24 
GEOLOGY OF THE FOURTH DISTRICT. 
York System over the grealer portion of the country between the grand Primary chain on 
the east and the Mississippi river. And it further appears that throughout all this extensive 
area, these formations are overlaid by no rocks more modern than the Coal formation, except 
in a few limited districts where the newer Secondary rocks or those of the Cretaceous group 
succeed the latter. Such wide development and generally undisturbed condition will cer¬ 
tainly afford the means of bringing to light many important facts regarding the formations, 
which could never have been known from the examination of limited districts, however perfect 
the sequence. As before remarked, the territory of New-York, from possessing the most 
complete series, and abundance of fossils, together with the undisturbed position of the strata, 
offers the most interesting field of investigation and reference, and will be found the best point 
of departure for the geologist who is making more extended researches. 
The geographical divisions in this table, though convenient for reference, nevertheless do 
not indicate any great natural divisions of the system as founded upon fossil characters. Such 
a mode of subdivision will follow only a perfect knowledge of the fossils, both in this State 
and elsewhere. From the commencement of the fossiliferous rocks, to the termination of the 
Hudson-river group, there seems to have been a uniformity of condition and a continuation 
of species which cease with this period, and cannot be found in any subsequent one. The 
two lower masses, it is true, so far as examined, contain few fossils, and those of species 
not known to extend upwards ; but these rocks must be considered as having been produced 
at the dawn of that era, and are emphatically the “ protozoic rockswhile, after living 
forms had become abundant, many of the same continued throughout the whole period unde¬ 
stroyed. 
Mr. Conrad is disposed to include in this period the Medina sandstone and Clinton group. 
The former possesses many analogies, though none of the fossils of the lower rocks; while, 
on the other hand, none of the fossils of the Medina sandstone are continued into the rocks 
above, and the apparent continuation of the Grey sandstone into the base of the Medina would 
argue in favor of placing the latter in the lower division. But in regard to the Clinton group, 
its great contrast with the Medina sandstone, both in lithological and fossil characters, seems 
an insurmountable obstacle in the way of uniting this with the lower division; particularly 
as in many points that group, or its fossils, pass into the next above, while we have not a 
solitary example of the passage of any fossil from the lower group into this. The termina¬ 
tion of the Medina sandstone, therefore, as far as regards the State of New-York, must be 
considered the termination of the lower division of the system. 
Throughout a part of New-York, and more particularly in Pennsylvania and Virginia, as 
we learn from Professors H. D. and W. B. Rogers, the Oriskany sandstone forms a marked 
line of separation, and might perhaps well be considered the limit of the Second great divi¬ 
sion. Nevertheless, the absence of this rock in western New-York, in Canada, and, so far 
as I know, in Michigan, Ohio and Indiana, would still render it an obscure point of reference. 
The “ Cliff limestone” of Ohio, which is there known as a single formation or group, includes 
rocks both above and below the Oriskany sandstone. This shows such a close analogy in 
the strata, that together with the absence of the rock on which the division is to be founded, 
