Geological Survey Bulletin 1359 Geology and Mineral Resources of the Northern Part of the North Cascades National Park, Washington

By MORTIMER H. STAATZ, ROWLAND W. TABOR, PAUL L. WEIS, and JACQUES F. ROBERTSON, U.S. Geological Survey

Great expanses of highly deformed metamorphic rocks, thick piles of weakly deformed volcanic rocks, and thin accumulations of sedimentary rocks are all intruded by the varied intrusive rocks of the Chilliwack composite batholith. The batholith forms a medial belt that separates metamorphic and volcanic rocks on the east from a different suite of rocks on the west.

ROCKS IN THE EASTERN PART OF THE AREA

CUSTER GNEISS OF McTAGGART AND THOMPSON (1967)

The Custer Gneiss of McTaggart and Thompson (1967) is the oldest rock unit known in the area. This unit, which is principally made up of biotite and hornblende gneisses, forms the bedrock in the southeast quarter of the area and from there northwestward is present as isolated remnants surrounded by batholithic rocks. These remnants indicate the approximate former extent of the gneiss. Unconformably overlying the gneiss are the Skagit Volcanics (fig. 3) in the northeast part of the area and the Hannegan Volcanics in a small area in the central part. The gneiss is bounded in the east-central part of the area by metagabbro. The structural and age relations of that contact are obscure.

FIGURE 3.—Custer Gneiss (cg) overlain by Skagit Volcanics (Ts) on west side of a spur of Glacier Peak.

Outcrops of the gneiss along the Canadian border were originally named the Custer Granite Gneiss by Daly (1912, p. 523-526). The name was modified to Custer Gneiss by McTaggart and Thompson (1967, p. 1205-1219), who mapped the rocks in British Colombia and traced them 30 miles north of the U.S. border. These or similar gneisses can be traced southeastward for at least 45 miles to Lake Chelan, where they have been correlated with the pre-Upper Cretaceous Swakane Biotite Gneiss (Cater and Wright, 1967). Misch (1952, p. 12-14) called these rocks Skagit Gneiss, but the name Skagit had been previously used by Daly (1912, p. 528-531) for the volcanic sequence along the Canadian border.

The Custer Gneiss of McTaggart and Thompson is a well-foliated rock, the foliation of which has a general northwesterly strike and a steep dip. The rock is made up principally of varying proportions of five rock types: (1) light-colored biotite and hornblende-biotite gneiss, (2) dark biotite and biotite-hornblende gneiss and schist, (3) amphibolite-gneiss, (4) marble and calc-silicate rocks, and (5) light-colored quartz diorite. The gneisses and marbles are commonly interlayered, and in many areas sills, dikes, and irregular masses of light-colored quartz diorite are intimately intruded into the older rock types (fig. 5B). Combinations of these different rock types form three distinctive units within the Custer Gneiss (pl. 1). These units are (1) light-colored gneiss consisting predominantly of light-colored biotite and biotite-hornblende gneiss with abundant light-colored quartz diorite dikes, sills, and irregular masses, (2) banded gneiss consisting of alternating layers of light-colored biotite and hornblende-biotite gneiss with dark biotite and biotite-hornblende gneiss that is intruded by numerous dikes and sills of light-colored quartz diorite, and (3) amphibole-rich gneiss consisting of alternating layers of amphibolite with the light-colored biotite and biotite-hornblende gneiss. The marble and calc-silicate rocks occur in the amphibole-rich gneiss, and where the marble is thick enough it is delineated on plate 1.

The five rock types that make up these three units are first discussed, and then a general overall description of each unit is given.

The light-colored biotite and hornblende-biotite gneiss ranges from a strongly foliated rock, with thin anastomosing folia of biotite and (or) hornblende between lenses of plagioclase and quartz, to a faintly lineated granitoid rock (fig. 4A, B). Typically, this gneiss is composed of about 15-30 percent quartz, 45-60 percent andesine, 10-15 percent biotite, 0-10 percent hornblende, 1 percent or less potassium feldspar, and 1-2 percent garnet, apatite, allanite, sphene, magnetite, and zircon. Plagioclase tends to form larger zoned crystals set in a matrix of finer quartz and biotite. Recrystallized mylonite in the light-colored gneiss crops out in several places, including areas on Luna Peak and Mount Redoubt (fig. 4C). In these rocks, subrounded plagioclase grains, as much as 5 mm long, are set in a streaked-out matrix of very fine grained quartz and plagioclase with thin lenses of biotite-hornblende aggregate. The theoretical petrology of the gneisses is discussed by Misch (1968).

