GEOLOGY. 231 



we consider it as partly siliceous.* They are replete with 

 organic remains. They are principally referrible to the pro- 

 ductae, terebratulse, &c. We saw none but what belonged 

 to bivalves. The existence of these fragments was observed 

 upon many elevations, over a considerable extent of coun- 

 tiy, while in the valleys no trace of them could be seen. 

 Generalizing the observations made during the three last days 

 of our journey previous to our arrival on the Missisippi, we 

 are led to admit that there are, or rather that there were for- 

 merly, two distinct formations of limestone in this country, 

 and that they were separated by a thick stratum of sand- 

 stone. Of these two limestone formations, the older one, 

 which we have already described with minuteness, we have 

 been induced to consider as coeval with, or analogous to, the 

 magnesian limestone of England. The superior formation 

 is distinguished by the circumstance of its containing harder 

 fragments or nodules of limestone, which alone remain to 

 establish the fact of its former existence : that it contained 

 no hornstone or flinty quartz, as observed in the former, we 

 are led to believe, because had they existed, they must 

 necessarily have resisted decomposition, as well or better 

 than the calcareous nodules w^iich are now found alone. 

 The much greater abundance of shells in these nodules, and 

 the total absence of the madreporites, appear to us to be very 

 characteristic distinctions between these and the underlaying 

 limestone, though perhaps too much weight ought not to be 

 assigned to the absence of the madreporites, as these, from 

 their loose and more porous texture, may have been unable 

 to resist the decomposing causes which appear to have 

 affected this formation. In some places a limestone bed was 



* There are localities occasionally on both sides of the river, where the 

 surface of the ground for a small distance is covered with these chert 

 fragments. 



