In the absence of definite paleontologic evidence, the correlation of the Sunrise formation with the similar graywacke-slate series of Prince William Sound must be made on purely petrographic and structural grounds. On this basis it appears that the Sunrise formation is probably the equivalent of the Valdez formation.

IGNEOUS PHENOMENA.

In addition to the greenstones, rhyolites, and accompanying pyroclastics, which form an integral portion of these rocks, other igneous rocks occur, probably as intrusives. They include quartz diorite, peridotite (lherzolite), and andesite. The quartz diorites on the south side of Knik Arm form bosses, whose relation to the surrounding sedimentary rocks was not seen. They are cut by andesitic dikes of greenstone habit. A mass of peridotite composed of augite, enstatite, and partially serpentinized olivine occurs in the mountain mass southwest of Eklootna Creek.

LOWER MIDDLE JURASSIC ROCKS.

DISTRIBUTION AND CHARACTER.

The oldest rocks of determined age comprise andesitic greenstones, quartz porphyries, tuffaceous sandstones, shales, and conglomerates. Some coal is found in the nonvolcanic portion of this series. The greenstones constitute the most characteristic member and attain a wide distribution, especially in the interior of the Talkeetna Mountains, where only the volcanic portion of the series appears to be

present.

The great bulk of the greenstones are clearly products of explosive volcanic activity. They occur largely in the form of stratified breccias (Pl. III, B), in which angular fragments of dark-blue porphyritic rock, varying in size from microscopic dimensions to several feet in diameter, are inclosed in a matrix of green color. This contrast of colors, which serves to emphasize the breccia character of the rock, is due to the fact that the matrix is abundantly changed into the green mineral chlorite, whereas the large angular fragments, being of a dense, fine-grained texture, have often withstood alteration more thoroughly. With the breccias are intercalated various sheets of amygdaloids, porphyries, and flow breccias. (See Pl. III, D.) The amygdaloids are frequently studded with numerous large amygdules, whose filling may consist of chalcedony, quartz, calcite, chlorite, or zeolite.

A prominent feature of the greenstones is the great amount of shattering and slickensiding which they exhibit. Where this internal movement has been of a more regular character a rude schistosity has been impressed on them. Extensive pyritization of a diffused char

acter was noted at several localities, but a thin irregular seaming with calcite appears to be the most usual form of mineralization.

Petrographically the greenstones are prevailingly andesitic, usually of hyalopilitic or pilotaxitic texture, and more or less completely chloritized. Examples more basic than andesite appear to be rare and were noted only on Boulder Creek, about 7 miles from its mouth, where greenstones displaying columnar structure make up the north wall of the valley. These prove to be diabase porphyries.

The thickness of the greenstones often exceeds 1,000 feet. The tuffs locally contain fragments of lignite, and at the eastern end of Sheep Mountain, in the upper Matanuska Valley, were found to carry marine shells in a fine state of preservation.

Under the convenient field term "quartz porphyry" have been grouped various rhyolites and dacites, including their tuffs, that are associated with the greenstones. T ese are widely distributed in the interior of the Talkeetna Mountains. The rhyolites are generally flinty-looking rocks containing glassy phenocrysts, but also include some splendid flow-banded phases. Devitrification is common in the rhyolites. Some of the dacites resemble the rhyolites in appearance, but others possess a greenstone habit. These acidic lavas appear to have been erupted either during or soon after the period of the andesitic extrusions, as they contain included in them fragments of microlitic andesite.

During the extravasation of the rhyolites conditions were not uniform over the province. While lavas were accumulating in some portions of the region sedimentation was progressing in others. On the divide between Hicks and Boulder creeks a white stratum 20 feet thick is prominent. It proves to be a feldspathic sandstone, with some irregular thin white bands, about a quarter of an inch thick, scattered through it. The microscope shows that these bands are composed of microspherulitic particles and fragments of devitrified rhyolite, with fine flow structure, indicating that showers of rhyolitic material were prevalent during this portion of lower Middle Jurassic sedimentation.

At various points, especially in the region east of Chickaloon Creek, the greenstones are overlain conformably by a series of sandstones and shales. Some finely flow-banded volcanics, now devitrified, are in places associated with the sediments, proving that eruptive activity had not entirely ceased during their deposition. This series, by its dominantly sedimentary character, can be readily distinguished from the underlying meta-andesite accumulation.

The sandstones are usually feldspathic, and under the microscope -how numerous small fragments of microlitic andesite no larger than the feldspar or quartz grains. The shales locally carry numerous

13070-Bull. 327-07-2

limestone nodules. At various localities the shales and sandstones associated with the greenstones are intensely shattered and crushed. This is especially notable in the gorge of Billy Creek. In this vicinity also occur several conglomerates not found elsewhere in the series. Some of these, perhaps, merit description. The conglomerate just above the head of the gorge is composed of well-rounded bowlders, up to 4 feet in diameter, of andesite porphyry and of amygdaloid. The matrix of this volcanic conglomerate has been severely slickensided. On the same stream another conglomerate 400 feet thick is revealed. It consists of various porphyries and white quartz pebbles embedded in a gritty, coherent matrix. A noteworthy feature of this conglomerate is the abundance of pebbles that show individual faulting.

STRUCTURE.

The general strike of the series is north and south (magnetic), and the dips are variable. Along one of the western tributaries of Hicks Creek a gentle synclinal folding is exposed; farther north on Hicks Creek a small steep anticline, trending N. 30° E. (magnetic), is shown. The beds are broken by numerous faults of small throw and, as is evident from the detailed description of the series, have been subjected to a large amount of internal disturbance. Detailed mapping will show the presence of a complicated system of more extensive faults.

FOSSILS AND AGE.

The fossils found in tuffs associated with the greenstones were submitted to T. W. Stanton, who reports as follows:

6 A. K. 201. Fossiliferous tuffs, associated with lavas and coarse pyroclastics, from the head of Matanuska River.

Rhynchonella.

Lima.

Pecten. Smooth species.

Pecten. Species of Vola type.

Trigonia.

Astarte?
Protocardia.

Pleuromya.
Sonninia?

The Jurassic age of this lot is clearly shown by the form of the Trigonia and of the ammonite (Sonninia?). The general aspect of the fauna is that of the lower part of the Enochkin, though it may be somewhat older than the fauna in lots 88A and 88B. With the exception of the Vola-like species of pecten there is nothing in it to suggest the lower Jurassic fauna of Seldovia.

According to this determination the age of the greenstones is lower Middle Jurassic."

@ Stanton, T. W., and Martin, G. C., Mesozoic section on Cook Inlet: Bull. Geol. Soc. America, vol. 16, 1905, pp. 391-410.