CORNIFEROUS LIMESTONE. 
167 
FEET. INCHES. 
Limestone in two courses, with an irregular layer of hornstone between . 2 2 
Thin-bedded limestone with much hornstone in nodules and flattened masses, all con¬ 
taining abundance of coralline fossils... 3 0 
Limestone with a large proportion of hornstone intermixed; contains some coralline 
fossils ... 10 0 
Limestone in two courses of ten inches each... 1 8 
From this point there is a fall of twenty or thirty feet in the stream, where the rock 
is indistinctly seen. From the point where it reappears, there are thin strata of 
hornstone with a small admixture of calcareous matter, the whole exceedingly 
rough and ragged . 24 0 
The last division contains in its upper part numerous coralline fossils usually replaced by 
silex, and in many instances they are even more durable than the mass of hornstone around 
them. The lower part of this mass is separated from the Salt group by six feet of light grey 
limestone, destitute of fossils, which, from its position, represents the Onondaga limestone. 
There is no other point known in the district, where there is so great a development of the 
hornstone of this rock, it usually being a very subordinate part of the whole. From the vici¬ 
nity of Le Roy, there appears to be a gradual diminution of the siliceous matter in a westerly 
direction, some points being nearly free from it. Within ten miles of the Niagara it again 
increases, constantly augmenting as far as Black Rock. In this intermediate space it occurs 
mostly as nodules embraced in the calcareous rock, and sometimes increasing so as to form a 
separation between the layers. The rock is usually of grey or greyish blue color ; but when 
within ten miles of the Niagara, it again assumes the very dark, almost black color of the mass 
in Seneca county. 
At Black Rock, where so well developed, the mass consists of calcareous matter in irregular 
wedge-form layers, separated by thin laminse of shale. In some instances the siliceous por¬ 
tions are equal to the calcareous, and both are blended in the most heterogeneous manner. 
It is separated from the terminating rock of the Salt group, by a layer of grey limestone about 
fourteen inches thick. 
It will be observed, that in all cases where this rock is highly developed, the Onondaga 
limestone, the next rock below, is but meagrely so. For all practical purposes, in the Fourth 
District, the two masses may be considered as one. They are intimately connected, forming 
together the Limestone Terrace, and there are few good localities where both are not to be 
seen. In lithological character, there is scarcely more difference between the Onondaga and 
the Corniferous, than in different parts and different localities of the latter rock. 
The fossils of the two rocks in many instances blend together, or more properly the Favo- 
sites and other corals of the Onondaga occur in the central part of this rock. The upper part, 
however, is always distinguished by its organic contents, as will be seen under that head. 
Localities. — The occurrence of this rock almost continuously through the district, offers 
numerous localities for its examination. A few of these may be indicated, as affording better 
