to a hard, dense coke. Besides this local effect the presence of thick sills in the coal-bearing rocks appears to have raised the grade or fuel value of the coal. As will be shown under the discussion of the mineral resources, the Matanuska coal field can be divided into two districts, of which one, the western, contains low-grade bituminous coal, approaching lignites in composition, while the other, the eastern, contains high-grade bituminous coal. The difference in character of the coals can not be explained as due to difference in intensity of folding in the two districts. Igneous intrusives are absent from the western district, but are prevalent in the eastern. This association of high-grade coal with abundant intrusives is at least suggestive. Furthermore, it is probable that the anthracitic character of the coal of Anthracite Ridge is due to the presence of unusually thick and abundant dikes in rocks of Kenai age.

COMPARISON WITH ADJOINING REGIONS.

The Kenai rocks of Matanuska Valley differ markedly from those of the type section of the Kenai as exposed on Kachemak Bay, on Cook Inlet, where the sandstones are soft and incoherent, the shales are plastic when wet, and the lignite seams form the resistant members. The beds on Kachemak Bay lie at low angles in undisturbed attitudes. The lignite is in general hard and glossy, but in the higher seams becomes dull and shows a more woody fiber. The lower seams have been buried under a depth of nearly 10,000 feet of strata. If the Kenai of Matanuska Valley be compared with the Kenai exposed at Tyonek, the contrast is even greater, though Tyonek is nearer than Kachemak Bay. The Kenai at Tyonek consists" of alternating beds of clay, sand which has hardened to sandstone in huge nodules, and lignite." According to Eldridge, the lignite in places hardly appears to be more than a compressed mass of carbonized wood. The strata are tilted at angles ranging from 35° to 60°. Though more sharply deformed, the coal is of lower grade than that of Kachemak Bay.

From the foregoing comparisons it appears that the Kenai of Matanuska Valley differs considerably from that of the adjacent regions in its higher degree of induration and in the superior quality of its coal seams. The conclusion can be drawn that the higher character of the Matanuska coal is due not to conditions of deeper burial or sharper folding, but to the prevalence of intrusives in the coal-bearing strata.

a Stone, R. W., Coal fields of the Kachemak Bay region: Bull. U. S. Geol. Survey No. 277, 1906, p. 58.

Spurr, J. E., Reconnaissance in southwestern Alaska in 1898: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 7, 1898-99, p. 102.

POST-EOCENE.

Overlying the older rocks unconformably is a series of nearly horizontal basalt flows, which, with their intercalated pyroclastics, attain a thickness of 1,000 feet. They have an extensive distribution throughout the Talkeetna Mountains and compose many of the peaks and summits of the region. They weather in tints of red and break down in characteristic erosional forms which lend a picturesque castellated appearance to the mountain crest lines.

The basalt flows with their interbedded tuffs and breccias present a stratified appearance. The surface upon which these volcanics were accumulated appears to have been one of gentle relief, cut across the upturned edges of the older rocks. The discordance with which the nearly horizontal basalts rest upon the underlying strata varies from 10° to 90°. About the headwaters of Chickaloon Creek this surface slopes gently northward. East of Chickaloon Creek its altitude is about 5,000 feet. It is somewhat broken by small faults, the maximum displacement noted being 50 feet. At many places the basalts rest upon a conglomerate occupying broad channels in the Lower Cretaceous limestone. The basal portion of the volcanic series here consists of 100 feet of tuffs and amygdaloid breccias, which interleave with the sandstone beds in the upper portion of the conglomerate. The conglomerate consists of rather coarse wash embedded in a gravelly matrix. The pebbles are well rounded, though poorly sorted and shingled, and cobbles up to a foot or more in diameter are common. The material is largely of volcanic origin, consisting principally of greenstone amygdaloids and eutaxitic rhyolites-extrusives of Jurassic age. Some irregular lenticular beds of sandstone are included in the conglomerate. It is 200 feet thick, and in the vicinity of Billy Creek fills a channel 1,000 feet wide, cut in the Cretaceous limestone and partly in the underlying sandstones. The arclike line of contact is finely exposed in the deep canyons tributary to Billy Creek. This conglomerate, whose fluviatile origin is thus apparent, has a very limited distribution and has been preserved only where buried under the covering of basaltic lavas. This conglomerate is later than Lower Cretaceous, but further than that its age is not known. Lithologic similarity suggests its correlation with the great mass of conglomerate occurring in the upper horizons of the Kenai in Matanuska Valley.

