Up Tsadaka Creek and along the ridge on its southern side sandstones may be observed dipping southeast, which is a direction opposite to that of the beds just described. Still farther east conglomerate beds dip to the southwest. These conglomerate beds seem to owe their elevated position to a series of block faults passing in a general northeast and southwest direction, and the Tsadaka Creek coal is probably cut off to the east. It seems probable that Eska Creek coal is separated by these faults from that of Tsadaka Creek. It will be noted that faulting or folding is present at nearly all the localities above described. Such conditions will make the cost of mining higher than it would be if the beds were less disturbed.

ANTHRACITE RIDGE FIELD.

GEOLOGY.

The most promising exposures of anthracite examined during the season were seen along the southern flank of Anthracite Ridge, between Boulder and Hicks creeks. The geology of this locality is somewhat complex. The western extremity of the ridge is formed by a large mass of augite dacite and brown porphyry. This latter appears to be intrusive into a series of interstratified sandstones and shales striking N. 35° E. (magnetic) and dipping vertically. Associated with them is a large accumulation of andesitic amygdaloids and pyroclastics, of somewhat chloritized aspect. At the base of the ridge along the trail, however, cross-bedded gritty sandstones, apparently belonging to the Kenai formation, are exposed, striking N. 75° E. and dipping 15° S. Some sills of coarse ophitic diabase, 50 feet and more in thickness, are intercalated between them. These observations indicate that the western extremity of the ridge is marked by a line of faulting along its base. And as marine fossils of Mesozoic age have been found on Anthracite Ridge, it has been

assumed by Martin that an eastward extension of this fault determines the boundary between the Kenai and the Mesozoic rocks. Martin obtained a few fragmentary fossils, partly from the summit and partly from an elevation of 3,450 feet on the southern flank of the ridge. These indicated a horizon as low as the Middle Jurassic or as high as Upper Cretaceous. From the northern flank the writer obtained a single form, concerning which Stanton reports: "Inoceramus cf. lucifer Eichwald. This species occurs in the lowest part of the Enochkin formation (Middle Jurassic) at Snug Harbor." It is thus plain from the faunal and lithologic evidence that lower Middle Jurassic rocks are present on Anthracite Ridge, especially in the western end as approached from the Boulder Creek side.

Farther east, along the numerous small creeks which rise on Anthracite Ridge, it is found that the dip of the strata, which is nearly horizontal in the floor of the valley, gradually steepens as the flank of the ridge is approached. The beds become folded into anticlines and synclines, which are more closely crowded together toward the summit of the ridge. Locally the limbs of the anticlines have been doubled together. Thick sills of coarsely crystalline diabase are prevalent, especially along the middle portion of the ridge, and heavy dikes, attaining a maximum thickness of 500 feet, cut across the strata. Their energetic contact action in baking the adjoining shales has already been referred to. Moreover, their presence seems to be favorable to the anthracitic character of the coal. On a creek a few miles east of Purinton Creek, at an altitude of 3,100 feet, the stream banks show a 50-foot sill of diabase, about 10 feet stratigraphically above a coal bed. The sandstone between the coal and the diabase, though elsewhere yellow, is black with material coating the cement between the quartz grains and in some places even invading the grains themselves. This suggests an impregnation of the sandstone during the distillation of the coal by the heat of the intrusive. The coal of this exposure is a fine lustrous anthracite. Higher up the same creek, near the summit of the ridge, a number of coal beds of bituminous character are exposed. No diabase intrusives are present in the immediate vicinity, but folding of a closely appressed character has affected the strata.

In the roof of an anthracite bed exposed in a creek somewhat west of Purinton Creek a few fragments of netted-veined leaves were found. Knowlton reports on these as follows: "6 A. K. 244. Matanuska Valley. Juglans nigella? Heer, no margin preserved. It looks like a Kenai dicotyledon, and not like any Cretaceous species known to me."

See analysis No. 22, p. 62.

The evidence from these plant remains shows, then, that at least this portion of the anthracite is probably of Kenai age, and that it does not occur in rocks of Middle Jurassic age, which are known from their marine fauna to be present in close vicinity and possibly to be infolded to some extent with the Kenai rocks, or irregularly faulted against them.

From a consideration of the structural and floral evidence it appears to the writer that the coal of this field is chiefly of Kenai age, and that its anthracitic character is due to the large amount of heat furnished by the unusually abundant diabase intrusions. The Anthracite Ridge field represents, according to this view, a portion of the Tertiary coal-bearing area which has been more highly affected by the heat of eruptive rocks. The amount of anthracite appears to be limited, and will not warrant an extensive outlay of capital for its exploitation. In addition, the complex folding of the strata and the presence of massive diabase injections will seriously increase the difficulties of mining.

DESCRIPTION OF THE COAL.

Martin has described the coal as follows:

This coal has the ordinary physical characteristics of most good coal of this kind. It is heavy, firm, hard, and not much fractured for surface coal, and has a high luster. Pyrite was noticed. The seams are not much broken by small partings of shale and bone.

Two sections were measured by Martin: One at an elevation of 3,410 feet on the south bank of Purinton Creek showed 38 feet of clean, solid coal, both roof and floor being concealed (see analysis No. 1, pp. 60, 62, which represents the entire thickness as measured). Martin says:

At this point the strike is N. 40° E. (magnetic) and the dip is 10° NW., or into the mountain. The rocks in the vicinity are chiefly graywacke and sandstone and show considerable variation of strike and dip. A short distance downstream is a good-sized mass of diabase occupying the axis of an anticline, which is in other places restricted by a fault. The anthracite is probably restricted to a zone along the face and in the mountains which is cut off from the valley plateau by a fault following the base of the mountains. Black streaks that are probably coal could be seen high up on the face of the mountain and could be followed by the eye for several miles.

*

About a mile northeast of the locality above described, at an elevation of 3,460 feet, a coal section gave the following measurements:

The area of anthracite was not estimated by the writer, and the amount available for economic mining and shipment may possibly not be sufficient to justify the necessary expenditures.

In addition to the sections of coal measured by G. C. Martin on Anthracite Ridge, the following sections were obtained somewhat farther east:

Diabase...

Section of coal on Anthracite Ridge, altitude 3,100 feet.

Sandstone and shale

Coal and shale_.

Coal..

Shale..

Coal

Shale

Sandstone

Strike N. 70° W. (magnetic), dip 40° S.

Section of coal on Anthracite Ridge, altitude 3.600 feet.

A syncline of coal 3 feet thick, cut off by a heavy diabase dike, is exposed in a small creek west of Purinton Creek (as shown in fig. 3).

FIG. 3.-Cross section showing relation of anthracite to intrusive diabase near Purinton

Creek.

NORTHEASTERN FIELD.

The coal-bearing rocks of this field include an area of about 500 square miles. Coal is found at various localities, but never in thick beds, the best discovered having a thickness of 3 feet. The rocks are of Jurassic age and from fossil evidence are divided into a Middle and an Upper Jurassic series. The character of the coal appears in a general way to be in accordance with this separation, i. e., the older rocks carry bituminous coal, and the younger, coal of a lignitic

character.

The Middle Jurassic rocks generally are severely shattered and crushed, or sheared and slickensided, and as a rule present a decidedly unfavorable appearance for the presence of workable seams of coal. Locally the strata are closely folded, as on Billy Creek (see fig. 4), where the coal has developed a strong cleavage and has assumed a semianthracitic character. That the high-grade coal is restricted to such belts of sharp deformation is rendered probable by