University of Washington Publications in Geology The Geology of the San Juan Islands

GENERAL STATEMENT

The sedimentary rocks of the San Juan Islands have been intruded, and in many places nearly destroyed, by dikes, sills, and irregular masses of igneous rocks. The latter range in composition from ultrabasic dunites and pyroxenites to the most acid aplites and pegmatites. The bulk of the igneous material was apparently derived from the late Jurassic batholith, and the character of the rocks composing the Turtleback complex would indicate that the roof of the batholith was fairly close to the present surface.

The pre-batholithic intrusions took the form of dikes, sills, and large irregular masses which are probably laccolithic in character. The later off-shoots from the late Jurassic batholith took the form of an injection breccia, and the study of the interrelationships of these rocks is an exceedingly complicated and difficult one.

The late Jurassic batholith outcrops along the shore of the Saanich Peninsula on the West side of Haro Strait.³² Many of the igneous off-shoots occurring in the Turtleback complex in the San Juan Island map-area are identical with dike rocks which cut the granodiorite of the Saanich batholith.

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³²Clapp, C. H., Geology of the Victoria and Saanich Map-Areas, Vancouver Island: Geol. Survey Canada, Mem. 36, pp. 71-93, 1913.

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In the eastern part of the San Juan Island map-area large masses of peridodite have been intruded into the sediments of the Leech River group.

The most widespread of all of the igneous rocks of this region are the Eagle Cliff porphyrites. Although these rocks are commonly ellipsoidal in structure, they are clearly intrusive into the older rocks and likely served as feeders to flow rocks which have since been removed by erosion. It is probable that these rocks are to be correlated with the Vancouver volcanics.³³

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³³Dawson, G. M., Report on a Geological Examination of the Northern Part of Vancouver Island and Adjacent Coasts: Geol. Survey Canada, Ann. Report, 1886, p. 10B, 1887.

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Effusive igneous rocks which have actually reached the surface are now probably lacking in the San Juan Island region.

PLATE XIX. Above: Ellipsoidal Eagle Cliff porphyrite at the east side of Davis Bay, Lopez Island. Below: Ellipsoidal Eagle Cliff porphyrite near Cape St. Mary, Lopez Island.

FIDALGO FORMATION

A number of intrusive masses of serpentized dunite, called here the Fidalgo formation, outcrop in the eastern portion of the map-area.

The Fidalgo formation occurs on Fidalgo Head and at several localities in the southeast part of Fidalgo Island. The following islands are composed entirely of this formation: Burrows Island, Young Island, Allan Island, Williamson Rocks, Saddlebag Island, Dot Island, and Hat Island. It forms the major part of Cypress Island.

Petrographic Details. The Fidalgo formation is composed of three distinct rock types which are invariably associated with each other: (1) Large irregular masses of extremely coarse-grained dunite, which weathers to a dark green or dark brown color. (2) Thin irregular off-shoots of fine-grained dunite, injected into the joint-cracks of the coarse-grained variety. These off-shoots weather first to a light brown or buff color, and on further disintegration, to a bright orange-red. (3) Both types of dunite are everywhere cut by thin stringers (rarely exceeding two inches in thickness) of serpentinized pyroxenite. The latter is composed almost entirely of altered diallage, the crystals often being as large as the width of the stringer permits. Of the three rock types the pyroxenite is the least resistant to alteration and erosion.

The injected fine-grained dunite often contains schlieren-like segregations of chromite. The latter have been mined to some extent on Cypress Island, in spite of the fact that each ore-body usually contains only a few tons of chromite.

At the southeast corner of Cypress Island, a cone-shaped hill, called Olivine Hill, rising to an elevation of 600 feet, is composed almost entirely of fresh vitreous olivine. Individual crystals of the olivine measure up to two inches in length.

A chemical analysis of the fresh olivine rock, made by the writer, gave the following composition:

COARSE-GRAINED TYPES

The coarse-grained dunites contain crystals of altered olivine that average half an inch in diameter. The freshly-broken surfaces are dark green in color, but on longer exposure they change to dark brown or black. When exposed, the rock surfaces are differentially etched by erosion, and the individual crystals stand out in relief. The spaces between the crystals, being the first to alter to serpentine, are more easily and quickly eroded. An occasional crystal of altered diallage is seen on the freshly-broken surfaces. The diallage has been altered largely to a lamellar variety of serpentine which possesses a silvery-gray color and a silky lustre.

