242 Part IIL Chapter 3. 
favorable to vegetation. The change in elevation during the Eocene 
suffhicient to drain the Eocene lakes, but the elevation attained was so small” 
that vast Miocene lakes covered a large part of what now constitutes Ri 
eastern slopes of the mountains and continued into the Pliocene‘). The climate. 
was still favorable to a rich forest vegetation of deciduous and conifer q 
trees, which were with a slight preponderance of present western. types the 
same, as covered the eastern part of the continent in the Appalachian Mounta 
and elsewhere. The changes which followed from the Pliocene down to 
present produced vast changes in the topography of the country and in’ 
vegetation. Great floods of trachyte, basalt and other lavas from many poi 
and fissures over a vast space of the Rocky Mountains and westward w 
poured out. During the Pleistocene, the territory occupied by the Great Ba 
in great part a desert was then a lake region. One of these seas, named‘ 
Bonneville occupied Salt Lake Valley, Utah and a contemporary’ lake cal 
Lake Lahontan, situated in northwestern Nevada and extending into Califo 
existed, 886 feet deep. The climate was more humid than at present, 
from the evidence that the basin itself affords, it seems safe to assume, 
the average rainfall over Nevada at the time it was transformed into a 1 
region was probably not in excess of ten or fifteen inches a year. The tin 
of marked expansion of these ancient lakes indicate an increase in m£ 
annual precipitation, and times of contracted water surface, a decrease of f2 
fall. During this period, a rather uniform forest covered the western moU 
tainous part of the continent, as far, as the Pacific. It will be remembered 
from what has preceded that the great ice sheet effectually separated WE 
practically uniform Miocene vegetation of North America into two great grol 
an eastern and a western, a change which had already begun when 
Mississippi Valley was submerged and after its elevation was tenanted by 
grass formation (see ante) accompanied by a steppe climate which still 
effectually isolated the two great floras. The vegetation of the Rocky Moun 
had many elements during the period of greatest lake extension which do f 
exist now for we have evidence from remains found in South Park that 5 
genera as Seguoia, Glyptostrobus, Myrica, Rıhus, Sapindus, Fieus, Plane 
Caesalpinia, Acacia, Zisyphus, Ilex etc. existed there. With the disappeät 
of Lakes Bonneville and Lahontan, the gradual recession of the glaciers WF 
the mountains which supplied these lakes with water and the disappea! Ä 
of the great ice sheet from the northern part of the continent, an arid clim2 
explanation. The whole region is marked by the scantiness, of absence 
ı) Dana, J. D.: Manual of Geology.. Foirth ed. 1895: 933. 