FIGURE 4.—Custer Gneiss. A, Uniform biotite gneiss exposed on the north side of McMillan Spire. B, Closeup of uniform biotite gneiss. This is a layer in the banded gneiss unit on the south ridge of Davis Peak. C, Recrystallized mylonite in the banded gneiss unit northwest of Mount Redoubt. Large rounded white masses are of light-colored quartz diorite; small white masses are of feldspar.

The dark biotite and biotite-hornblende gneiss and schist are generally brown or green and are finer grained and more schistose than the light biotite and hornblende-biotite gneiss. Although the rocks range in composition from biotite-quartz schist to a biotite-hornblende gneiss, they are mostly made up of 20-50 percent biotite, 10-25 percent hornblende, 50-80 percent plagioclase, and 10-30 percent quartz. Some contain as much as 10 percent garnet. Potassium feldspar is rare. Magnetite, apatite, sphene, and zircon are minor accessory minerals.

The amphibolite is a dark-green rock with a well-defined lineation. It consists principally of hornblende and plagioclase and grades into biotite-hornblende gneiss by an increase of biotite and decrease in hornblende content. This rock consists of 25-55 percent hornblende, 30-70 percent plagioclase, and minor amounts of quartz chlorite magnetite, sphene, and ilmenite.

The marbles and related calc-silicate rocks form relatively thin layers or lenses that are rarely more than a few hundred feet thick. Marbles are white to gray, fine to coarse grained, and commonly banded. Marble layers in upper Pass Creek (east of Whatcom Pass) contain as much as 10 percent reddish-brown andradite that occurs in aggregates as large as an inch in diameter. The calc-silicate rocks consist principally of clinopyroxene, andradite, idocrase, calcic plagioclase, and epidote with accessory sphene, apatite, graphite, and magnetite.

The light-colored fine- to coarse-grained quartz diorite forms dikes, sills, and irregular masses (fig. 5) that are the most ubiquitous and distinctive rocks in the Custer Gneiss. Although this rock type is common, only locally does it make up more than about 30 percent of any unit. The quartz diorite is made up almost entirely of plagioclase and quartz, and it owes its white color to its very low content of mafic minerals, which seldom exceed 5 percent of the rock. The contacts with other rock types range from gradational (fig. 5A) to sharply crosscutting (fig. 5B).

FIGURE 5.—Custer Gneiss. A, Swirled banded gneiss made up of layers of dark and light-colored biotite gneiss. White rock under pick handle and in upper left-hand corner is light-colored quartz diorite. Picture taken on south ridge of Davis Peak. B, Fairly uniform biotite gneiss crosscuts banded gneiss on the south ridge of Davis Peak. Dikes and sills are light-colored quartz diorite. C, Banded gneiss made up of layers of dark biotite gneiss in light-colored biotite and hornblende gneiss on the south ridge of Davis Peak. Whitest layers are light-colored quartz diorite.

Two or more of these rock types make up the three units in the Custer Gneiss that are described in the following paragraphs.

LIGHT-COLORED GNEISS

Although the light-colored gneiss is predominantly light-colored biotite and biotite-hornblende gneiss and light-colored quartz diorite, it locally contains some dark biotite and biotite-hornblende gneiss.

The light-colored gneiss varies from place to place. In some areas, such as in the Crescent Creek and Terror Creek cirques, the rock is fairly uniform, consisting mainly of light-colored biotite and hornblende-biotite gneiss. In other areas, the light-colored biotite and biotite-hornblende gneiss forms a migmatite with the quartz diorite. The contacts of these two rock types may be either gradational or crosscutting. Some of this light-colored gneiss contains small irregular bodies of darker rock. Those bodies rich in biotite may have been derived from an older gneiss; those rich in hornblende may be pieces of metamorphosed dikes. Swirls and contortions of the light-colored gneiss suggest plastic deformation. Crosscutting contacts and rotated inclusions give evidence of movement.

