15* 



UKOLOGICAL l.'El'OWT. 



[1232, 283 



or less, ol tit, and generally also ol alts. This 



is the case even w itli many of the solid sandstones, which on exposure to the 

 weather become covered with efflorescences of salts ; and it is to this peculiarity, 

 no doubt, i 1 want of durability is to a great extent to be attributed. 



Moreover, th [uently contain small concretionsof iron pyrites, 



which vitriolesce when the rock is exposed, and thus rend even large blocks. 



The iit, and the deposits containing it are 



always quite limited. Even in these, I have never detected even a trace of 

 murine foss 



232. iitcrop, perhaps, is more 



charad nts within a small space so many peculiarities of the 



formation, a- that from which it has taken its name— that forming the Bluff at 

 Grand Gulf, on tl River, where it is overlaid by the calcareous silt 



of the Blufl btained by myself, 



OF THE BLUFF \T GRAND CLAIBORNE COUNTY. 



233. The character of the minor ledges of thi very cha 



that, fifty yards from the precise point where these data were taken, the aspect 

 of the lower portion of the profile especially, mi eatly. It is only the 



upper ledge (No. 11) which possesses the peculiar structure which charael 

 the " Grand Gulf Sandstone" proper, viz : Grains of pellucid quartz, constituting 

 rather a coarse sand, imbedded in ue, white, enamel-like mass of silex, 



