29 
shore lines. The alkali plains and salt flats, mark the places 
of dried up lakes ; all of these still existing being excessively 
salt. This is the state of things at the north. In the south, 
the great Colorado plateau is without mountain barriers or 
local basins, and there are few traces of extinct lakes. This 
arid district was once a beautiful and fertile plain, drained by 
the Colorado which, on the western margin poured over a 
precipice five thousand feet or more high, into the Gulf of 
California, which then reached several thousand miles farther 
north than it does now. In time the river cut its way farther 
back through the subjacent rocks, and at last formed that 
remarkable gorge, nearly a thousand miles long and three to 
six thousand feet deep. As the channel deepened, the 
country around became dryer, until it was the arid plain we 
find it now. Almost no rain falls on this plain, therefore the 
walls of the cafion remain sharp-cut precipices unaffected by 
moisture. 
On the east of the Rocky mountains, is the great plateau 
country of the plains, which differs from the country to the 
west, by not being bordered on its east by a mountain chain, 
but sloping gradually to the Mississippi. Its surface was also 
covered by great fresh water lakes, larger if not more numer- 
ous than those now existing on our northern boundary. From 
the northern portion of this plateau, Dr. Hayden has brought 
his specimens, and he has there obtained a harvest of scientific 
truth, which will form for him an enduring and enviable 
monument. He has studied the deposits which accumulated 
in these lakes, and they are very rich in specimens of both 
animal and vegetable life. The vertebrate remains have 
been studied by Dr. Leidy, who has published his investiga- 
tions in the splendid monograph so well known, and which 
forms a contribution to paleontology, not second in value or 
interest to that made by Cuvier, by his illustrations of the 
fossils from the Paris basin, nor to that of Falconer and 
Courtly, descriptive of the Sewalik hills of India. The first 
instalment of the plants have been described by Dr. 
Newberry, in the Report of Col. W. F. Reynolds, U.S. A., 
not yet published. The descriptions are , published, in the 
