76 GEOLOGICAL REPORT. [1T116 



III. THE ROTTEN LIMESTONE GROUP. 



116. The general character of this formation in Mississippi va- 

 ries little from that given by Tuomey and others to the same strata 

 in Alabama. The surface of its territory is generally level or but 

 slightly undulating; when high ridges do occur, their main mass is 

 the limestone itself, on which the Orange Sand formation is want- 

 ing, or present only to an inconsiderable thickness, or in patches ; 

 the surface formation being mostly stiff clays, which underlie the 

 prairies. Hence a great dearth of naturally available water dur- 

 ing the dry season, characterizes the region in an economical point 

 of view. The material of the formation itself is of great unifor- 

 mity — a soft, chalky rock, of a white or pale bluish tint, with very 

 little sand ; consisting of variable proportions of fat, tenacious 

 clay, and white carbonate of lime in crystals extremely minute, and 

 with some shells of infusoria. The stratum is of great thickness 

 and uniformity of character on its southwestern border, borings of 

 700 to 1000 feet being no uncommon occurrence in S. Chickasaw. 

 E. Octibbeha, Noxubee and N. E. Kemper. In consequence of its 

 dip, the stratum here thins out norlheastivard further N., easticard; 

 but besides, there is a general thinning out to the northward, bo 

 that in S. E. Tippah, the maximum thickness is only about 350 feet, 

 at Blackland, in Tishomingo county, 150, and on the Tennessee 

 line, from 70 to 100 feet — perhaps much less. At the same time, 

 in this northern portion of its territory, its materials lose their 

 uniformity, consisting at times of white or bluish, tenacious, calca- 

 reous clay — called by the inhabitants, from its massy cleavage, 

 "joint clay" — alternating with strata of the common "Rotten 

 Limestone," and sometimes — though rarely — with layers more or 

 less sandy. 



Its lowest portions, where it adjoins the Toinbigbee sand strata, are generally 

 white clayey sands — as at the outcrop on Okalilly Creek (1J112), and stratum 

 No. 10 at Plymouth Bluff, (Ull-i, Sec. 10). Among its fossils, those rarely 

 wanting in any of its outcrops, are Exogyra costata, Gnjphaea mutahilis, ccn- 

 vexa, incurva, Pitcheri, Plucuna scabra, Janiia qtn'yiqvecostata. The occur- 

 rence of the Gryphaeae, of Ostrea falcata, of Radiolites, Ichthyosarcvlites, and 

 Belemnitdla mucronaia, which appear to be wanting in the Tombigbee sands, 

 form prominent and convenient palacontological marks of distinction from the 

 latter group ; [the Gryphaeae do, however, occur in the Ripley group also.] The 

 species of the genus Inoceramus (numerous in both) also afford convenient 

 landmarks, altho' 1 hav-i not thus far determined them specifically. 



