NIAGARA GROUP. 
81 
The members of this group are : — 
1. Argillaceous, or (in many localities) argillo-calcareous shale. 
2. Limestone, presenting several different varieties. 
l. Niagara Shale. 
The lower part of the Niagara group exhibits a great development of dark, bluish shale, 
which on exposure gradually changes to grey or ashen color, and forms a bluish or greyish 
marly clay. In this state, it is undistinguishable from the ordinary clays ; and its outcropping 
edges, where long weathered, are often considered as clay beds. This character is well ex¬ 
hibited at Lockport, on the northern slope of the terrace where the canal and railroad have 
been excavated; a d also at numerous localities in Wayne and Monroe counties. The depth 
of tint in the clay differs according to degree of exposure, the outer portions becoming of the 
usual yellowish brown color of the ordinary soils. 
When freshly excavated, the mass is tough, and breaks irregularly, some portions only 
exhibiting a slight tendency to slaty structure. After weathering for a short time, it cracks 
in all directions, and soon falls into innumerable angular fragments, when the disintegration 
goes on till it forms the soft clay. This change seems due to the intimate mixture and de¬ 
composition of iron pyrites in the rock ; and its presence is also indicated by the production of 
sulphate of alumina/on decomposition in favorable situations, and upon calcination. In color, 
aspect, manner of weathering, and other properties, it closely resembles the shale of the upper 
part of the Hamilton group in the Fourth District. Neither are micaceous, and both are 
slightly calcareous, probably from the great amount of organic matter. The Niagara shale, 
however, is destitute of those spheroidal concretions, which in the Hamilton group are more 
or less common, and in many places abundant. The only approach to a concretionary form 
seen in this shale, is in the increased thickness of some layers of impure limestone ; and this 
appears rather due to a greater development of corals or other fossils, around which the mud 
accumulated more freely than elsewhere. A few such examples may be seen in the banks of 
the Genesee at Rochester. 
The lower part of this shale is mostly free from calcareous bands ; while towards the middle 
and in the upper part, we find numerous thin, wedge-form or continuous layers of impure 
limestone, mostly composed of corals and other fossils, and their surfaces covered with the 
same, forming beautiful and interesting specimens for the cabinet. The perfect similarity of 
these with specimens from Dudley in England, together with the identity of many of the 
organic forms, renders the conclusion unavoidable that the two are formations of the same 
age. These layers are from half an inch to two inches thick ; and from the decomposition 
and sinking down of the shale, they are usually found broken into fragments. One of the 
most striking features of this rock is the abundance of its fossils, which will be enumerated 
in another place. Scarcely a locality can be examined where they do not occur in great 
perfection. 
Geol. 4th Dist. 
11 
