more abundant. The rock, in short, has about the same average character as that around and between the granitic inclusions but is more uniform.
On the south side of the valley, in the vicinity of Glacier Point, the granodiorite has a highly distinctive character, which is given chiefly by the character of the biotite. This mineral forms unusually large crystals, some of them as much as an inch in diameter, which in close many small grains of feldspar and other mineralsthat is, they are poikilitic. The discrimination between these two varieties in the dark eastern zone has been helpful in working out the glacial geology near Glacier Point and Sentinel Dome, for boulders of granodiorite resting on the rock platform just east of Sentinel Dome consist of rock like that of Glacier Point, rather than like the nearest outcrops of granodiorite, and therefore must have been transported southwestward.
HALF DOME QUARTZ MONZONITE
The head of the Yosemite Valley, all of Tenaya Canyon and the Little Yosemite, and all the domes and other eminences surrounding them are carved from a single continuous body of granitic rock that extends far into the High Sierra. This rock was named by Turner82 the Half Dome quartz monzonite, as Half Dome is the most remarkable eminence composed of it.
The monzonite is a medium-grained light-gray rock that bears considerable resemblance to the western type of the Sentinel granodiorite, being composed of plagioclase, quartz, orthoclase, biotite, and hornblende. In the monzonite, however, the dark minerals are somewhat less abundant than in the granodiorite, and they are distinguished by exceptionally regular crystal forms. (See pl. 52, A.) The biotite commonly occurs in hexagonal tablets or in little prisms from one-eighth to one-fourth inch in height. The hornblende occurs in prisms one-eighth of an inch to nearly an inch in length and generally clean-Cut save at the ends.
As the rock crumbles the biotite tablets and hornblende prisms usually fall out entire and form conspicuous, gleaming objects in the sand. Such is true especially on the summit of Half Dome, in and about the Little Yosemite Valley, and in the Illilouette Basin. Probably no granitic rock in the Yosemite region affords better opportunity to become acquainted with the ferromagnesian mineralsbiotite and hornblendethan the Half Dome quartz monzonite.
To the geologist, however, the most significant and reliable distinction between the monzonite and the granodiorite consists in the fact that the potash feldspar is considerably more abundant in the monzonite, this being, indeed, the reason for classing the rock as a monzonite. This feldspar is recognized rather readily in the monzonite, being more transparent than the plagioclase and generally pinkish. Instead of forming only ragged interstitial particles, as it does in the granodiorite, it tends here and there to assume its own crystal form.
Microscopic analysis shows that plagioclase is the preponderant mineral. Quartz is somewhat less abundant and is about equaled in quantity by the potash feldspar, which is chiefly microcline. Biotite is a trifle more abundant than hornblende. Titanite is a conspicuous accessory mineral and in many places occurs in well-shaped crystals.
In the eastern part of the area shown the Half Dome quartz monzonite is obscurely porphyritic, owing to the development of large crystals of potash feldspar. The transition from the nonporphyritic to the porphyritic facies is readily observed on the northwest shore of Tenaya Lake, where the surface of the rock is swept clean by the ancient glaciers and is in part still polished. In passing along the road toward the head of the lake the attentive observer is likely to become aware of a gradual development of grayish dappling in the rock due to phenocrysts. These phenocrysts have, however, rather indefinite outlines and are rarely more than an inch in diameter. They are prominent on some weathered surfaces, from which they project as little knobs.
The Half Dome quartz monzonite is younger than the Sentinel granodiorite, as is clear from the tongues which it sends far into that rock. The nearly plane surface of contact between these rocks, which is traceable on the walls of the upper part of the Yosemite Valley, dips in general westward at an angle of about 30°, and as a consequence the monzonite for considerable distances forms the lower parts of the cliffs, and the granodiorite forms the upper parts. This relation is strikingly expressed in the sculptural detail of the cliffs under Glacier Point, on the south side of the valley. The granodiorite, above, is jointed and rugged; the monzonite, below, is unjointed and smooth.
The Half Dome quartz monzonite is also younger than the Mount Clark granite and the porphyritic rocks of Gray Peak and Red Peak. On the other hand, it is older than the Cathedral Peak granite, whose mass it completely surrounds. It is significant that the porphyritic facies of the monzonite invariably lies next to the Cathedral Peak granite, which is itself conspicuously porphyritic. The zone of porphyritic texture in the monzonite is on the average about half a mile broad.
