158 Association of American Geologists and Naturalisis. 
The lower rocks exhibited by the section, and which were well de- 
veloped in New York, he had met only at a few points westward. 
They occur at Frankfort, Ky., and according to Dr. Owen, on the Mis- 
sissippi at Prairie du Chien, and at the mouth of the Wisconsin. 
Their development at Frankfort cannot well be ascertained. The same 
fossils which typify these rocks in New York are found in Kentucky 
and at the mouth of the Wisconsin. We thus have a uniform compo- 
sition, nearly similar developments and like fossils, extending this great 
distance ; and must, therefore, admit a uniform condition in the depth 
and bottom of this primeval ocean. Already do we know that this ex- 
tent east and west was not merely a margin, but that the same rocks 
extend into Canada and stretch west beyond Lake Huron; and Profs. 
H. D. and W. B. Rogers have identified them in Pennsylvania and Vir- 
ginia. (Specimens from some of the different localities were then ex- 
hibited.) : 
The next was the Hudson River group, made up of shales, shaly 
sandstones, and sandstones with little calcareous matter. This also is 
seen in Ohio, Indiana, Kentucky, and on the Mississippi aboye Dubuque ; 
but with a change of character—having become more calcareous, 80 
much so as to have received the name of Blue Limestone. Its thiek- 
ss is apparently less than in New York. In New York its typical 
fossils are of the Conchifera, while with some exceptions the Brachio- 
poda are rare. At the west, the latter are the predominating fossils; 
while the fossils which are characteristic in New York, are the least 
prominent at the west. Corals and crinoidea are also far more abun- 
dant throughout the group to the west, and indicate a source of the cal- 
careous matter. The crustacea also appear in greater numbers, and tri- 
lobites different from those of the Trenton. Of the Oneida conglome- 
rate, the Medina sandstone, and the Clinton group, we have scarcely 
any definite traces at the southwest. The Niagara group, which is 
next in order, consists in New York of shale and limestone, both being 
highly fossiliferous, the former containing corals, crinoidea, shells and 
trilobites—the latter, chiefly corals. At the west the shale has disap- 
peared with its fossils, and the limestone much increased in thickness, 
and, as in New York, abounds in corals. Here we haye the calcareous 
matter increasing as We go west. This mass in the centre of New York, 
only a few feet thick, is two hundred and fifty feet at Niagara. Falls, 
and in the western states little less than a thousand. The Onondaga 
salt group, which in New York is upwards of a thousand feet thick, 
has thinned out to the west so as to become almost insignificant, _Suc- 
ceeding these bai have the Helderberg limestones, an extensive group 
Tee in fossils. They, however, except the two upper limestones, 
ppear before we leave this state. And these two are well devel- 
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