HAMILTON GROUP. 
J85 
character, the cleavage is irregular, and oblique to the planes of stratification. On weathering, 
where the edges are exposed, there is manifested a slight tendency to slaty cleavage. 
From the wide and even distribution of the materials of this group, it was evidently pro¬ 
duced during a period of great tranquility, when the finely levigated mud was transported over 
wide areas by gentle oceanic currents. The great profusion and variety of organic forms 
proves also the quiescent condition of the ocean, which, together with the slightly calcareous 
nature of the mud, favored the growth and distribution of the Testacea. 
The upper part of this group, in the Fourth District, bears a very close analogy in its litho¬ 
logical nature to the shale of the Niagara group ; and in abundance of organic remains, it is 
even more prolific. The forms of the latter are, however, of entirely dilferent species, though 
corals and shells of similar genera abound, and triiobites of the same and different genera. 
Concretions or septaria, in well defined and often fantastic forms, are common in every part 
of this group. In many instances the calcareous matter has concreted around some organic 
body, or a nodule of iron pyrites seems to have been the centre of attraction. In such in¬ 
stances, we often find numerous fossils imbedded in them, or attached to their outer surface. 
The greater number, however, are well defined spheroidal masses, with or without seams of 
crystalline matter, and not containing any organic body. Others, and particularly in the lower 
part of the shale, are small, spherical or elongated, and with a small perforation through their 
centre, in the manner of the common nodules or other concretionary forms in recent clay beds. 
Organic forms abound throughout the group, but they vary somewhat in the different parts. 
In the lower division, the most abundant are those of Orthis, Atrypa and Strophomena, with 
some spiral univalves ; while above this portion, great numbers of Avicula, Cypricardia, Nu- 
cula, and other similar forms abound, with fewer of the genera Orthis, Delthyris, &c. In the 
next division, Delthyris, Strophomena and Atrypa abound, to the almost entire exclusion of 
the forms before mentioned. In the same situation with these we find numerous species of 
corals : Cyathophylli, Favosites and other forms, are abundant; while fragments of crinoidal 
columns are every where scattered through the mass, or spread evenly over the surface, form 
thin layers of themselves. Many of the species of this division are discontinued, and their 
place supplied by others of the same genera in the higher part of the group. The principal 
characteristic forms of each division will be found under the head of Organic remains of the 
group. 
Although this group is so widely spread and evenly distributed, and of uniform character 
over the western part of the State, still at its eastern extremity the lithological character is 
widely different. The shales are more or less arenaceous, and some parts are well marked 
sandstone. The proportion of siliceous and argillaceous earth is nearly reversed from what it 
is in the same rocks farther west. The mass varies from sandy shale to shaly sandstone, and 
even tolerably pure sandstone. This character gradually changes to the westward, the sand 
diminishing and the clay increasing. The features presented by this group at its two extremes, 
and along its whole length, offer one of the most instructive exhibitions of the varying character 
of mechanical deposits. The facts prove the origin of the materials to have been at the east 
or southeast. The force of the current which drifted them into the ocean was sufficient only 
[Geol. 4th Dist.] 24 