Of all the granitic rocks of the Yosemite region the Half Dome quartz monzonite is the most prevailingly massive. It is wholly devoid of joint fractures over considerable areas and characteristically forms huge exfoliating monoliths. Not only Half Dome but North Dome, Basket Dome, Mount Watkins, Mount Starr King, and many other more or less dome-shaped prominences are composed of it. In some places, however, it is traversed by many closely spaced joints and possesses a sheeted structure that has greatly facilitated the erosion of canyons and gulches.
CATHEDRAL PEAK GRANITE
With the Cathedral Peak granite the reader has already been made acquainted in the account of the glacial history of the Yosemite Valley (p. 56), for, as is explained in that account, it is largely upon the presence of ice-transported boulders of this highly distinctive granite that the glacialist depends for the certain identification of the older and more obscure moraines. It indeed is fortunate, from the glacialist's point of view, that in a mountain region where so many closely similar granitic rocks occur there should be one of unusual and quite unmistakable aspect in the very area from which the main glaciers radiated.
The distinguishing feature of the Cathedral Peak granite is that it contains unusually large phenocrysts of feldspar. Whereas in most porphyries the phenocrysts are only a fraction of an inch in length, in the Cathedral Peak granite they measure not uncommonly 2 to 4 inches in length and from 1 inch to 2 inches in breadth. Where the rock is smoothed and polished by the glaciers these large, oblong crystals present an appearance suggestive of lumps of domino sugar, their whitish color contrasting with the somewhat darker tone of the groundmass. (See pl. 52, B.) On weathered rock surfaces also they are conspicuous, for, owing to their greater resistance to decomposition than the groundmass, they project from it in high relief. (See pl. 37, B.) As the rock finally disintegrates into granular sand, the phenocrysts are freed entirely from the groundmass, and as a consequence on many mountain sides in the High Sierra such loose crystals are found lying about in large numbers.
Many of the phenocrysts are twinned in accordance with the Carlsbad law, as is readily apparent from the flash of their surfaces in the sunlight. Others show no evidence of twin structure. Their color in many localities is slightly pinkish. Inclosed in them as a rule are numerous small grains of the other minerals present in the groundmass, including biotite and hornblende. These grains are most prevalent in their outer portions, and as a consequence the phenocrysts when entirely weathered out from the groundmass have as a rule rough surfaces, yet even so they are much more clean-cut than the phenocrysts of the porphyritic variety of the Half Dome quartz monzonite.
The groundmass of the Cathedral Peak granite consists mainly of small grains of quartz and feldspar and contains a fine sprinkling of biotite and hornblende. The crystals of these dark minerals are much smaller and less regular in form than they are in the Half Dome quartz monzonite. The feldspar is as a rule pure white, and the general tone of the rock is lighter than that of any of the rocks described on the preceding pages. The microscope shows that plagioclase is somewhat more abundant in the groundmass than potash feldspar, but the latter is slightly more abundant on the whole, if the phenocrysts are taken into account. The accessory mineralsmagnetite, apatite, titanite, and zirconare present in small quantities.
The Cathedral Peak granite is fairly uniform in character, but in some places, especially near its contacts with the older Half Dome quartz monzonite, the phenocrysts of feldspar are crowded in swarms and the crystals of biotite and hornblende are distributed in groups or streaks.
The intrusive body as a whole appears to be rather compact, but its limits are not yet completely known. The mass is in general elongated from north to south. The southern tip is about a mile northeast of Merced Lake, and the greatest breadthabout 8 milesis attained in the vicinity of the Tuolumne Meadows. To judge from the course of the outer boundary, as traced on the walls of deep canyons, the intrusive body broadens downward and is shaped somewhat like a broadly elliptical, dome, diversified by no great off-shoots or irregularities.
Wherever its periphery has been examined the Cathedral Peak granite is found to be in contact with the porphyritic facies of the Half Dome quartz monzonite. It has many characteristics in common with that rock, yet the boundary between them is everywhere sharp, and there is ample evidence that the Cathedral Peak granite is the younger of the two.
JOHNSON GRANITE PORPHYRY
The Johnson granite porphyry forms a relatively small body situated entirely in the High Sierra, at a considerable distance from the Yosemite Valley, yet its description here seems appropriate, for it is the youngest of the Tuolumne intrusive series, the other members of which have just been described. It occupies an area about 7 miles in length from north to south and 2 miles broad from east to west, centering about the Tuolumne Meadows. The name is derived from Johnson Peak, south of the meadows, which is entirely composed of this rock.
