INTRODUCTION. 7 



the fossil characters, we have little evidence of any great physical change. 

 In the State of New- York, the Chazy limestone, which succeeds the 

 Calciferous sandstone, often presents in its lower divisions evidences of 

 a continuation of the same conditions as prevailed during the deposition 

 of the preceding rock. The lithological aspect of some of the beds is 

 precisely similar to that of the Calciferous sandstone, showing that the 

 waters charged with the same materials still flowed over the ocean bed of 

 the Chazy period. Nevertheless in some other localities within New- York 

 there appears a slight unconformity between the Calciferous sandstone and 

 the succeeding rock ; but this appearance may be attributed to the absence 

 of a portion of the limestone beds, permitting the Birdseye limestone to 

 rest directly upon the sandstone. 



In the northwest, on the contrary, there are many evidences of physical 

 change. There, the Calciferous sandstone is often, through a thickness of 

 forty or fifty feet or more, a huge mass of breccia. The materials of the 

 rock appear to have been broken up, while partially indurated ; the in- 

 terstices are often filled with sand ; and fragments of friable sandstone, 

 from the weight of an ounce to several pounds, are found mingled with 

 the broken rock itself. In some instances these fragments of sandstone 

 present lines of deposition, and sometimes of discordant lamination, 

 showing that they have been torn from masses of rock previously in- 

 durated. These phenomena occur at several points along the Mississippi 

 river ; and however local they may be in their extent, they point to a 

 disturbed condition in the surrounding ocean, which must have been highly 

 unfavorable to the continuance of the previously existing fauna. 



From the circumstance that these conditions of the rock have been 

 noticed only in the west, we are prepared to find the source of disturbance 

 in that direction, and probably beyond the limits to which our examina- 

 tions have extended. 



It should not be forgotten, moreover, that at the west, and throughout 

 the great exposvire of the Calciferous sandstone in the Northwestern States, 

 it is everywhere succeeded by a homogeneous light-colored sandstone. 