Thin sections of the dunite occurring at Olivine Hill on Cypress Island show that 90 to 95 per cent of the rock is composed of olivine. Small scattered crystals of diallage, enstatite, and chromite make up the balance of the rock. The rock is remarkably fresh, and the average section shows almost no trace of serpentine. The diallage is likewise fresh, and shows maximum extinction angles of 38 to 45 degrees. Chromite occurs in subhedral crystals, but more commonly it forms irregular grains.

In all other localities the dunites have been serpentinized. The freshest samples obtained show the alteration to have reached the point where the serpentine exceeds the olivine in abundance. Each crystal of olivine has been crossed by cracks which have been produced by the pressure of crystallization of the serpentine. The alteration has proceeded outward in every direction from the cracks, and isolated remnants of olivine surrounded by serpentine, are all that remain of the former olivine crystals. In many places the alteration has reached the point where all of the olivine has disappeared.

Near the surface, the iron oxide released in the alteration of olivine to serpentine is partially changed to limonite. The chromite oxidizes sufficiently at the surface to kill off the plant life where it is not deeply rooted. The diallage and enstatite alter to bastite, chlorite, and antigorite.

FINE-GRAINED TYPES

The fine-grained dunites are identical in mineral composition with the coarse varieties. They are intruded into the earlier dunites along zones of weakness, particularly along joint cracks, and their widespread distribution indicates that the coarse dunites had not solidified to any great depth when the intrusion took place.

In general the fine-grained dunites contain a larger percentage of chromite. Apparently the magma supplying the dikes was derived from the lower portions of the original magma, whose chromite had been enriched by gravitative differentiation.

The two types of dunite appear to be almost identical in thin section. The large crystals of the coarse varieties are so badly cut up by cracks filled with serpentine that they are almost indistinguishable from the fine-grained dunites. In both cases the typical section shows small remnants of olivine crystals completely surrounded by serpentine. Iddingsite is often present in small quantities, especially in the more highly altered specimens.

The weathered surfaces of the fine-grained dunites are usually buff-colored, and on still further weathering they become orange-red in color. The reddish-colored hill-sides are noticeable at all localities where the Fidalgo formation comes to the surface. The color is due to the chromium chlorite, kotschubeite, together with iddingsite and a chromium-hearing limonite.

PYROXENITE DIKES

The dunites of the Fidalgo formation are everywhere cut by thin stringers of coarse-grained diallage rock which is now largely altered to a variety of serpentine resembling bastite. In was first supposed that the stringers were composed of chrysotile formed during the process of the alteration of the dunites. Some of the stringers of serpentine no doubt formed in this manner, but the majority of them apparently had a quite different origin.

An examination of thin sections of the materials composing the stringers reveals the presence of unaltered remnants of diallage and hornblende. The serpentine occurs typically in the form of a pseudomorph after diallage. It is quite apparent, therefore, that the stringers of this type were derived from pyroxenites intruded into the dunites in the form of an injection breccia. The pyroxenites were evidently charged with a large amount of volatile constituents at the time of their intrusion.

Age Relations. The rocks of the Fidalgo formation are intrusive into the sediments of the Leech River group and consequently they are at least post-Carboniferous, and probably post-Paleozoic in age.

On Fidalgo Head, across the channel from Burrows Island, a dike of the Eagle Cliff porphyrite has intruded the Fidalgo formation. Similar intrusive dikes occur at several localities on Cypress Island. Since the Eagle Cliff porphyrites are probably of late Triassic or early Jurassic age, it appears that the Fidalgo formation was intruded during the Triassic period.

The fresh vitreous dunite occurring on Olivine Hill on Cypress Island is apparently much younger than the serpentinized dunites. It is possible that the fresh dunite is equivalent to the dunites of late Tertiary age which occur in the Skagit and Hozomeen Ranges.

EAGLE CLIFF PORPHYRITE

Intrusive dikes of porphyrite, referred to here as the Eagle Cliff porphyrite, are to be seen in all parts of the map-area where pre-Cretaceous rocks are exposed. The porphyrites are the most widespread and abundant of all igneous rock types occurring on the San Juan Islands.

At Eagle Cliff, on the north end of Cypress Island, the ellipsoidal porphyrites form immense dikes, cutting the Leech River slates and graywackes on one side and the Fidalgo formation on the other.

