SUPERIOR SANDSTONES. 423 



ingredient; the others are comparatively rare, 

 and almost always in^very small quantity, but on 

 them chiefly depend the numerous varieties of 

 character which this rock presents. 



As these sandstones alternate with shales and 

 with limestones, they sometimes also pass into 

 them by insensible gradations ; but the transi- 

 tions into the former, are more frequent than 

 those into the latter rock. 



No independent minerals, except pyrites, are 

 found imbedded in these sandstones. Cobalt, 

 oxidulous iron, bitumen, and manganese, are too 

 rare to deserve being reckoned among substances 

 capable of modifying the character of the rock. 



Many organic substances, both vegetable and 

 animal, are found occasionally included in some 

 of these sandstones, as are fragments of carbonized 

 wood, and of coal. Moreover, there often occur in 

 them, forms which appear to have been produced 

 by the previous existence of vegetable bodies, of 

 which, nearly all but the traces have vanished. 

 There are, generally, cylindrical substances, often 

 of considerable dimensions, with scaly surfaces, 

 or with regular impressions resembling those 



