GEOLOGICAL SKETCH. 
The rocks which inclose the fossils described on the following pages 
occupy a series of detached areas extending interruptedly from Nova Scotia 
to North Carolina. They are in the form of basins, with their longest diam- 
eters northeast and southwest, parallel with the bearing of the ridges of the 
Alleghany Mountain belt Indeed, they seem to have been deposited in 
troughs lying between the most easterly and lowest of these ridges; troughs 
that were for ages occupied by fresh or brackish water lakes or estuaries, 
the surface drainage of the adjacent country. 
After the Carboniferous age the whole region between the Mississippi 
and the Atlantic was raised above the ocean level, where it has remained 
with little variation of altitude to the present time. Of this belt of elevated 
country, which reached from the Green Mountains and the Adirondacks to 
the Gulf, the most easterly portion was much the older. This was formed 
by the Blue Ridge and the Hudson Highlands, with one or more parallel 
ridges on the east, which have since been depressed. The Alleghanies 
proper were added toward the close of the Carboniferous age. In-Mesozvie 
times all this broad belt of highland was suffering erosion, and the material 
removed-was carried away by the draining streams, both east and west, 
either in suspension or in solution. That which was dissolved flowed off 
into the somewhat distant oceanic basins, where it was deposited by organic 
agencies as limestone or flint, as it was lime or silica; while the suspended 
material was spread as sand, clay, and gravel cver what are now the plains 
of the Mississippi Valley and what were then water-filled basins along the 
Atlantic coast. As I shall show farther on, the first of the Mesozoic strata 
to which we have access—composed of materials removed from the ancient 
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