Locally the conglomerate is capped by an angular wash consisting of fragments of sandstone and cellular lavas, upon which rests the stratiform series of basalts and pyroclastics. At one point a breccia of unusual constitution was found near the base of the volcanic series. This breccia consists of large angular blocks of highly amygdaloidal

lava, blocks of sandstone containing Aucella, and a few blocks of the Cretaceous limestone characteristic of the region. A basalt block 8 by 10 feet was the largest noted. The matrix in which this angular material is embedded is composed of small, well-rounded gravel. These features are interpreted as indicating a volcanic mud flow occupying an ancient stream course.

The overlying basalts display a wide variation in habit and texture. Highly glassy types, amygdaloids, porphyries, and dolerites are variously represented. They belong to the acidic end of the basalt family and are in general non-olivinitic. The coarse-grained basalts consist of doleritic aggregates of labradorite (Ab,An,) and augite. Plagioclase is the dominant constituent, and is in only a few specimens poikilitically inclosed in the augite. The basalt glass from the head of Chickaloon Creek, in spite of its holohyaline aspect, is found to contain sporadic phenocrysts of plagioclase (Ab,An,) and augite, and some small, irregularly shaped doleritic aggregates of labradorite and augite. The glassy base is of a deep, clear, brown color, and contains numerous augite and very minute feldspar microlites, the latter usually having forked terminations.

The basalts overlie a conglomerate of post-Lower Cretaceous origin and of probably late Kenai age. Their petrography suggests that they are the effusive equivalents of the diabase dikes and sills that are so common in the Kenai of the Matanuska basin. These volcanics of the Talkeetna Mountains can be correlated with the great series of Tertiary volcanics of the upper Nizina basin, described by Schrader and Spencer, and with an important volcanic series on Nabesna River, described by Brooks. Both these series are described as comprising andesites, rhyolites, and pyroclastics, and were deposited unconformably upon a general uniform surface. The volcanics of the Talkeetna region, however, show a greater petrographic uniformity and consist exclusively of basalts and their fragmental accumulations. In this respect they show a closer resemblance to the late Tertiary lavas of the Alaska Peninsula and the Aleutian Islands, which have been described by Spurr as andesitic basalts.

QUATERNARY DEPOSITS.

Extensive deposits of glacial and fluvioglacial gravels are prevalent. They extend entirely around the head of Cook Inlet, forming bluffs 50 to 100 feet high. Below Knik the gravel consists largely of subangular to rounded pebbles of hornblende-biotite granite and graywacke, with occasional muscovite-biotite granite, pyroxenite, and andesitic porphyries. Enormous angular blocks are frequently in

a Spurr, J. E.. A reconnaissance in southwestern Alaska in 1898: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 7, 1900, p. 234.

cluded in this stream-worn material. Many of these have smooth, polished surfaces several feet square, much scratched and grooved. The gravel sheet at Tyonek, on the west side of Cook Inlet, with its extension up Susitna Valley as far as Chulitna River, has been recognized as divisible into an older and a younger formation. The older apparently consists of the ground moraine of an advancing ice stream," and the younger includes fluvioglacial gravels deposited upon the earlier sheet during the recession of the ice. These distinctions can not be drawn in the Matanuska region. The gravel sheet in Matanuska Valley is of considerable areal extent and mantles the old valley floor, especially in the region west of Chickaloon Creek. The gravels near the mouth of Kings Creek have a maximum thickness of about 300 feet. They are greatly waterworn, and the icerafted bowlders so conspicuous in the vicinity of Knik Arm were not observed. However, the fact that the bluff near this exposure in Matanuska Valley is composed of shales shows that gravels were deposited on a surface of irregular relief, or possibly in channels. This is also shown by the fact that the walls of the Matanuska gorge are in places composed of bed rock, and in other places of gravel.

