186 
GEOLOGY OF THE FOURTH DISTRICT. 
to carry onward the coarser particles to a certain distance, where they were deposited. The 
finely levigated mud was carried beyond this point, being floated by less force than the sand. 
Some portion of the clay was deposited with the sand toward the central part of the State, and 
but little of the latter extended beyond this point. Finally the current became more gentle, 
and the clay was deposited to a certain extent, beyond which the power of the current was 
insufficient to carry even this material, and the deposit consequently thinned out in that 
direction. 
Nothing could be more analogous than this to the simple operation of a powerful stream 
conveying detritus into the ocean. The force of the stream is at once checked as it flows into 
the larger body of water, and the coarser materials are thrown down. From this point its 
force is gradually diminished as it progresses, and the coarser particles of the remaining matter 
are precipitated, until finally it loses its force entirely, and the materials before in suspension 
fall to the bottom. Matter like finely levigated clay, however, falls slowly to the bottom of a 
deep ocean, and in this way it may be carried on hundreds of miles, although the current be 
comparatively weak. 
This is not an isolated example; for the same character is observed in all our mechanical 
deposits, which are vastly thicker in one place than another, showing that the ancient ocean 
had a limit, and that these materials were brought in like the common detritus of rivers, and 
spread over the bottom, gradually diminishing as the force of the current was neutralized by 
the greater body of surrounding water. 
This change in the nature of the materials is accompanied by an equally marjted change in 
the fossils imbedded in the different parts of the group. In the eastern part of the State, 
Avicula and Cypricardia with Nucula, &c. prevail in immense numbers ; while at the extreme 
western margin, though in precisely the same position, they are of the rarest occurrence, 
while numerous forms of Delthyris and Atrypa abound. In the present state of our know¬ 
ledge of the limits of this ancient sea, it is impossible to say positively what influence proxi¬ 
mity to land or shallow water may have had upon these forms ; but it seems, nevertheless, 
true that the kind of bottom or bed of the ocean had much to do in modifying the character of 
the testacea inhabiting it. Or, perhaps we should say that the different kinds of bottom were 
more favorable to the growth of certain species and genera than to others, and that as the kind 
of deposit changed, so also did the organic beings inhabiting it.* Of this fact we shall find 
sufficient proof as we investigate this formation. 
*In the present sea we know that its littoral inhabitants, at least, congregate in certain situations and upon certain bot¬ 
toms, and that forms abundant in some situations are rare in others, though different species are abundant. Even the 
fishes that approach the shore frequent different kinds of bottom, doubtless in search of their food; and different portions of 
our Atlantic bays are known as “ haddock ground,” and “cod ground,” the former always being caught on sandy bottom, 
while the. cod frequent stony and rocky bottoms. These facts may very well explain why one part of a geological forma¬ 
tion may abound in certain forms, while another, varying somewhat in character, may contain a very different congregation 
of fossils. 
The lithological character of rocks, therefore, is an element to be taken into consideration when we undertake to identify 
strata by their contained fossils. 
