Eee PNT ne Nh eye te ee 
J. LeConte—Extinct Volcanoes about Lake Mono. 41 
canic cones in recent times. I suppose, therefore, that it is the 
result of sand and ash eruptions since the recession of the lake 
waters. 
8. We have already described the material of the largest | 
island as being composed wholly, except a portion of the eastern 
part, of a fine infusorial earth, horizontally stratified with 
lamine of slightly different colors, so thin as to give specimens 
an almost agate-like beauty. This material was evidently depos- 
ited in the middle and deepest part of the lake, beyond the reach 
of coarser sediments, at a time when the place of the island was 
still a lake bottom. Now, that this occurred during or after 
the epoch of great glaciers, is demonstrated by the fact that 
scattered sparsely through this fine laminated material, and 
lying on its surface, having been washed out by erosion, I found 
many bowlders, both worn and angular, of Sierra granite and 
slate, and also of obsidian. These could have been brought 
there only by the agency of floating ice, either as icebergs or as 
shore ice. If by icebergs, of course during the epoch of great 
glaciers; if by shore ice, either during that time or still later, 
for manifestly the bowlders were brought down to the shore 
rom the Sierra during that time. It is evident, therefore, that 
the stratified mud was formed and the bowlders were dro 
during the period of great glaciers or later. But still later the 
island itself was upheaved by volcanic action, as shown by the 
anticlinal position of the strata at the base, and by the solfa- 
taric action still going on. The formation of this island I sup- 
ose to have been coincident with the last eruptions of the 
volcanoes on the plains. ’ 
4. Within the craters of several of the voleanic cones on the 
plains, I found pebbles and angular fragments of granite of a 
peculiar reddish color from the presence of a rose-colored feld- 
spar. Whitney observed the same, and accounts for them in 
the following manner: They could not, he thinks, have been 
brought by glaciers or by water, for this is inconsistent with 
the perfect shape of thecones. He rightly concludes therefore 
that they must have been ejected from the voleanoes. But if so, 
he says, “ they must have been torn off from the underlying granite, 
through which the eruptive matter has forced ws way, as is seen 
everywhere in the Sierra.”* On the contrary, I account for them 
in a wholly different way. The fragments which I saw were 
some of them angular—true; but most of them were well-worn 
pebbles. There is not the slightest doubt that these were pebbles 
of the drift-layer which everywhere underlies the loose sand of the 
plains. The eruptive forces broke through this drift-layer, and 
the ejected pebbles fell back into the crater. They demon- 
strate that the cones and craters, where they are found, not only 
* Geol. Survey of Cal. vol. i, p. 455. 