In its upper course Matanuska River has cut a V-shaped gorge in the deposits underlying the southwest corner of the Copper River Plateau. The gravels here consist of angular blocks, scratched and striated, inclosed in a blue, clayey-sandy matrix. The river has cut down 300 to 400 feet in this glacial till, but does not yet reveal bed rock. Farther north, in the Copper River Plateau, stream bluffs on a tributary of the Nelchina show coarse gravels of subangular to well-rounded character. With this stream-worn material are associated numerous glacially scored bowlders and occasional angular blocks up to 10 feet in size. The gravel shows considerable variety in its petrographic charcter, including numerous fragments of granite, gabbro, gneiss, schist, slate, graywacke, and vein quartz. None of these rocks are found in the drainage area of this stream, so that the fragments appear to have come from the Alaska Range.

GEOLOGIC HISTORY.

PRE-QUATERNARY.

The oldest records of the geologic history of the region are preserved by the albite and garnetiferous mica schists. They appear to represent a series of calcareous argillites and plagioclase arkoses, recrystallized under conditions of deep burial. Subsequent to their metamorphism they have been uplifted, intruded by pyroxenite dikes, and subjected to a second period of dynamic activity. The dikes

Spurr, J. E., op. cit., p. 102.

were thus sheared and the component minerals of the schists were crushed. The highly crystalline character of the schists and their isolated occurrence as a single narrow belt indicate the completeness of the geologic revolution which the region has undergone since their formation. On the basis of petrographic similarity these schists are correlated with those of the Yukon-Tanana region and assigned to a pre-Silurian age.

The time interval between the deposition of these oldest rocks and that of the Susitna slate is not known, but appears to have been of considerable duration, as is indicated by the far less advanced metamorphism of the latter. The Susitna comprises fissile slates, foliated graywackes, and microcrystalline hornblende and biotite schists. The possibility, however, that they may be contemporaneous in origin with the rocks of the Sunrise "series " is not excluded.

In late Paleozoic or early Mesozoic time sedimentary conditions prevailed over the region now occupied by the Chugach Mountains, and an extensive series of shales, arkose, and ferromagnesian sandstones derived from the destruction of a quartz-diorite land mass were laid down. Volcanic activity interrupted the normal sedimentary sequence and caused the accumulation of greenstone tuffs, associated with some massive basaltic greenstones. Rhyolitic lavas were also poured out, and the accompanying fragmental material was mingled with the sediments derived from the erosion of the granitic land area. Subsequently these deposits were closely folded and metamorphosed to graywackes, slates, and, locally, to phyllitic schists.

At the beginning of Middle Jurassic time marine conditions were inaugurated. Eruptive activity was then widespread throughout the region of the Talkeetna Mountains, and andesitic greenstone breccias with intercalated amygdaloids were accumulated to a thickness of 1,000 feet. The relations of land and sea appear to have been unstable during this period, and a portion of the volcanics were doubtless extruded upon uplifted land areas. The exceedingly coarse volcanic conglomerates show the effectiveness of erosional agents, and the composition of the succeeding sandstones indicates their derivation from an andesitic land area.

Toward the close of the Middle Jurassic, quartz diorites forming the core of the Talkeetna Mountains were intruded and the Talkneeta area was uplifted. A vigorous denudation ensued, the sedimentary roof arching over the plutonic rocks was stripped off, and the granolites were uncovered.

By the beginning of the Upper Jurassic the granitic land mass thus laid bare furnished large amounts of material to the conglomerates then accumulating. Some andesitic eruptions occurred at this time. The relief of the land mass was soon subdued, and the succeeding sediments became normal sandstones and shales, interstratified with