201 KXPEDITION TO THE 



vious to our arrival on the Mississippi, we are led to admit 

 that there are, or rather that there were formerly, two dis- 

 tinct formations of limestone in this country, and that 

 they were separated by a thick stratum of sandstone ; of 

 these two limestone formations, the older one, which wc 

 have already described with minuteness, we have been in- 

 duced 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 re- 

 main 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 which are now found 

 alone. The much greater abundance of shells in these no- 

 dules, 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 observed upon the sand- 

 stone, but these depositions were so partial, and in all cases 

 the ground was so much overgrown with bushes, that we 

 were unable to examine their characters with any degree 

 of minuteness. This striking difference, however, we 

 observed, and we are led to consider it as constant, that 

 the inferior limestone, whenever it appears exposed, is 

 covered with small scales or fragments of the hornstone 

 nodules whose existence has already been alluded to, while 

 none of the flat, calcareous fragments, abounding in shells, 



