224 Hague and Iddings— Voleanoes of 
rom the boundary between Nevada and California, occurs Las- 
sen’s Peak, lying, as already mentioned, along the direct line of 
the Sierras, where the granite of the main range has been ab- 
ruptly broken down. The region has been the seat of great 
voleanic activity lasting through long periods of time, the 
present mountain forming but a remnant of former extrusions, 
and so far as geological interest is concerned is probably unsur- 
passed by any other mountain. Lassen’s Peak, however, is by 
no means as conspicuous an object as many of the volcanoes, 
being surrounded by other cones of considerable elevation, all 
of them rising out of a great volcanic table. The altitude of 
Lassen’s Peak is given at about 10,500 feet. It isa broad, 
irregularly shaped mountain with four prominent summits and 
abundant evidences on the slopes of comparatively recent ex- 
trusions of lava. 
s the rocks brought back may be considered as represent- 
ing the principal types of the ejected lavas from the different 
flows, a large number of thin sections have recently been pre- 
pared for the purpose of comparative study with the volcanic 
rocks of the Great Basin, and their microscopic examination 
has been followed up by chemical investigation. While a cur- 
sory examination of these rocks shows certain special charac- 
teristics in color, habit and form of crystals which in many 
cases easily identifies hand specimens with one or the other of 
the volcanoes, a comparative study of their mineralogical, struc- 
tural and chemical features shows the closest identity in the 
nature of the ejected material from the four volcanoes. In 
their lithological characters they possess so many features 12 
common that a general description might be given which 
would be applicable to them all. 
. These four great cones, which may be taken as typical of the 
chain, are all andesite volcanoes, with extrusions of basalt break- 
ing out upon their slopes and along the edges of the plain 
extending in all directions for long distances. From the com- 
pact basic basalts of normal type to the more porous acidic 
andesites similar variations in mineral composition and minute 
details of structure are found at each of the volcanoes. 18 
similarity holds good not only in macroscopic characters, but 18 
ill more marked in microscopic structure. In size, color an 
distribution of porphyritic crystals, in structure of groundmass, 
