Geological Survey 8th Annual Report (Part I) Geology of the Lassen Peak District

GEOLOGY OF THE LASSEN PEAK DISTRICT.

By J. S. DILLER.

The geological survey of the Cascade Range, under the direction of Capt. C. E. Dutton, was commenced in 1883. At that time the writer was sent out to make a general reconnaissance of the range in the northern part of California and Oregon. A party outfitted at Red Bluff, and after ascending Lassen Peak and Mt. Shasta proceeded northward through Oregon along the eastern base of the range. From this line of travel detours were made to examine the summit of the range at Crater Lake, Mt. Scott, Union Peak, Mt. Thielson, Diamond Peak, and the Three Sisters. Crossing the range at Mt. Hood, the party returned along its western base to California. Since then three field seasons have been devoted to the study of volcanic phenomena in the vicinity of Mt. Shasta and Lassen Peak.

The preliminary survey of the Cascade Range clearly indicated that the point at which to begin a study of its details was in the vicinity of Lassen Peak. This locality was considered of particular interest on account of the great variety of its lavas, the recency of some of its volcanic eruptions, and the vigorous solfataric action which still survives. It was anticipated also that a careful study of this region would solve one of the perplexing geographic problems of the Pacific coast, viz., the relations between the Coast, Cascade, and Sierra Nevada Ranges.

The district has been found replete with a great variety of interesting phenomena, only part of which can be discussed at present. This initial report will be restricted to a consideration of the geologic history of the region as chronicled in its stratified deposits. Special reference will be made to the development of the Sierra Nevada Range and the relation of its great structural features to the volcanic phenomena in the vicinity of Lassen Peak, leaving the particular discussion of the latter for another occasion.