A faint lineation formed by spindle-shaped aggregates of biotite and hornblende is the most consistent structural element. Foliation where detectable parallels this lineation. Divergent structural elements are also common, especially in areas of multiple intrusion. For instance, in the northern part of the Picket Range a conspicuous sub-horizontal layering, made by swarms of light-colored quartz diorite dikes, crosscuts the steeply southwest dipping foliation.

BANDED GNEISS

The banded gneiss is the most common unit in this area. It is particularly conspicuous in the Mount Redoubt area, where hornblende-rich layers are commonly stained with iron oxides. In some areas, as on Luna Peak and in the Mount Redoubt area, the banded gneiss has been thoroughly granulated before or during metamorphism (fig. 4C). Multiple intrusion of quartz diorite into the gneissic rocks formed migmatites in the banded gneiss that are similar to but somewhat darker than those formed in the light-colored gneiss.

Foliation in the banded gneiss is parallel to the compositional layering; locally both are contorted. Fold axes of the contortions are relatively uniform, plunging gently southeast. They parallel both the lineation formed by several sets of joints and the alinement of sausage shaped rods of light-colored quartz diorite that commonly are found along fold crests.

AMPHIBOLE-RICH GNEISS

The amphibole-rich gneiss is commonly banded and is gradational with the banded gneiss. The two were arbitrarily separated in mapping on the basis of the hornblende content. Light-colored quartz diorite intrusions are not common in this unit, although on the ridge west of Pierce Creek (east of Sourdough Lake) thin crisscrossed dikes of quartz diorite have produced a spectacular "breccia." The amphibole rich gneiss is also somewhat limy, and it is in this unit that the marble and calc-silicate rocks occur. Limy rocks crop out sporadically in a belt from the head of Pass Creek (east of Whatcom Pass) to Ross Dam (pl. 1). Fine- to coarse-grained marble crops out in discontinuous lenses as much as 200 feet thick near the center of the unit. An anthophyllite-phlogopite schist in Arctic Creek may have been formed by metamorphism of dolomitic rocks.

Like the banded gneiss, the amphibole-rich gneiss is characteristically well-foliated parallel to compositional layering. Although most of the folds plunge southeast, on the ridge north of Arctic Creek foliation and compositional layers are folded into broad northwest-plunging folds.

ORIGIN AND AGE

The history of the Custer Gneiss of McTaggart and Thompson has been long and involved. As Misch (1952, p. 12) previously noted, the rocks appear to have a sedimentary and volcanic origin. Some of the layers, such as marble, calc-silicate rock, and amphibolite, were obviously derived from such a source. Interlayered biotite and hornblende-biotite gneiss and other rocks also appear to be primary compositional layers. The light-colored quartz diorite that is intimately intermixed with these layers may have been derived from an external igneous source or they may have formed during the intense metamorphism by being segregated from the gneiss itself.

Broken and granulated rocks indicate that the deformation was severe and that it occurred after most metamorphic crystallization. Most of the metamorphism preceded the intrusion of the Chilliwack batholith, although some recrystallization is due to heating by this underlying batholith. Obvious thermal metamorphism, such as the formation of fine-grained directionless aggregates of biotite, quartz, and plagioclase, is limited to a few hundred feet from the contact or to relatively small roof pendants or embayments, as on Pioneer Ridge or east of Wiley Lake (east of Whatcom Peak).

The Custer Gneiss is one of the oldest rock units in this area. Little, however, is known of its age. In the northern part of the area, it is cut by intrusions of the Chilliwack composite batholith that are of early Tertiary age. Near Lake Chelan, F. W. Cater (oral commun., 1967) noted that the correlative Swakane Biotite Gneiss is intruded by quartz monzonite of Late Cretaceous age. Hence, it is at least as old as Cretaceous and probably much older.

