172 
THE GEOLOGICAL MIDDLE AGE. 
face the eastern end of the range was raided 
slightly, lifting the lower or Valengian deposits 
out of the water, so that they remain uncovered, 
and the next set of deposits, the Neocomian, is 
accumulated along their base, while these in their 
turn are slightly raised, and the Urgonian beds 
are accumulated against them a little lower 
down. They follow each other from east to west 
in a narrower area, just as the Azoic, Silurian, 
and Devonian deposits follow each other from 
north to south in the northern part of the United 
States. The Cretaceous deposits have been inti- 
mately studied in various localities by different 
geologists, and are now subdivided into at least 
ten, or it may be fifteen or sixteen distinct peri- 
ods, as they stand at present. This is, however, 
but the beginning of the work; and the recent 
investigations of the French geologist, Coquand, 
indicate that several of these periods at least are 
susceptible of further subdivision. I present here 
a table enumerating the periods of the Cretaceous 
epoch best known at present, in their sequence, 
because I want to show how sharply and in how 
arbitrary a manner, if I may so express it, new 
forms are introduced. The names are simply 
derived from the localities, or from some circum- 
stances connected with the locality where each 
period has been studied. 
