HAMILTON GROUP. 
187 
The valleys of Seneca and Cayuga lakes are both excavated, for more than half their length, 
in the shales of this group; and the banks of these lakes, with the lateral ravines, afford the 
best facilities for examination. The group appears also to be better developed in this part of 
the State than farther west, there being in that direction a gradual thinning of the different 
masses. 
Along the banks of these lakes I have been able to trace the following subdivisions, which 
hold good over considerable areas, but which cannot be relied on in every instance; for to¬ 
ward the western extremity of the State some portions are lost or merge into others, so that 
the same lines of subdivision cannot be recognized. These divisions, which have been enu¬ 
merated in the annual reports, are the following: 
1. Dark, slaty fossilferous shale, which rests directly upon the Marcellus shale, there 
being, as before remarked, no very well defined line of separation between the two. This 
part of the group is not very abundant in fossils ; it may be seen in the towns of Varick and 
Fayette in Seneca county, and on Flint creek and Mud creek in Ontario county. 
2. A compact calcareous blue shale, often passing into an impure limestone. The mass is 
quite thin, and worthy of notice only from being somewhat persistent, and marking the point 
of separation between two more important shaly masses. This is visible on the banks of 
Cayuga and Seneca lakes, on Flint creek, and w r as traced as far as the west side of the Ge¬ 
nesee river. 
3. An olive, or often bluish fissile shale rests upon the last named mass. It is marked by 
small concretions, and contains a few Cyathophylli and some other fossils. Toward the upper 
part, this shale becomes very fissile and of an olive color, often stained by oxide of manganese. 
It can be seen on the shores of both the lakes just mentioned. 
4. Ludlowville shales. The latter mass passes by insensible gradation to a more compact 
rock, which contains an admixture of sand, and often separates in large masses which resist 
the action of the weather for a long time. Above this part the rock is a soft bluish grey shale, 
which is more calcareous and fissile, and contains a different association of fossils. These two 
kinds, with the next below and the two above, appear at and near the village of Ludlowville, 
in Cayuga county; and since these rocks are of the same age as those of the Ludlow forma¬ 
tion of England, described by Mr. Murchison, this is a desirable locality to perpetuate the 
name. 
5. Encrinal limestone. This rock is a thin bed, usually of impure limestone, with a great 
abundance of crinoidal columns. Sometimes the mass is a compact shale, held together by 
columns of great size and length. It is a persistent mass, holding only one position in the 
group, and continuous as far as Lake Erie. It is a convenient point of reference, and will be 
frequently mentioned in the description. It rests upon the last named shale, separating it 
from the Moscow shale. 
6. Moscow shale. This mass succeeds the encrinal limestone, and is a well defined and 
persistent mass throughout the district. It is of a greyish blue color, and scarcely laminated ; 
it separates into irregular fragments, which are rarely slaty. It is slightly calcareous, and 
abounds with fossils, many of them unlike those below, 
24* 
