ACADEMY OF SCIENCES. 173 
Such is a general sketch of the results of glacial action at the head of 
American Valley—results which could only have been produced under totally 
different climatic conditions than a mere diminution of the mean annual tem- 
perature. It is evident that the formation of the large moraine across the 
head of the pass, from the glacier coming from the head of Lake Valley, 
could only have taken place when the surface of the mountain at the head of 
the pass was uncoyered by snow, at least duringa part of the year; or, in 
otber words, at the time that the glacier in Lake Valley had attained a thick- 
ness of more than a thousand feet, there was no permanent glacier at the 
head of the pass. At present, the snow by the end of the winter is from ten 
to twenty feet deep at the head of the pass, and from four to eight feet deep 
in Lake Valley, and it has meltedin the valley six to eight weeks before it 
disappears from the head of the pass. With a colder climate, in which, 
however, the relative temperature of the summer and winter should be the 
same as at present, it is evident that long before the Lake Valley glacier had 
attained a thickness of one thousand feet, a glacier some hundreds of feet 
thick must have occupied the head of the pass, so that the moraine matter 
brought down by the southern tributaries of Lake Valley glacier could not 
have been deposited there, but must have been carried down the valley of 
the American River as soon as the Lake Valley glacier was thick enough to 
force the ice stream in that direction. The most probable climatic conditions 
under which such a deposition of moraine matter as is found at the head of 
the American Valley could take place are, a colder winter with a very heavy 
snow-fall, and a hot summer, during which the snow would be removed from 
the surface, even at an elevation of 7,000 feet, when not fed by glaciers. The 
gradual filling up of Lake Valley by ice, was the result of the many glaciers 
coming into it on all sides, as has been shown by Prof. J. LeConte, and 
which had their origin in mountains from 1,500 to 2,000 feet above the level 
of the pass. That these ice streams were pouring into Lake Valley when the 
head of the American Valley was comparatively free from ice, is proved, also, 
by the formation of the large lateral moraine, from Echo Lake glacier, on the 
north side of the valley. Another fact that would indicate the rapid disap- 
pearance of ice at the upper part of the American Valley during the height 
of the glacial epoch is, the comparatively slight longitudinal extension of the 
glaciers down the American Valley. Although there is undoubted evidence 
that in Lake Valley, and at the head of Johnson’s Pass, the ice attained a 
thickness of six hundred feet above the level of the pass, yet the larger part 
of the terminal moraine matter has been deposited within six or seven miles 
of the head of the pass, and at an elevation of only 1,000 feet below the top . 
of the pass. Now, the rapid disappearance of this American Valley glacier, 
fed, as it was, by the Echo Lake glacier, and also by the vast ice stream from 
Lake Valley, would indicate that it must have been exposed to a much higher 
summer temperature than prevails at present. The topographical formation 
of the American Valley would also favor the melting of the ice, as the valley 
opens directly on the heated plains of the Sacramento, and thus affords a 
channel for the hot air of the plains during the summer, and for the moisture- 
