as Time-boundaries in Geological History. 229 
With no great epochs of revolution to fix limits to the Silu- 
rian, and none to give bounds to the Devonian, the heights of 
the Appalachians loom up majestically as a time-boundary to 
the Paleozoic. 
It is fitting that the raising of one of the two border-chains of 
the continent—the eastern—should thus mark one of the grand- 
est of the transitions in geological history. The transition was as 
abrupt in the life of the continent and globe as in its formations; 
for it was the time when its ancient features were to a great 
extent lost:—when Trilobites, Cyathophylloid and other old styles 
of corals, and the Sigillarie and Lepidodendra of the old forests 
tory has its appropriate monument in the ky Mountains, the 
western border-chain of the continent. The Rocky-Mountain 
its rise, as has been stated, just before Cenozoic time began. 
The elevation was not completed at once, but continued in 