METAGABBRO

West of Ross Lake, along the northeast side of the Custer Gneiss, is an elongate mass of rock approximately 5,000 feet wide that is predominately metamorphosed hornblende gabbro. This rock body can be traced northwestward for 7-1/2 miles from Ross Lake across the headwaters of Skymo and Noname Creeks to Arctic Creek.

The metagabbro lies between the Custer Gneiss and the phyllite and schist of Ross Lake; north of Arctic Creek it is cut off by quartz diorite of the Chilliwack batholith (pl. 1).

The metagabbro is along the Ross Lake fault zone, and continued movement since the emplacement of the metagabbro is indicated by extensive shearing within the unit. The contact with the Custer Gneiss is interpreted as an intrusive contact, but as it is not exposed and is in an area of shearing, it might well be a fault. Along its northeast side the metagabbro is faulted against the phyllite and schist of Ross Lake and in places is imbricated with it.

In addition to the predominant metamorphosed hornblende gabbro, the mass contains irregular bodies of pyroxene gabbro, hornblende diorite, schistose amphibolite, and ultramafic rock, all of which, at one place or another, have been intruded by dikes of light-colored quartz diorite and dark-gray porphyry. The close association of gradational contacts between these rocks indicates that they are probably all facies of the same gabbroic intrusive.

Most of the hornblende metagabbro is massive; locally, it is gneissose or schistose or sheared in zones a few inches wide. The gabbro consists of 50-60 percent labradorite (locally bytownite) surrounded by large pale-brown crystals of hornblende partially altered to actinolite. Relict clinopyroxene and hypersthene in the amphibole probably were the dominant mafic minerals in the original rock. Commonly the plagioclase is crushed and the amphibole squeezed into well-defined microshears.

The pyroxene gabbro is generally fine grained and consists of about 50 percent labradorite and 50 percent clinopyroxene. In many places the pyroxene is in part pseudomorphically replaced by pale-brownish-green hornblende and actinolite, and in other places it is replaced by clay minerals and calcite.

Hornblende diorite makes up most of the metagabbro along Ross Lake. This rock consists of plagioclase, brown hornblende (partially altered to actinolite), sphene, ilmenite, and chlorite. The rock is commonly sheared and crisscrossed by thin zones of mylonite. Zones of fine-grained schistose amphibolite occur in some of the crushed diorite.

Ultramafic rocks are found north of Noname Creek, where large pods of plagioclase-bearing pyroxenite several hundred feet across occur in the hornblende gabbro. North of Skymo Creek small pods of dunite a few inches to several feet across and larger irregular masses of dunite of unknown extent crop out near the eastern contact of the metagabbro. Olivine, minor phiogopite, ilmenite, and magnetite are primary minerals. Late alteration products are prehnite, cummingtonite, calcite, and chlorite.

Light-colored quartz diorite similar to that found in the Custer Gneiss cuts the metagabbro on the ridge southeast of Arctic Creek and contains inclusions of hornblende gabbro and metamorphosed dark-gray porphyry.

The age of the metagabbro is not known. It is older than the rocks of the Chilliwack batholith of Tertiary age, which cut it north of Arctic Creek. It is also older than the regional metamorphism and the light-colored metamorphosed quartz diorite dikes. These metamorphosed dikes cut only the Custer Gneiss and the metagabbro; thus the metagabbro is probably one of the older rock units in the area.

PHYLLITE AND SCHIST OF ROSS LAKE

Phyllite and schist crop out along the broad Ross Lake fault zone between the Hozomeen Group of Carines (1944) to the east and the metagabbro (pl. 1). These rocks form a belt, which for most of its length of 5.5 miles is 2,000-3,000 feet wide and which extends from Ross Lake northwest to the ridge south of Arctic Creek. The rocks are separated from the metagabbro and from most of the Hozomeen Group by faults. A fault within the unit separates the more phyllitic rocks to the northeast from the more schistose rocks to the southwest. Other parts of the unit also have been sheared by fault movement. South of Skymo Creek the schist and phyllite are in normal contact with the Hozomeen Group. There phyllites, similar to those seen in the phyllite and schist unit, are interbedded with the greenstones in the upper part of the Hozomeen. On the ridge south of Arctic Creek, the phyllite and schist are cut off by rocks of the Chilliwack batholith (pl. 1).

