522 
GEOLOGY OF THE FOURTH DISTRICT. 
Notwithstanding all this, however, the unity of the series is never broken; and although, 
during certain periods, all living things inhabiting the ocean were exterminated, the violence 
and disorder that prevailed at the sources of these deposits never prevented the gradual and 
quiet deposition at distant points. The change from one rock to another is rarely abrupt : 
there is usually a mingling of the products of both; and lime, sand and clay are found in all 
their varying proportions. Still the great uniformity in mineral character of certain deposits 
is very remarkable. The black and green shales of some of the higher rocks of the New- 
York System can be traced over an extent of a thousand miles, while specimens from either 
extremity are scarcely distinguishable. 
The conditions of this ancient ocean bed are as clearly proved by the nature of these 
deposits, as is the character of any small stream from the nature of the detritus it transports. 
The varying depth is marked upon the strata, and the clear or turbid state of its waters is 
indicated by the character of the formation and the kind of living things that then flourished. 
We are able to recognize portions which were raised above the surrounding ocean, and even 
the direction of the waves is as perfectly preserved as upon a modern sea beach. 
The evidences of this condition have been pointed out as we progressed. We have seen 
that the strata from the Hudson to the Mississippi are composed of materials which could 
only have been deposited from water, and that they contain organic remains which could only 
have lived upon the bed of an ocean. It is necessary, then, to conceive of a period when 
all that portion of our continent, now occupied by these rock formations, was covered with 
water. 
The chief source of our sedimentary deposits must have been from the east or southeast; 
and from this direction the sand of the earliest formation was distributed over the bed of 
the ocean, together with fragments and pebbles of rock. Calcareous springs issued from 
the margins of the sea, and mingled their products with the arenaceous matter. For a period 
the abrading and transporting process was in a measure suspended, and corals and shells took 
possession of the limpid waters of the ocean; these constructed their habitations, and lived 
quietly through myriads of ages. Their perfect structures and comminuted remains constitute 
a limestone formation which flanks the great primary nucleus of New-York, and, extending 
into Canada, spreads out westward over almost the entire width of the continent. This con¬ 
dition ceased with the deposition of the Trenton limestone ; the bed of the ocean subsided, and 
a deposit of carbonaceous mud rendered the waters turbid with its wide diffusion, while few 
living forms existed during the period of its accumulation. Again the transporting power 
became more energetic, or new sources were opened, and sand became mingled with the mud ; 
the latter gradually ceased, and gave place to sand alone ; and towards the close of this period, 
the disturbing influences and the transporting power had reached their acme, and the forma¬ 
tion is terminated by a coarse conglomerate, indicating a period of storms and tempests. 
During this time were deposited the shales and sandstones of the Hudson-river group, 
terminating with the Grey sandstone and Oneida conglomerate. 
