38 J. LeConte—Hextinct Volcanoes about Lake Mono. 
Steam and boiling water issue in many places in this rocky 
rtion of the cereus and in the shallow water in the vicinity. 
visit, but according to Wiuteey, they are webolid sea and 
the largest of them is 300 feet high, and is a well-defined vol- 
canic cone. 
The general conclusion, at which I arrived from my examin- 
ation of the largest island, was that the basaltic portion was 
first formed at the bottom of the lake, or else subsequently 
submerged ; then the diatomaceous mud was deposited, cover- 
ing it up completely ; then the fine mud-bottom was raised 
into an anticline and exposed as an island by the fall of the 
lake level,and finally erosion sculptured the whole, and in part 
exposed the underlying basalt. 
Voleanoes on the Plains.—We have already alluded to a con- 
an group of volcanic cones situated on the level plain 
sout the lake. These are twenty or thirty in number, 
Seiendin | in a line from near the margin of the lake to a dis- 
tance of ten to fifteen miles, and vary in height from 200 to © 
* 700 feet above the plain. Partly from the recency of their 
In many cases I observed a very perfect ¢ one-and-ra mpart 
structure, such as is known to be Sate uce 
followed by smaller ones ; or haps in — cases by te “ 
ent of the crater in nae ! 
perfect es! of this Kind is foanad in . small and ane 
accessible =~ oe See the aiken Fig. 2 is an aes . 