On Lopez Island the porphyrites are practically free from the late Jurassic intrusives. On San Juan and Orcas islands they have been metamorphosed and in places destroyed by later intrusions. In the eastern part of the map-area, the porphyrites are usually free from the later intrusions, and in such cases the rocks are remarkably fresh.

The rocks occurring on the following islands are composed largely or entirely of the Eagle Cliff porphyrites: Turn Island, Cliff Island, Spindle Rock, Black Rock, Small Island, Fortress Island, Ram Islands, southern part of Decatur Island, Richardson Rock, Castle Island, Boulder Island, Bird Rocks, Belle Rock, the larger or eastern members of the Cone Island group, and Vendovi Island. Excellent outcrops of the Eagle Cliff porphyrites occur on Lopez Island, Orcas Island, San Juan Island, Blakeley Island, Cypress Island, Guemes Island, Lummi Island, and on many of the lesser island and reefs.

Petrographic Details. The Eagle Cliff porphyrites are typically ellipsoidal in structure, the pillow-shaped masses varying from two inches to several feet in diameter. They clearly occur as dikes and sills, and probably served as feeders to flow rocks that have since been removed by erosion. The rocks resemble basalts and basic andesites, both microscopically and structurally, and it is apparent that they solidified near the surface. From their occurrences, it seems probable that the surface of the ground at the time of the intrusion was not far from the elevation of the present surface.

On the southern part of Lopez Island there are excellent exposures of the contacts of the porphyrites where they intrude the sediments of the Leech River group. The sedimentary rocks are fractured and silicified on both the upper and lower contacts of the porphyrites.

On San Juan and Orcas islands, where the porphyrites sometimes cut the limestones of the Orcas group, the latter are silicified and irregular nodules of flint make their appearance. More rarely, crystals of grossularite, epidote, vesuvianite, and wollastonite are present in the limestones, but some of these minerals are probably due to the late Jurassic intrusives.

The porphyrites are usually medium to fine-grained in texture, and in areas where they have not suffered from later intrusives they are green in color. In the vicinity of the intrusive dikes of the Turtleback complex the porphyrites are often reddish-brown in color, and sometimes highly metamorphosed. The ellipsoidal masses are fine-grained at the margins and they become increasingly coarse in texture as the center is approached. The spaces between the ellipsoids are usually filled with fine-grained andesitic or basaltic material which is often silicified. Small radially arranged vesicles are almost invariably present within the ellipsoids.

Thin sections of the porphyrites at Eagle Cliff show a typically basaltic texture. The interstices between the long reticulated laths of acid labradorite or andesine are filled with augite. Both common augite and titaniferous augite are usually present in the porphyrites. Although olivine is present in the rocks at the type locality, it is generally lacking in the other parts of the map-area. Basaltic hornblende occurs in some of the porphyrites and ilmenite and pyrite are usually present in relatively large amounts. In many instances the coarser-grained porphyrites contain varioles of mingled fine-grained plagioclase and pyroxene. The varioles are relatively rich in pyroxene.

While the porphyrites appear to be quite fresh in the hand specimen the microscope reveals the fact that a considerable amount of alteration has taken place. The plagioclase has been somewhat altered to saussurite and the pyroxene has partly altered to chlorite, serpentine, and epidote. The alteration of the olivine has reached the point where about 50 per cent has changed to serpentine. Streaks of calcite, probably derived from the alteration of the andesine or labradorite, occur in the typical section. Where vesicles occur they are usually filled with serpentine and calcite, but in some cases, chalcedony or zeolites form the chief filling material.

The filling between the ellipsoids is composed of glass with plagioclase microlites. The margins of the ellipsoids contain a considerable amount of glass in the interstices between the plagioclase laths. Towards the center of the pillows the rocks are holocrystalline but never coarse-grained.

The Eagle Cliff porphyrites belong, on the whole, to the more acid type of basalt porphyrites, and they grade into diorite or andesite porphyrites. Some of the more acid facies, e.g., the ellipsoidal porphyrites on the north end of Lummi Island, show a semi-andesitic texture and a relatively large percentage of acid or intermediate andesine. In many places in the western part of the map-area the rocks are porphyritic with rounded phenocrysts of andesine or labradorite up to three-eights of an inch in diameter. The groundmass in the porphyritic varieties is identical in texture and composition with the non-porphyritic types. Vesicles are occasionally present in the porphyritic types.

