128 CHAMPLAIN DIVISION. 



Potsdam sandstone are not extensions downward of repeated beds confonnable thereto, 

 but they are throughout non-confonuablc, of divers kinds, following each other in suc- 

 cession, and forming together an immense thickness far superior to all the rocks of the 

 New- York system, embracing even all the masses up to the coal of Pennsylvania. For 

 this reason, we say that those who maintain that the silurian rocks are merely altered 

 rocks of the Champlain group, maintain that which is not far removed from an absurdity. 



But to return to the consideration of the distribution of the Champlain group. I have 

 already spoken of numerous insulated patches of some of these rocks. These are usually 

 the Calciferous sandstone ; and inasnmch as they frequently resemble the Sparry limestone, 

 they are very liable to be mistaken for it, especially when they occur in the neighborhood 

 of the latter. This mistake has been, and is still, very likely to be committed in the eastern 

 towns of Rensselaer and Washington counties, where there are heavy beds of Calciferous 

 sandstone with the fossils peculiar to the same. 



I may call the attention of geologists to the limestones in Hoosic in the former county, 

 where the Madurea has been found by my friend Mr. L. Wilder. Generally those 

 masses of limestone are not extensive ; and even some are so limited, in which these fossils 

 occur, that the entire mass has been removed, showing conclusively that they do not form 

 a constituent part of the rocks upon which they rest ; and moreover their dip and strike 

 do not conform at all to the slates upon which they repose. 



Another small range of the Hudson-river rocks occurs between Chatham centre and 

 Chatham four-corners. The Great Western Railway passes over and through many oi 

 these thick-bedded masses, which are clearly the same kind of rocks as those which ap- 

 pear in the northern face of the Helderberg range. They lie in deep troughs ; and as the 

 thickness exposed in the railway cuttings are never deep, no lower rocks appear, but those 

 which belong to the superior part of the Champlain group. 



The superior rocks of the Champlain division border the west side of the Hudson, from 

 Coeymans to New-Jersey. On this river, upon either shore, not a single member of any 

 of the superior divisions exists. At Hudson city, on the western side, the Helderberg division 

 forms the surface rocks over a limited area, but they are removed two and a half to three 

 miles from its banks. At Coeymans, the same rocks are at least two miles west. At 

 Kingston point, the Pentamerus rock is within about one mile, which is the nearest ap- 

 proach of this rock to the river. 



The Shawangunk range, farther south, is a distinct western boundary of the Hudson- 

 river group to the New-Jersey line ; at least, neither the Ontario or Helderberg division 

 appears on the east of this very remarkable range. The limit of these rocks, however, 

 may be better understood by a direct reference to the accompanying map. 



Important developments of the upper members of the Champlain division exist at the 

 nortliern termination of the Mohawk valley, the valley of Oneida lake, and of Salmon 

 river. It is interesting to notice some of the differences in these masses. At or near Rome, 

 it is a tolerably clear gray sandstone, free from slate comparatively ; and from its consoli- 



