— they are subordinate, and with whi 
in the Geological History of North America. 339 
tures of the land ; that the same cause which originated the one, 
impressed peculiarities on the other; that the two had a parallel 
ward, the continents upward; in other words, that they have 
g 
in my address before this Association last year—evidence derived 
from the extent and nature of the Potsdam sandstone, the earli- 
est of the Silurian strata, showing that this ee rock was laid 
down over a large part of North America by a sea which just 
bathed its surface—thus proving that the continent was 
made, and indicating in part its water level. 
- The relation between the extent of the oceans and the border 
features of the continents, which has been pointed out, 1s not 
simply a relation of fact, but of effect and dynamics, pomting 
should have in general a common system of structure. : 
A unity of cause there must be for the great Reenonntie a 
eg Such is nature in all her p unasoonne gate za ee 
means by which we penetrate to ee . 
x cause is once roabied and fully apprehended, the details have 
0 
of their beauty fois the rising trunk and os branches to 
C 
ony. i 
-neiple of development through which 
What then is the a structure ‘os features have been 