On Orcas Island where the porphyrites have been cut and largely destroyed by intrusions of the Turtleback complex, the textures are somewhat coarser. The alteration has been such that little remains of the original rock except its texture. The feldspars have altered to kaolin, calcite, and zoisite, and later they became silicified and albitization took place. The augite and other femic minerals have changed, first to uralite and later to chlorite with minor amounts of serpentine and epidote. Tremolite and actinolite are usually present, especially near the contacts with later intrusives.

Age Relations. Owing to the fresh appearance and ellipsoidal nature of the porphyrites in the eastern part of the map area, the author first supposed that they belonged to the Metchosin volcanics (middle or upper Eocene). However, a contact with the sandstones and conglomerates of the Chuckanut formation (lower Eocene), occurring on the north end of Lummi Island, shows that the latter were laid down upon the eroded surfaces of the dikes of the Eagle Cliff porphyrites.

The rocks of the Leech River group serve as the most common host for the porphyrite dikes and sills. The texture and ellipsoidal nature of the porphyrites would naturally indicate that they were flow rocks. Nevertheless they clearly occur as dikes and sills intruding the Leech River and Orcas sediments. The sediments are metamorphosed and brecciated on both the upper and lower contacts of the intrusive sills.

The field evidence indicates that the porphyrites were feeders to former flow rocks, and the present outcrops represent the uppermost portions of the dikes.

The porphyrites intrude the Fidalgo formation at several localities on Cypress and Fidalgo islands. On Orcas and Blakeley islands the metamorphosed remnants of somewhat coarser-textured ellipsoidal porphyrites are intruded by the off-shoots from the late Jurassic batholith. It is probable that the Eagle Cliff porphyrites are of late Triassic or early Jurassic age and related to the Vancouver volcanics.

TURTEBACK COMPLEX

Throughout areas of considerable size the Paleozoic sediments have been intruded by igneous rocks of so many different types and ages that it is impossible to map them individually. For this composite group of intrusive igneous rocks, which are well exposed on Turtleback Mountain, the name Turtleback complex will be used.

In the areas mapped as the Turtleback complex, the Paleozoic sediments were first intruded by dikes and sills of the Eagle Cliff porphyrite. Intrusions of the still earlier rocks of the Fidalgo formation are seldom found in the areas mapped as Turtleback complex. If these were formerly present they have been almost entirely destroyed by later intrusions.

Some of the rocks belonging to the Eagle Cliff porphyrites are apparently related lithologically to the typical Vancouver volcanics occurring on Vancouver Island. These intrusions did not all occur at the same time. It is probable that they were intruded intermittently during a great lapse of time.

Following the intrusions of the Eagle Cliff porphyrites the region was cut by a great variety of intrusive materials, the greater part of which were probably derived from the late Jurassic batholith. The invaded rocks were so badly broken and shattered that the later intrusions took the form of an injection breccia. The igneous rocks occurring on Turtleback Mountain have the appearance of a scrambled mass or net-work of both acid and basic intrusives in a matrix furnished by the remnants of the Eagle Cliff porphyrites. On Blakeley Island the matrix is usually formed by a fine-grained gabbro diorite which may be laccolithic in nature.

The Turtleback complex is composed of a confused net-work or injection breccia containing the following rock types:

(1) Dunites of the Fidalgo formation.

(2) Basalt and andesite porphyrite belonging to the Eagle Cliff porphyrites.

(3) The Wark gabbro diorite.

(4) The Colquitz quartz-diorite.

(5) Scattered off-shoots of diorite porphyrite.

(6) Scattered off-shoots of rhyolite porphyry.

(7) A series of granodiorite porphyry off-shoots together with aplites, pegmatites, and igneous quartz veins.

(8) A series of lamprophyric off-shoots, ranging from the more basic porphyrites to the ultrabasic pyroxenites and hornblendites.

DUNITES OF THE FIDALGO FORMATION

A few scattered remnants of serpentinized dunite occur on Mount Pickett Range and also on Blakeley Island. These remnants presumably belong to the Fidalgo formation. In lithology they are similar to the fine-grained types of dunite seen on Cypress Island.

EAGLE CLIFF PORPHYRITES

In the western and northwestern portions of the map-area the matrix of the injection breccia is generally formed by the Eagle Cliff basalt and andesite porphyrites. Where they are intruded by the later off-shoots from the late Jurassic batholith the Eagle Cliff porphyrites are often red in color and altered almost beyond recognition.

