George Otis Smith.
XV. THE ROCKS OF MOUNT RAINIER
By GEORGE OTIS SMITH
Director George Otis Smith of the United States Geological Survey was born at Hodgdon, Maine, on February 22, 1871. He graduated from Colby College in 1893 and obtained his Doctor of Philosophy degree from Johns Hopkins University in 1896. He had begun his geological work in 1893 and from 1896 to 1907 he was assistant geologist and geologist of the United States Geological Survey. Since 1907 he has been director of that important branch of the Government work.
He had been studying the rocks of Mount Rainier before he joined Professor Russell in the explorations of 1896. The record of those studies was published at the same time as Professor Russell's report in the Eighteenth Annual Report of the United States Geological Survey for 1896-1897. With his permission the record is here reproduced in full. So far as is known to the present editor it is the most complete study yet published on the rocks of Mount Rainier.
The earliest geological observations on the structure of Mount Rainier were made in 1870 by S. F. Emmons, of the Geological Exploration of the Fortieth Parallel. The rock specimens collected at this time were studied later by Messrs. Hague and Iddings, of the United States Geological Survey. [27] This petrographical study showed that "Mount Rainier is formed almost wholly of hypersthene andesite, with different conditions of groundmass and accompanied by hornblende and olivine in places." The only other petrographical study of these volcanics is that of Mr. K. Oebbeke, of Munich, [28] upon a small collection made on Mount Rainier by Professor Zittel in 1883.
On the reconnaissance trips on the northern and eastern slopes of Mount Rainier, during the seasons of 1895 and 1896, the writer had opportunity to make some general observations on the rocks of this mountain, and the petrographical material then collected has since been studied. The observations and collections were of necessity limited, both by the reconnaissance character of the examination and by the mantle of snow and ice which covers so large a part of this volcanic cone.
Two classes of rock are to be discussed as occurring on Mount Rainier: the lavas and pyroclastics which compose the volcanic cone and the granitic rocks forming the platform upon which the volcano was built up.
Volcanic Rocks
GEOLOGIC RELATIONS
On Crater Peak a dark line of rock appears above the snow, and here the outer slope of the crater rim is found to be covered with blocks of lava. A black, loose-textured andesite is most abundant, and from its occurrence on the edge of this well-defined crater may be regarded as representing the later eruptions of Rainier. Lower down on the slopes of the mountain opportunities for the study of the structure of the volcanic cone are found in the bold rock masses that mark the apexes of the interglacial areas. Examples of these are Little Tahoma, Gibraltar, Cathedral Rock, the Wedge, and the Guardian Rocks. These remnants of the old surface of the cone, together with the cliffs that bound the lower courses of the glaciers, exhibit the structural relations very well.