88 ROCKS BESTING UPON 



glazed shales for four or five feet in width. To depend on lithological characters for the 

 determination of such a mass, would be unsafe ; and it was only after much search in 

 these beds for fossils, and after I had finally succeeded in obtaining those peculiar to the 

 Hudson river shales, that I felt at ease on the subject. 



From the above remarks and facts, it will not appear strange that the district through 

 which this section runs should have been set down as wholly occupied by the Hudson river 

 shales, particularly when it is known also that heavy beds of drift conceal much of the 

 rock, that there is a resemblance between the two slates, and that it is only in a very few 

 places that their relations are so exposed as to excite the inquiry whether they may not be 

 diflferent slates in juxtaposition ; and even when this question is raised, it is difficult to 

 make real advances towards its solution ; so much so, that we are frequently disposed to 

 let it remain still unsettled. 



There is one circumstance, however, which I have often observed, and which has been 

 of some service in tlie examination of a country, viz : Where two systems lie in contiguity, 

 there is a wide space in which no rocks appear : neither one nor the other ; but a space 

 which appears to have been a deeper depression than usual, and filled with soil and drift : 

 those on one side plunging deeper beneath the surface ; on the other, rising up from 

 deeper depths below. I do not state this as an absolute or universal fact, but only as one 

 which I have frequently observed, and of which several instances have been noticed in 

 this report. 



Between the Taconic slates and the rocks belonging to the Champlain division, we 

 find similar relations to those that exist between the Primary schists and the Taconic 

 rocks ; that is, the latter members come in longitudinally, or in the direction of strike, 

 and in separate portions of the same rocks, from the former, as in the case of granite in 

 section. In all our examinations of the strata of such districts, we are continually in 

 danger of committing mistakes, especially if we leave much of the surface unexamined, 

 and have ventured upon generalizations incautiously, or without due regard to the systems 

 and to the country under consideration. 



Now in regard to those rocks which succeed the Taconic slates one mile west of Chat- 

 ham four-corners, they can be regarded in no other light than as overhanging unconformable 

 masses to the Taconic system. First, the graptolites arc the same as those of Norman's kill 

 on the west side of the Hudson, one and a half miles south of Albany ; and no diflference 

 can be discovered between the thick-bedded sandstone, and that which crops out on the 

 northern slope of the Helderberg. Proceeding a few miles farther west, we pass over 

 another outcrop of green taconic slates of great thickness, without beds of sandstone as 

 in the Hudson river rocks. The Hudson river rocks are prolonged southwardly towards 

 Hudson, on the railway" or adjacent to it. The lower limestones are not exposed; but 

 three or four miles west of Hudson, the birdseye is exposed, and the upper portions of the 

 Champlain group disappear beneath the Manlius waterlimes of Becraft's mountain. 