WARK GABBRO-DIORITE

On Blakeley Island and other smaller islands the matrix of the injection breccia composing the Turtleback complex is formed largely by the Wark gabbro diorite³⁴ These rocks are identical in lithology with those occurring in the vicinity of Victoria, B. C., though as a rule they are somewhat finer-grained. In several localities on Blakeley Island the Wark gabbro diorites are intruded by long and extremely thin parallel stringers of diallagite and hornblendite. (See Plate VII-B.) Near the contacts of these stringers the gabbro diorites are profoundly altered and crystals of diallage occur disseminated through them. Where the contact action has been intense, the minerals composing the gabbro diorite are largely replaced by pyrope, andalusite, and vesuvianite. The diallagites and hornblendites were apparently very fluid and contained a high percentage of volatile matter.

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³⁴Clapp, C. H., Geology of the Victoria and Saanich Map-Areas, Vancouver Island: Geol. Survey Canada, Mem. 36, pp. 57-71, 1913.

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The Wark gabbro diorite occurs in all of the larger areas mapped as Turtleback complex. It is particularly abundant on Blakeley Island, but it also occurs on Frost Island, Willow Island, Armitage Island, Pointer Island, Orcas Island, San Juan Island, Guemes Island, Huckleberry Island, and along the shore of Burrows Bay on Fidalgo Island.

COLQUITZ QUARTZ-DIORITE

The Wark gabbro diorite is usually accompanied by later intrusions of quartz-diorite known as the Colquitz quartz-diorite.^{35, 36} These rocks apparently form a part of the great series of off-shoots derived from the late Jurassic batholiths.

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³⁵Clapp, C. H., Southern Vancouver Island: Geol. Survey Canada, Mem. 13, pp. 96-101, 1912.

³⁶Clapp, C. H., Geology of the Victoria and Saanich Map-Areas, Vancouver Island: Geol. Survey Canada, Mem. 36, pp. 57-71, 1913.

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DIORITE PORPHYRITE

Occasional small intrusive masses of diorite porphyrite are seen in the injection breccia composing the Turtleback complex on Orcas Island, Blakeley Island, and Fidalgo Island. The diorite porphyrites have the same mineral composition as the more acid facies of the Wark gabbro diorites, and it is assumed that they are related in origin.

PLATE XX. Above: Turtleback Mountain and Orcas Knob as seen from the entrance of West Sound. Below: Mount Baker as seen from Friday Harbor, San Juan Island. (Photograph by J. A. McCormick).

RHYOLITE PORPHYRY

Small isolated intrusions of rhyolite porphyry occur in the Turtleback complex on Orcas Island. They also occur on Jones Island and Barren Island, the latter being composed entirely of rhyolite porphyry.

The rhyolite porphyry is brownish-gray in color with vitreous phenocrysts of quartz. At the surface it weathers to a white color with a pink stain. The groundmass is aphanitic and somewhat glassy in the fresh specimens. The phenocrysts of quartz are usually rounded by resorption, and they measure up to a quarter-inch in diameter.

Under the microscope the rock is seen to consist of a glassy groundmass containing large rounded phenocrysts of quartz and smaller ones of orthoclase. The quartz crystals are cracked by the pressure of crystallization, and they generally contain minute needle-like inclusions of rutile. The orthoclase crystals are much smaller than the quartz phenocrysts, the amount of silica in the rock exceeding 75 per cent. The orthoclase crystals are somewhat altered to kaolin but they have not been resorbed to any extent.

The groundmass in the rocks on Barren Island is glassy and filled with microlites. On Jones and Orcas islands the rocks usually have a dense aphanitic groundmass showing occasional minute crystals of orthoclase.

The rhyolite porphyries appear to be among the youngest of the off-shoots composing the Turtleback complex.

The Turtleback complex includes a series of porphyry and lamprophyric off-shoots thrown out by the late Jurassic batholith. The earlier intrusions were coarse-grained and very inequigranular quartz-diorite porphyrites. The succeeding off-shoots followed two distinct types of differentiation; the one became increasingly acid and culminated in the aplites, pegmatites, and quartz veins; the other became increasingly basic and formed a series of lamprophyric intrusions.

ACID OFF-SHOOTS

Porphyry Intrusions. The earlier porphyries are very inequigranular and usually coarse or medium-grained. They are composed largely of plagioclase and hornblende.

The earlier off-shoots consist chiefly of andesine, hornblende, orthoclase, and quartz. Biotite occurs in some of the rocks but its presence is not consistent. The plagioclase and hornblende occur in crystals of various sizes, but the orthoclase and quartz crystals are relatively small. The hornblende is generally somewhat altered to chlorite and serpentine.