The phyllite and schist are principally metamorphosed sedimentary rocks that were cut by igneous rocks before both were metamorposed. Most of the sedimentary rocks were originally argillites with some arkose beds. Metamorphism is complex and appears to be in part dynamic and in part thermal. The grade of metamorphism gradually increases from northeast to southwest, with a change from phyllite to various types of schists. Across faults, however, changes are commonly abrupt.

Most of the phyllites along the northeast side of the belt are black and strongly foliated. Foliation generally parallels bedding, and both are tightly folded. Some rocks, however, have several crosscutting foliations. Typical phyllite contains mostly quartz and feldspar.

Near the center of the belt, biotite schist is commonly found with the phyllite. Both are brown to black and have strong foliation. The schist contains quartz, plagioclase, sericite, pale-green to brown biotite, chlorite, and magnetite. Although these rocks have been recrystallized, some rocks have rounded composite grains of quartz and feldspar that may be relict sand grains.

More thoroughly metamorphosed rocks in the southwestern part of the belt are separated by a major fault from phyllite and schist to the northeast. Here, fine-grained biotite and hornblende-biotite schist are made up of quartz, plagioclase, biotite, green hornblende, calcite, apatite, and magnetite. Biotite commonly occurs as large crystals, which are set in a finer grained matrix. Some of the rocks have been granulated and later recrystallized. Only a few of the rocks have relict sedimentary clastic textures.

Scattered along the southwest side or the belt are outcrops of cordierite (?)-garnet-sillimanite schist, andalusite-biotite schist, and sillimanite-garnet-biotite schist. Some of these high-rank schists are associated with quartz diorite intrusions. Most of these schists have also been granulated and partially recrystallized. During this process the sillimanite and garnet crystals were crushed and locally replaced by biotite; andalusite crystals are partially or completely pseudomorphically replaced by sericite and muscovite.

Amphibole-rich rocks on the ridge south of Arctic Creek range in composition from fine-grained hornblende-plagioclase schist that is rich in sphene and calcite to a fine-grained anthophyllite-rich rock.

Several bodies of foliated and nonfoliated metamorphosed quartz diorite intrude the schists and phyllites. The intrusive character of this rock is demonstrable in a body in the southern part of the area just west of Ross Lake that contains numerous schist inclusions and in a body on the ridge south of Noname Creek that has chilled margins and has metamorphosed porphyry dikes extending from the quartz diorite body into the surrounding schist. The quartz diorite is composed of 50 percent plagioclase, 30 percent quartz, 5 percent potassium feldspar, and 15 percent biotite and hornblende, as well as accessory sphene, apatite, magnetite, and zircon. The dike offshoots have essentially the same mineralogy and texture, but they are finer grained and more schistose. Nonfoliated quartz diorite within or intimately mixed with schist just west of Ross Lake contains cordierite, garnet, and sillimanite. Clots of white mica surrounding spinel dot the rocks.

Light-colored metaporphyry dikes and sills, ranging from rhyolite to dacite in composition, occur throughout the southwestern part of this unit. Although these rocks have been recrystallized, the dikes commonly contain relict phenocrysts that are either broken crystals of potassium feldspar, zoned plagioclase, or aggregates of quartz and plagioclase The metaporphyry sills and surrounding schist are commonly tightly folded.

The phyllite and schist unit is also known east of Ross Lake, though no schist is present there (Staatz and others, 1971). From Ross Lake the unit can be traced eastward along Canyon Creek around the south end of the outcrop of the Hozomeen Group of Cairnes. Its metamorphic grade decreases eastward, and the rock gradually changes from a phyllite to an unmetamorphosed argillite, which is part of the thick "plagioclase arkose and argillite sequence" that underlies much of the Pasayten Wilderness (Staatz and others, 1971, pl. 1). Misch (1966, p. 115) also noted this gradual change from argillite to phyllite and referred to the phyllitic part of this sequence as the Jack Mountain Phyllite. The plagioclase arkose and argillite sequence has been traced northward into British Columbia, where Coates (1967) collected fossils from the sequence that ranged from Early Jurassic to latest Early Cretaceous in age.

