£3 J. LeConte—Eatinct Volcanoes about Lake Mono. 
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erupted, but were wholly formed, after the epoch of the pebble — 
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I think, therefore, there can be no doubt that all of these — 
voleanoes erupted, and many of them were wholly formed after — 
the epoch of great glaciers (Champlain). Whether any of 
them preceded that epoch is doubtful. I have never seen any 
undoubted evidence that they did. If the bowlders found on 
_ the island were carried there by icebergs, then volcanic action 
preceded the epoch of icebergs, for many of the fragments are — 
voleanic; but they may have been carried by shore ice at a 
later time. Again, I believe the rocky part of the island is 
older than the sedimentary part, for the latter seems to have 
been deposited on the former. If the sedimentation was Cham- 
plain, then the rocky part was probably pre-glacial ; but the 
sedimentation may have been later. 
Sequence of Hvents—Assuming that the island strata belong 
to the epoch of great glaciers, then the order of events was 
something like this: 
1. Volcanic eruptions on the plains producing obsidian, 
fragments of which were afterwards carried by ice anddropped ~ 
in mid-lake. At the same time also, the basaltic part of the 
islands was formed. 
2. Then followed the period of great glaciers and flooded 
lakes, or Champlain epoch. The lake was nearly 700 feet higher — 
than now. Its waters covered the whole plains and washed 
against the Sierra; and glaciers from this range ran far into — 
the lake and formed icebergs, which floated over its surface 
and dropped rock-fragments over its fine mud bottom. 
3. Volcanic forces, acting quietly like the solfataras and fuma- 
roles still existing, heaved up the stratified mud-bottom of the 
mid-lake into a gentle mound with quaquaversal dip of the 
strata, but not rising to the surface. Coincident with this were 
the eruptions of the plains volcanoes. 
4. The lake then dried away gradually to its present level, 
leaving the terraces as its old flood-marks, and exposing the 
rounded mud-island ; and erosive agents then sculptured this 
into its present turreted form and cut away its margin to its 
present limits, and exposed the mud-covered older basaltic — 
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_ Lake rising again.—The existence of salt and alkaline lakes _ 
shows an extreme dryness of climate. But the climate of the — 
desert region has not always been dry. During the Champlain 
epoch the interior plains were covered with immense sheets of © 
water, of which the present saline lakes are the isolated resi- 
dues. Gilbert has shown that at that time Great Salt Lake con-— 
tained 400 times as much water as now, and that it drained — 
northward through the Snake and Columbia Rivers into the © 
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