The intrusion is completely surrounded by the much larger mass of Cathedral Peak granite, into which it has broken; and, as the latter is almost completely surrounded by the older Half Dome quartz monzonite, whose outer margin, again, is in contact in most places with the still older Sentinel granodiorite, it will be seen that the Johnson granite porphyry is centrally placed with respect to the older members of the Tuolumne intrusive series. It is also the most highly siliceous member, intrusion having taken place in the order of increasing silicity throughout the series.
The Johnson granite porphyry has various aspects. Its major portion resembles an aplite, as it has a fine sugary texture, but it is distinguished by widely scattered phenocrysts of microcline. These are remarkably clean-cut in form and very large compared with the fine grain of the groundmass, many of them exceeding 2 inches in length. Scattered phenocrysts of quartz and of soda-lime feldspar about a quarter of an inch in diameter also occur in the groundmass. The rock contains quartz, oligoclase, and microcline nearly equal proportions and also a small amount of biotite.
A part of the mass has the characteristics of typical granite porphyry, for it contains, in addition to the large phenocrysts of microcline, many small crysts of quartz, plagioclase, and biotite.
The boundaries of the intrusive body are irregular and difficult to trace, a number of flat-lying sheets penetrating far into the surrounding Cathedral Peak granite. However, the mass as a whole appears to fall off steeply on the east side and to slope outward at a moderate angle on the west side.
The Johnson granite porphyry is, like aplite, much more resistant to weathering than the adjoining granite and consequently retains much of its ice-polished surface in localities where the granite has already lost most of its polish. Glaciated surfaces of the porphyry may be seen near the Tuolumne Soda Springs, but the finest exhibit of glacier polish is to be found in the high saddle between Johnson Peak and Rafferty Peak.
MOUNT CLARK GRANITE
The granite of which Mount Clark, the sharp peak at the north end of the Clark Range, is composed is readily distinguished from all the other granitic rocks of the Yosemite region by the fact that it is essentially devoid of ferromagnesian minerals, such as biotite and hornblende, and therefore is not speckled, It is a very siliceous, light-colored granite, or alaskite, composed almost wholly of feldspar and quartz. It is moderately coarse grained and even textured and in these respects resembles the Taft granite. The feldspar is snow-white where fresh but yellowish where it is weathered, and in not a few places it assumes a beautiful rose tint. The quartz is generally brownish gray, sufficiently dark in tone to contrast with the feldspar.
The microscope shows that quartz, alkali feldspar, and plagioclase are about equally abundant. The alkali feldspar is in part orthoclase, in part microcline, with which more or less albite is intergrown; the plagioclase is partly albite and partly oligoclase. Accessory minerals are magnetite, zircon, titanite, apatite, and allanite. Minute grains of hornblende occur here and there.
The Mount Clark granite is clearly older than the Half Dome quartz monzonite, being penetrated by sheets of that rock, but its relations to the granitic rocks of which Gray Peak and Red Peak are composed are not yet certain.
Like the Half Dome quartz monzonite, the Mount Clark granite is in the main sparsely jointed and tends to exfoliate in curved shells. That Mount Clark is not dome-shaped but sharply triangular in form is due to the fact that it has been glaciated on three sides. The exfoliating habit of the rock is most prominently displayed on the sides and at the head of Clark Canyon.
Because of its high silica content the Mount Clark granite is exceedingly slow to decompose, and this accounts for the fact that erratic boulders of it, dropped on the divide east of Mount Starr King by very early glaciers, still remain in a fair state of preservation, although nearly all the other ice-borne materials there have long since disintegrated into sand and have been washed away.
PORPHYRITIC ROCKS OF CLARK RANGE
The Clark Range as seen from Glacier Point has a ruddy appearance, as compared with the rest of the Sierra landscape, which is almost uniformly pale-hued. Gray Peak and Red Peak in particular stand out by reason of their distinctive coloring, as their names well indicate. One might suppose that these peaks are composed of other than granitic rocksof metamorphic rocks of sedimentary or volcanic origin, such as those which occur on the lower Sierra slope. In fact, however, the two peaks consist of granitic rocks, very similar in character, that form what may be regarded as an intrusive body produced in installments. A number of distinct surfaces of contact may be observed on the walls of the peaks, between intrusive rocks which differ somewhat from one another yet have so strong a family resemblance that they can scarcely be mapped separately.
