251 
1 877] . Prof. Hayden's Explorations. 
great Rocky Mountain region as early as the Jurassic period, 
and that their differentiation had become great and clearly 
defined as early as late cretaceous and early tertiary times. 
Other observations suggest the probable lines of geographical 
distribution, during the late geological periods of their evo- 
lutional descent, by one or more of which they have probably 
reached the Mississippi River system, and culminated in the 
numerous and diverse forms that now exist there. 
The work of the past season shows very clearly the har- 
monious relations of the various groups of strata over vast 
areas ; that although there may be a thickening or a thinning 
out of beds at different points, they can all be correlated 
from the Missouri River to the Sierra Nevada Basin. The 
fadt also that there is no physical or palaeontological break 
in these groups over large areas from the cretaceous to the 
middle tertiary is fully established. The transition from 
marine to brackish water forms of life commences at the 
close of the cretaceous epoch, and without any line of sepa- 
ration that can yet be detected, continues on upward until 
only purely fresh-water forms are to be found. Dr. White, 
an eminent palaeontologist and geologist, says that the line 
must be drawn somewhere between the cretaceous and ter- 
tiary epochs, but that it will be strictly arbitrary, as there 
is no well-marked physical break to the summit of the 
Bridger group. 
