ARCHEAN AND PALEOZOIC GEOGRAPHY 455 
beds extend from New York state southward to Georgia, and 
occur throughout the entire Appalachians. In the east they are 
sandy, as would be expected in the neighborhood of the coast, 
and in the west they become finer and finer, until they are actu- 
ally no longer shales, but limestone, with little clay impurity. 
This is also a result of the geography of the times, for in the 
quiet water at a distance from the shore, even fine clayey parti- 
cles of rock cannot be transported. 
The Clinton beds are marked throughout the greater part of 
their extent by the presence of iron, which in some places is 
sufficiently concentrated to serve as mines of iron. ‘This is true 
at Clinton, N. Y., at Birmingham, Ala., and in various intermedi- 
ate places. For some reason during the time of deposit of these 
layers, the ocean waters were furnished with considerable iron, 
some of which was perhaps directly gathered into the iron veins, 
but most of which was scattered through the accumulating rocks. 
By later changes this iron has in some places been gathered into 
veins. As for the source of the iron, no definite statement can be 
made; but it may have been derived from lavas furnished by 
voleanic action in the mountains of the east, which were supply- 
ing the materials out of which the beds were being constructed. 
The next epoch, the Niagara, indicates another change of con- 
ditions in New York; for above the shales of the preceding time, 
occurs a limestone, which in the western part of the state attains 
a thickness of eighty-five feet. The variation from the shale to 
the limestone is somewhat gradual in places, though at the Falls 
the change is abrupt. In fact, the very existence of Niagara 
depends upon the rapidity of this change from friable shales to 
the overlying compact limestone (p. 167). 
This limestone, resting as it does upon shale, shows a decided 
change in conditions. For some reason, very probably the deep- 
ening of the sea, the waters which had up to that time been 
depositing clay and sand, derived from the land, were unable to 
do so longer, and began to build limestone rocks. 
Diverging for a moment from New York state, it may be said 
