ARCHEAN AND PALEOZOIC GEOGRAPHY 453 
We can hardly find a more impressive lesson than 
this, of the great changes taught by geological study. 
What a vast amount of time must have been required 
for the accumulation of the great deposits of the Cam- 
brian and Ordovician strata, reaching a depth of several 
thousand feet! And then what immense time must 
have been needed to have uplifted these into the form 
of lofty mountains, and then for their extensive de- 
struction and planing down almost to sea-level ! 
While this mountain folding was in progress in the east, there 
were apparently changes of great import in the west; but our 
studies of this region have not yet progressed far enough for any 
detailed statement of these changes. 
At the same time as the formation of the mountains 
in the east, a gentle uplift of the bottom of the sea 
occurred on the site of the present states of Ohio, 
Kentucky, and Indiana, while a smaller one occurred 
in Tennessee. The former has been called the Cincin- 
nati arch. This was not a really mountainous growth, 
but a broad uplift of a part of the ocean bottom, which 
reached above sea-level. 
Silurian Geography. — This period and the succeed- 
ing, the Devonian, have been studied with great care in 
New York; and since this study has revealed the his- 
tory of this part of the country, it will be stated in 
more detail than has been given to the preceding 
epochs. The changes recorded in the Silurian strata 