The succeeding intrusions are less coarsely-textured and the crystals are less inequigranular. Accompanying this change the plagioclase becomes more and more acid and finally culminates in albite or soda orthoclase. The proportions of quartz, orthoclase, and soda orthoclase increase, but the hornblende decreases and is partly displaced by primary biotite.

Each set of off-shoots exerted its effect on the metamorphism of the earlier intrusives.

Later Acid Intrusions. The later porphyry off-shoots become finer and finer grained and grade into the equigranular aplites. The igneous rocks which intrude the aplites are confined to the pegmatites, quartz veins, and the coarse diallagites.

The later porphyry off-shoots consist chiefly of albite, soda orthoclase, and quartz, the latter making up over 25 per cent of the rock. An acid oligoclase sometimes occurs to the extent of about ten per cent. Small crystals of biotite, now largely altered to chlorite, make up the remainder of the rock. Magnetite and pyrite are usually present in small amounts.

The true aplites are medium to fine-grained rocks with an equigranular or sugary texture. In this region they always contain soda-bearing feldspars as the predominating constituents. The percentage of quartz in the aplites is generally well above 25 per cent.

The pegmatites, which are far more abundant than the aplites in this region, are composed of the same minerals as the latter. The variations in the relative proportions of the minerals is much greater in the pegmatites and their textures are usually much coarser. Occasionally the graphic texture is seen in the pegmatites. Some of the pegmatites contain more than 50 per cent of quartz and apparently grade into the igneous quartz veins. The feldspars of the pegmatites are more altered to kaolin than is usually the case with the aplites.

The youngest igneous rocks composing the Turtleback complex are the igneous quartz veins which cut all of the other rocks of the region. It is probable that the quartz veins which almost everywhere cut the sediments of the Leech River group, are a part of the last off-shoots from the Late Jurassic batholith.

LAMPROPHYRIC OFF-SHOOTS

The earlier porphyry off-shoots contain considerable percentages of hornblende. With the earlier off-shoots as a starting-point, a series of basic intrusives have arisen which form an analogy with the acid series. The lamprophyres were apparently thrown off from a portion of the basic differentiate within the batholith. In each succeeding set of off-shoots the percentage of hornblende became larger and larger. As in the case of the acid series the succeeding off-shoots were less inequigranular than the preceding ones.

The later lamprophyres vary greatly in mineral composition. They usually contain hornblende, diallage, biotite, acid plagioclase or soda orthoclase, and occasionally quartz, olivine, and basic plagioclase. Diallage tends to replace hornblende as a primary mineral in the later lamprophyres. Magnetite is the most persistent accessory mineral and it is often quite abundant. Titanite and apatite are usually present in small amounts.

The typical fine-grained equigranular lamprophyres, with a texture similar to that of granulated sugar, apparently correspond with the aplites in the acid series. They were thrown off a little earlier than the aplites which usually cut them.

The succeeding basic off-shoots, which generally cut the aplites, are composed largely of diallage and hornblende with minor amounts of enstatite and olivine. These may be considered as ultrabasic pegmatites, the crystals of which frequently measure six inches in length. At the time of their intrusion these diallagites and hornblendites contained large quantities of volatile constituents which altered the invaded rocks for considerable distances from the contacts. Where these rocks have intruded into the Wark gabbro-diorites, the later have been more or less completely replaced by magnesian garnet, and alusite, quartz, vesuvianite, diopside, and other contact minerals. It is difficult to understand how some of these contact minerals originated. It is very remarkable that sufficient alumina was provided to form andalusite, which is so abundantly present near these contacts. The occurrence of thin parallel ribbon-like stringers of diallagite and hornblendite in the Wark gabbro-diorite is equally astonishing. (see Plate VII-B.)

The diallagites sometimes show all of the transitional phases from fresh unaltered diallage to light green hornblende pseudomorphic after diallage, and finally to a variety of serpentine resembling bastite. Primary and secondary hornblende may occur in the same rock.

Age Relations. Owing to the fact that the Cretaceous and Eocene sediments occurring in the San Juan Island region are not cut by any igneous materials and since the conglomerate occurring in these sediments include boulders composed of rocks from the Turtleback complex it is evident that the igneous rocks composing the Turtleback complex are pre-Cretaceous in age. The great series of igneous off-shoots must therefore be related to the late Jurassic batholiths and not to those of late Miocene age.