22 CRETACEOUS GROUP 



by boring to be at least 500 feet thick. The more ele- 

 vated bluff at Erie is chiefly composed of the same rock, 

 which is here very friable and well characterized by fine 

 specimens of Pecten quinquecostatus, as well as abun- 

 dance of Exogyra costata. A short distance north of 

 Erie, the Cretaceous rocks terminate, following the course 

 of the Black Warrior ; and at Tuscaloosa the old Red 

 sandstone with bituminous coal form the bed of that river. 

 The Tombeckbe and most of its tributaries runs entirely 

 through a region, the substratum of which is the Creta- 

 ceous group, although it is probable that their sources 

 originate in the Carboniferous limestone, which may ex- 

 tend into the north-east section of Mississippi. We 

 learn from travellers, that the Cretaceous rocks chiefly 

 compose the countries of the Chickasaws and Choctaws, 

 and it is highly probable that nearly the whole state of 

 Mississippi is of the same formation. It is worthy of re- 

 mark, that all the prairies of Alabama and Mississippi 

 have a substratum of the older Cretaceous rock. The 

 newer Cretaceous strata prevail only in the southern por- 

 tion of Alabama, are never covered with a prairie soil, 

 and have not been observed north of the central parts of 

 Clarke and Monroe counties. 



JVummulite Limestone. — For the following account of 

 this highly interesting deposit, I am indebted wholly to 

 Mr. Conrad, who examined it personally : 



u After crossing the Alabama river at Claiborne, I tra- 

 versed a level alluvial country for two or three miles, 

 when the surface becomes broken into gravelly hills co- 

 vered by a pine forest. Near Suggsville the hills are 



