J. LeConte—Extinct Volcanoes about Lake Mono. 39 
section and half perspective view of this cone. It consists 
of a low sand cone about 200 feet high, with a perfect cireu- 
lar crater one and a half to two miles in circumference, 
from the center of which rises a trachytic cone and crater of 
much smaller dimensions, to about the same height. From the 
shattered condition of the inner cone, Mr. Muir suggested to me 
the possibility of the engulfment of the upper je J portion 
into the lower sandy portion of a once much higher cone. 
But, in many other cases observed, this explanation is evidently 
untenable ; for in some cases we found several small cones sur- 
rounded by one rampart. Such could only be formed by sue- 
cessive eruptions, — 
plains of Mono are covered to a depth of many feet with a 
nearly white volcanic sand, mingled with fragments of pumice 
and obsidian 
ag’ 
that epoch, which must have entirely destroyed their form. 
The remarkable perfection of their conical forms and of their 
craters is therefore strongly presumptive if not demonstrative, 
: f the 
of the fact of their eruption since the disappearance o 
iers. 
2. All the streams, which run from the Sierra into Lake 
Mono, cut into the level plains 100 to 150 feet deep. Fine 
sections of the materials of the plains are thus exposed. Fig. 3 
is the upper portion of such a section about eighty feet perpen- 