The quartz diorite and metaporphyry intrusions are somewhat younger than the phyllite and schist, but they preceded the metamorphism.

HOZOMEEN GROUP OF CAIRNES (1944)

The Hozomeen Group of Cairnes (1944) consists of a thick sequence of slightly metamorphosed mafic lavas (greenstones) with subordinate chert, phyllite, argillite, and mafic intrusives The Hozomeen characteristically forms rubbly dark-green cliffs, locally marked by white slashes where chert is present. Weathered outcrops are commonly yellowish brown. These rocks extend in a northwest-trending belt diagonally across Ross Lake, from about 1 mile north of Ruby Creek northward into Canada. The greater part of this unit lies on the east side of Ross Lake; west of Ross Lake it is exposed in a belt 2.5 miles wide for a distance of 8 miles (pl. 1).

This thick sequence of rocks was originally named the Hozomeen Series by Daly (1912, p. 500) after the prominent mountain of that name on the east side of Ross Lake. Later the name was modified to Hozomeen Group by Cairnes (1944). In British Columbia, the Hozomeen has been divided into four lithologic units by McTaggart and Thompson (1967, p. 1199-1205), who made the most detailed study of these rocks to date.

In the United States, the greater part of the Hozomeen Group of Cairnes is composed of greenstone that is generally dark green, but is locally greenish gray, dark purplish gray, reddish gray, or brown. In places differences in texture and composition produce a layering. The greenstone which is mostly fine grained, is made up principally of plagioclase, hornblende, and actinolite, and lesser amounts of calcite, quartz, chlorite, zoisite, epidote, magnetite, and ilmenite. Prehnite is common in some places, and quartz, calcite, and zeolites fill veinlets. The greenstone on the west side of Ross Lake appears to be of a slightly higher metamorphic grade than that on the east side, for on the west side blue-green hornblende is found and actinolite is more common. Only a little of the original texture remains in the greenstones inasmuch as many of these rocks are highly sheared and in some the original minerals have been entirely converted to hornblende or actinolite.

Here and there in the greenstone gray to white banded chert forms layers and lenses that range in thickness from a quarter of an inch to several inches (fig. 6). North of Arctic Creek, near the contact with quartz diorite of the Chilliwack batholith, the chert is schistose and contains tiny red garnets. In addition to chert, lesser amounts of marble, metatuff, volcanic breccia, dark siltstone, and sandstone are part of this sequence. Also included in the greenstone along the west edge of Ross Lake are several masses of diabase and gabbro that may be sills. The coarser grained diabase and gabbro retain some of their original texture as well as such primary minerals as brown hornblende and clinopyroxene.

FIGURE 6.—Thin-bedded chert in the Hozomeen Group that crops out on the ridge between Noname and Skymo Creeks.

Bedding in most of the Hozomeen Group is obscure, but it can be defined by the chert (fig. 6), phyllite, metatuff, and argillite layers. As may be seen on plate 1, the Hozomeen rocks are folded into a broad northwest-plunging syncline. The Hozomeen is considered to lie conformably on the schist and phyllite of Ross Lake and hence to be younger than that unit, whose age is Jurassic to Early Cretaceous. The Hozomeen Group is clearly older than the Skagit Volcanics and the Chilliwack batholith, which are largely if not entirely of Tertiary age. The contact of the Hozomeen Group with the schist and phyllite of Ross Lake is a fault at most places; however, near Skymo Creek the contact appears to be normal, and the presence of phyllite in that area in both units suggests a depositional gradation of one into the other.

