400 GEOLOGY OF THE SECOND DISTRICT. 



deemed it right to place lliem in tliis rock rather than in tlie loraine shales which succeed. 

 The mincralogical 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 ihc utica slate is destitute of fossils, I tliink 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 Jlohavvk, 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 arc 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 agi'iculture, 

 producing abundant crops of grass and grain, and affording good pasturage. 



To determine the whole thickness of this rock in .lefferson 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 thiiming 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. 