These rocks may be described collectively as porphyritic intrusive rocks having phenocrysts of potash feldspar and a groundmass of feldspars, quartz, biotite, and, in most places, hornblende. Despite their differences in texture and composition, these rocks look much alike, especially on weathered surfaces, where the texture is brought out by atmospheric etching: the phenocrysts are rather closely crowded together, and many of them show nearly square cross sections; and the dark minerals are clustered into somewhat mosslike forms. These textural characters have served to identify many erratic boulders in the lower part of the Illilouette Basin.
The coarsest material, which occurs in the cirque north of Gray Peak, is a porphyritic granite whose phenocrysts are about an inch in length. The summit and southern flank of Gray Peak and the bulk of Red Peak consist of a finer-grained rock which might more correctly be called a granite porphyry. Its phenocrysts hardly exceed half an inch in length, and the groundmass is moderately fine grained. It somewhat resembles the porphyritic granite which forms the summit of Mount Hoffmann.
The ruddy color of the rock in Red Peak appears to be due to iron oxide derived from one of the constituent minerals. A certain facies of the porphyry seen on the west spur of Red Peak contains abundant magnetite, and a relatively small quantity of such rock, distributed in the form of dikes, might stain a great bulk of the light-gray porphyry. The oxidation of the iron minerals and the dissemination of the resulting red pigment must have been facilitated by the thoroughly fissured state of the rock.
APLITE AND PEGMATITE
Dikes of aplite and pegmatite are plentiful throughout the Yosemite region. They range from a fraction of an inch to scores of feet in thickness and cut all the major bodies of intrusive rock. Some extend in nearly straight sheets, following apparently old joint fractures; others are crooked and branch irregularly, like fantastic trees. They are as a rule easily distinguished from the other rock bodies by their lighter, creamy color and may be traced by the eye for long distances on the bare walks and domes of the Yosemite region.
Each major intrusion of granitic rock seems to have been followed by the injection of highly siliceous materialaplite or pegmatiteinto open fissures. The siliceous material was derived, apparently, by differentiation from the parent magma and possesses a slight family resemblance to that magma, yet on the whole the injections following different major intrusions are so nearly alike in composition that separate description of them is here unnecessary.
The aplites and pegmatites of the Yosemite region consist almost wholly of alkalic feldspars and quartz, with biotite in very small amounts. In some dikes garnet occurs instead of biotite. The fine sugary texture characteristic of aplite passes gradually into coarse pegmatitic textures. Well-defined graphic intergrowth of quartz and feldspar is not common, but striking examples of it occur in a few places, as, for instance, in blocks of pegmatite mingled with Sentinel granodiorite in the talus west of the Royal Arches. Somewhat similar intergrowths of garnet with feldspar occur near the castellated spur east of the Cathedral Spires.
The most remarkable outcrop of pegmatite in the neighborhood of the Yosemite Valley occurs a few hundred feet west of the junction of the Pohono Trail and the Chinquapin Road. The mass appears at first glance to be composed wholly of clean milk-white quartz, but closer examination reveals a few huge crystals of flesh-colored feldspar, some of which measure as much as 2 feet in length. The aggregate is clearly a true pegmatite, exceptional in its very coarse texture and its great preponderance of quartz. A similar but smaller outcrop is situated about half a mile southeast of the one described, and there are others in the Illilouette Basin and on the Cathedral Range. These very coarse pegmatites form no dikes of great extent but relatively thick, irregularly shaped bodies.
Dikes of aplite and pegmatite are especially abundant in the Half Dome quartz monzonite, as may readily be observed in the Little Yosemite, in Tenaya Canyon, and on the surrounding heights. Their superior hardness causes these dikes to stand out in relief from weathered surfaces. On the curving back of Half Dome, for instance, especially in a grazing lightthat is a light striking the surface at a very slight anglethey appear like the branching veins on the back of a man's hand. On the summits of Indian Rock and Mount Watkins interlacing dikes of aplite remain in fantastically shaped masses as a result of the more rapid disintegration of the granite between them. In the gabbro and diorite of the Rock Slides, in the lower part of the Yosemite Valley, dikes of pegmatite are almost as abundant as in the Half Dome quartz monzonite. Many of them fill old joint fractures and extend in nearly horizontal sheets. They are the more conspicuous because of the contrast between their whiteness and the somber tones of the basic rocks.
Last Updated: 28-Nov-2006