Inasmuch as few firm data are available on the age of the Hozomeen Group, several age correlations have been suggested. Some geologists (Smith and Calkins, 1904, p. 23; Daly, 1912, p. 502) have tentatively correlated the Hozomeen Group of Cairnes with the Cache Creek Series because of the presence of greenstones and fine-grained sedimentary rocks in both units. The Cache Creek, which occurs in southeastern British Columbia, has been dated as Permian, although the lower part may be Pennsylvanian (Armstrong, 1949, p. 50). Misch (1966, p. 133) considered the Hozomeen Group to be Permian in age, and he explained its position on top of the Cretaceous clastic rocks to be a result of thrusting. Daly (1912, p. 512) and Cairnes (1924, p. 42) also tentatively correlated the Hozomeen with the Chilliwack Series because of the presence of greenstones, pyroclastic rocks, and fine-grained sedimentary rocks in both units. The Chilliwack, which occurs 24 miles west of Ross Lake, ranges in age from Middle Devonian to Middle Permian (Danner, 1965; McGugan and others, 1964, p. 109). Misch (1966, p. 116), on the other hand, believes that only the pyroclastic and sedimentary rocks of the western part of the Hozomeen outcrop belt correlate with the Chilliwack and that these rocks underlie the Hozomeen Group. We believe that a post-Early Cretaceous age for the Hozomeen is supported by the synclinal position of the Hozomeen on top of the schist and phyllite of Ross Lake and by the presence of identical phyllite beds in both units.

SKAGIT VOLCANICS

An irregular elongate body of volcanic rocks extends for 6.5 miles along the Canadian border just west of Ross Lake (pl. 1). Flows, tuffs, and volcanic breccias form the precipitous Glacier Peak and part of the steep ridges on either side of Silver Creek. Daly (1912, p. 528-531) named the rocks Skagit Volcanic Formation after the Skagit River to the east, and we have modified this name to Skagit Volcanics.

On the east side of Depot Creek and south of Glacier Peak, the Skagit Volcanics overlie the Custer Gneiss (pl. 1), and southwest of the mouth of Silver Creek they overlie the Hozomeen Group. Dikes of quartz diorite of the Chilliwack composite batholith cut the Skagit Volcanics southwest of the mouth of Silver Creek and at the Weezie prospect on Silver Creek, 1.5 miles west of Ross Lake. However, a dike of volcanic rock cuts quartz diorite on the south side of Silver Creek.

The Skagit Volcanics are generally massive; their structure and thickness are poorly known. On the ridge north of Silver Creek, however, some thin-bedded tuffs dip from 50° NE. to 15° NW. The thickness of the Skagit from Silver Creek to the crest of the main ridge to the north is approximately 4,000 feet.

Most of the Skagit Volcanics along both sides of Silver Creek is massive gray ash-flow tuff-breccia. Lava flows also occur within the tuff-breccia and make up a great part of the unit in the Glacier Peak area. Thin-bedded light-colored air-fall tuffs are found along the ridgetop on the north side of Silver Creek.

The tuff-breccia is a hard welded rock with 5-30 percent angular rock fragments that range in width from about 1/16 to 3 inches. The rock fragments are mainly of fine-grained volcanic rocks set in a matrix of ash and small mineral fragments. Plagioclase is by far the most abundant mineral. The flow rocks are all dark gray and consist of small crystals of plagioclase, generally with some hornblende, biotite, chlorite, and magnetite, set in a fine-grained groundmass of feldspar, quartz, and mafic minerals.

Air-fall tuffs are white, tan, light gray, or pale green; some are distinctly bedded. They consist of small angular fragments of volcanic rocks—commonly glass and minerals set in a matrix of ash. The mineral fragments are principally plagioclase, but quartz and epidote are also present. Glass shards are common in some of the ash.

Stain tests on the volcanic rocks showed the presence of potassium feldspar in the groundmass; thus, the composition of most of the rocks is probably that of a rhyodacite or dacite, although some may be andesite.

The Skagit Volcanics are probably early Tertiary in age. As they are only gently folded, they evidently were deposited after the more intense folding of the Lower Jurassic to Lower Cretaceous phyllite and schist of Ross Lake to the south. The Skagit is older than much of the quartz diorite of the Chilliwack batholith, as shown by the fact that it is cut by dikes of quartz diorite in several places. In one place on the ridge between Perry and Silver Creeks, however, a dike of Skagit Volcanics cuts the quartz diorite; thus, although the Skagit is for the most part younger than the Chilliwack batholith, it is in part older.