1117, 118] ROTTEN LIMESTONE— LOCALITIES. 77 



117. Whore the Rotten Limestone appears on the surface, or is covered by 

 pervious strata, it appears white or yellowish-white, and generally preserves 

 the same tint to some depth, varying with the perviousness of the mass, from 

 2 to 18 feet. Below this there is often a very marked change of color into 

 bluish gray, which when wet looks quite dark and is therefore very commonly 

 distinguished as "blue rock" from the "white prairie rock" on or near the sur- 

 face. The rock is the same, however, both in composition and fossils, the dif- 

 ference of color being caused merely by the oxidation of a trace of carbonaceous 

 matter, or protoxide of iron, or at times both. This circumstance causes 

 great difficulty in the study of the strata, where the records of well-borings 

 have to be relied on for information, since both the Rotten Limestone and the 

 dark colored clayey sands of the Tombigbee Sand Group are often indiscriminately 

 styled "blue" or "black rock" or "dirt," by the well -borers and inhabitants, 

 rendering their accounts extremely perplexing and apparently incompatible 

 with any regular stratification. Fortunately, notwithstanding the general le- 

 velness of the surface, outcrops are quite common in the Rotten Limestone re- 

 gion ; the channels of the creeks are often cut into the rock itself, and from its 

 resistance to denudation, it has not formed so many rounded subterranean hilL?, 

 but comes to the surface where a stratum ends, through the surface materials 

 forming "bald prairies" and "bald hilltops" — in which the limestone is too 



the surface to allow of the growth of trees or other deep rooted plants, and 

 not unfrequently forms white areas many acres in extent, strewn with fossils 

 (especially oysters) washed out of the mass, and only here and there a patch of 

 Verbena, or Cassia (C. obtusifolia, occidentalis, marilandica). 



118. Localities of the Rotten Limestone group. — At Breuton's contract on the 

 Memphis & Charleston R. R., near Chawalla Station, a cut exhibits about 17 

 feet of yellowish white calcareous clay, overlying a blue, micaceous, compact, 

 clayey, slightly effervescent sand ; near the surface of the latter, the calcareous 

 clay contains abundance of Ostrea falcata ; above, some Ezogyrae and Gry~ 

 phaeae are found, but chiefly Inoccrami, Mytilus, Tellina, &c. This whitish, 

 calcareous, "joint" clay occurs in numerous cuts between Chawalla and Cor- 

 inth, and also E. of the latter place ; the cretaceous strata being from 3 to 20 

 feet beneath the surface over the whole region, while N. of Corinth, in Ten- 

 nessee, bald prairies strewn with shells are said to exist. At Farmington, the 

 "blue rock" is passed through, into loose water-bearing sand, at 40 to 50 feet ; 

 at Corinth, at 70 ; while at Mr. Tate's, S. 7, T. 1, R. 7 E., in Tennes ;ee, N. of 

 Chawalla, a bore of 356 feet did not strike any loose sanl, but secm;d to con- 

 tinue in the unchanged "blue rock." Due S. of this, also, near Bone Yard and 

 Kossuth, and on the E. half of R. 6 E. down to T. 5 generally, we!!s are very 

 deep, and outcrops very scarce ; I have been unable to ascertain how much of 

 the "blue rock" belongs to the Rotten Limestone, anl how much, if any, to the 

 Tombigbee Sand Group, which the dark micaceous material seen at Breuton's 

 contract resembles exceedingly. On Parmeechee Creek, S. 33, T. 2, R. 6 E. t 

 there is an outcrop of very micaceous, sandy marl, the she'ls of which (vjry 

 imperfectly preserved) seem to place it within the Ripley group ; bciw^ea 



