California, Oregon and Washington Territory. 225 
in the nature of the glass base, and in the transitions from a 
nearly pure glass to a crystalline condition, many rocks from 
one volcano may be easily correlated with those from one or 
more of the other peaks. 
Their similarity in chemical composition is strikingly shown 
by the following analyses of typical rocks from each of the four 
volcanoes. — Roe bi & , are taken from the report of the 
Geological Exploration of the 40th Parallel. No. IV, as well 
as nearly all the rest of the chemical work referred to in this 
paper, was done by Mr. P. W. Shimer under the supervision of 
Dr. Thomas M. Drown, in his laboratory at Easton. 
TABLE I. 
5 a: itt, 1V. 
Mt. Rainier. Mt.Hood. Mt.Shasta. Lassen’s Peak. 
Sid, 61°62 63°28 60°44 62°94 
Al.O3 16°86 17°96 18°12 18°14 
Fe.0; 1°18 
eO 6°61 3°16 5°16 3.82 
Sad 57 6°34 6°43 6°28 
MgO 217 2°50 3°43 3:06 
Na,O 3°93 3°81 4°09 3°83 
2 1°66 2°06 1:26 1°22 
Ignition ° 0°12 0°89 
TiO, 1 
: P20; 0°10 
99°42 . 100°04 99°79 100°40 
While the rocks from these voleanoes in general present the 
closest resemblances, there is a wider range and a greater 
variety of structure in the more acidic types from Lassen’s 
Peak and Mount Shasta. On the other hand, judging from 
the collections, the range in the character of the extrusions is 
Most restricted at Mount Rainier. 
ee these rocks then may be classified under the following 
ads: 
_ Basalt, composed of plagioclase, augite and olivine, as essen- 
tial minerals, 
Hypersthene-Andesite, having plagioclase, hypersthene and 
augite as essential minerals. 
ornblende-Andesite, with plagioclase, hornblende and pyrox- 
ene as essential minerals. 
belong in general to the type found throughout the great basin, 
that is, they are more or less dense rocks, sometimes quite 
Am. Jour. Scr—Tump Series, VoL. XXVI, No. 153.—SEpr., 1883. 
