S6 



On the Weathering of the Subcarboniferous Limestones of 

 Southern Indiana. 



By E. R. Cumings. 



The subcarboniferous (Mississippian) limestones of southern Indiana 

 comprise three formations Icnown in the ascending order as the Harrods- 

 burg, Salem (Bedford) and IMitcIiell Hmestones, and having a combined 

 thickness of at least 350 feet. These rocks are in the main very pure car- 

 bonate of lime. Some shaly layers are to be found in the Harrodsburg 

 and Mitchell limestones Avhich may contain very little lime; and the 

 Harrodsburg is rather lower in the per cent, of lime carbonate than the 

 other two formations. Analyses of the Salem limestone show from 97.9 

 per cent, to 98.4 per cent. CaCoa, with the balance consisting of magne- 

 sium carbonate, and oxides of iron and aluminum, with traces of silica 

 and other substances. Analyses of Mitchell limestone show from 96.65 

 per cent. CaCOs to 99.04 per cent., with the balance consisting of mag- 

 nesium carbonate, iron, aluminum, and silica as in the Salem limestone. 

 Satisfactory analyses of the Harrodsburg limestone are not at hand. Of 

 these limestones the Salem is the most constant in composition and is on 

 the average the highest in i^er cent, of CaCoj. 



In texture the three limestones vary widely. The Harrodsburg is 

 rather thin bedded, coarse-grained, fossiliferous, in some cases decidedly 

 crystalline in structure, and contains geodes abundantly, in the lower por- 

 tion especially, and bands and knots of chert. There are layers and 

 lenses of shale. The Salem limestone, on the contrary, is, as is well 

 known, almost without bedding planes. It is a massive, oolitic or gran- 

 ular-crystalline, close-grained rock frequently cross-laminated and quite 

 free from geodes and chert. Its fossils are usually minute, foraminifera 

 and small ostracods predominating. The Mitchell limestone is in the 

 main thin-bedded, hard, fine-grained, sometimes almost lithographic, with 

 frequent alternations of shaly layers. It is in general unfossiliferous. 

 Bands and knots of chert are very common, but geodes are infrequent. 



All these limestones are conspicuously jointed. The Mitchell shows 

 the cleanest and most numerous joint planes; but the best examples of 



