400 
GEOLOGY OF THE SECOND DISTRICT. 
deemed it right to place them in this rock rather than in the loraine shales which succeed. 
The mineralogical characters of the slate continue up to this line ; but immediately above, 
the shaly layers commence, which are very barren of fossils. Above the plane upon which 
these fossils rest, a change soon appears ; but up to this plane, the mass is homogeneous, and 
characterized by the fossils which I have introduced. The remark made by one of my col¬ 
leagues, that the upper part of the utica slate is destitute of fossils, I think is based upon the 
fact, that the mass which contains those described above, and which I consider as belonging 
to that part of the rock, is washed away from the valley of the Mohawk, or does not appear 
there. But in my district I find these peculiar fossils in a position by which they are pro¬ 
tected in the gorges of Rodman and Loraine. And besides this, I know that in many places 
the rock is uncommonly barren of fossils ; and to me there is in reality no discrepancy in our 
observations, when we compare the two fields or districts together, and the varying character 
of the masses. 
In this State, and particularly in Jefferson, I have been unable to find a commingling of 
the fossils of the trenton limestone; for instance, those which follow, are strictly limited to 
the limestone and its shaly part, viz. Orthis testudinaria, Strophomena alternata. Lingula 
ovata, Favosites lycopodites, Isotelus gigas, Calymene senaria, and Graptolites dentatus. 
Of these, the last only have I discovered in the utica slate ; and I would suggest, that when 
the preceding fossils have been supposed to occur in this rock, whether, in reality, they 
were not found in the slaty part of the trenton limestone ? For if any fossils are characteristic 
of the trenton limestone, these certainly are the ones. 
The surface of the country underlaid by the utica slate is hilly, but rounded. It is also 
more or less intersected by steep ravines, which follow the course of the creeks; a result 
which arises from the disposition of the rock to disintegrate, wherever it is exposed to water. 
Where this rock prevails, the soil, being both rich and deep, is highly favorable to agriculture, 
producing abundant crops of grass and grain, and affording good pasturage. 
To determine the whole thickness of this rock in Jefferson county, I made several careful 
examinations of it in the deep gorges of Loraine. I was the more desirous of making an accu¬ 
rate estimate here, as the rock is elsewhere so much deranged and confounded with slaty masses 
of the loraine shales, that it is difficult to obtain satisfactory results. From the estimate, as well 
as measurements at some points of exposure, I am satisfied that its thickness never exceeds 
seventy-five feet. This is less, I know, than the estimates of other geologists ; and it is highly 
probable that, in other districts, the rock is thicker. It is not improbable, too, that the mass 
in this direction is really thinning out, being situated near the edge of the great basin in which 
the rocks composing the Champlain group were deposited, and hence its thickness will increase 
towards the Mohawk valley. But I have sometimes been suspicious that the thickness of 
rocks has been over-estimated, partly from their deranged condition, when it is impossible to 
determine the limits of a mass where it is deranged or concealed partially beneath the surface. 
