430 A. F. FOERSTE — LIMESTONES OF TENNESSEE AND KENTUCKY 



conglomerate. This sug-gests a soiitlieni source for the sandy and con- 

 glomeratic material. 



CAUSE OF EROSION SHOWN AT THE SILURIAN-DEVONIAN CONTACT 



In the preceding pages it has been shown that the Devonian limestone 

 and, especiall3% the Devonian Black shale rest on the inclined edges of 

 the various Silurian formations. Reasons have been given favorino; the 

 view that these subdivisions once extended much farther up the flanks 

 of the anticline, but were eroded before the deposition of the Black shale 

 and of the Devonian limestone. It would be interesting to know whether 

 this erosion was due to marine or subaerial causes. The number of facts 

 known at present is inadequate to determine this matter. The Devonian 

 limestones are of marine origin, and their basal layers in Kentucky and 

 Tennessee are not known to give any indication of residual soils or any 

 other evidence of land conditions. Since the Silurian-Devonian erosion 

 must have taken place before the deposition of the Devonian limestones 

 of Kentucky and Tennessee, the general absence of evidence of subaerial 

 erosion at the base of the Devonian limestone is important, although the 

 general absence of marine rocks between the Louisville limestone and 

 the middle Devonian limestone along the flanks of the anticline slightly 

 favors the view that land conditions were present. 



In the case of the Black shale, the evidence of land conditions or of 

 fresh-water conditions is more favorable. At many points through its 

 entire extent it has retained remains of land ])lants. Its strongly car- 

 bonaceous character, which gives rise to the black color of the shale, does 

 not necessaril}^ indicate the presence of land plants, although the pre- 

 sumptive evidence is in favor of this view. At various localities the re- 

 mains of animals have been i)reserved in this shale. Some of the brachio- 

 pods, such as species of Lingala, Barrlosella, Schizoholus, and Orbiculo'idea 

 may have lived in brackish waters, and these are the forms usually found 

 in Kentucky and Tennessee. They occur usually only near the base of 

 the formation, but in northern Indiana near Delphi, an undoubted marine 

 fauna, including species of GoniatUes and Orthocerash^s been discovered 

 by Mr K. M. Kindle. This fauna occurs within 20 feet of tlie base of 

 the Black shale. This is quite near the base when the total thickness 

 of the Black Shale deposits of northern Indiana is considered. As a rule, 

 however, it may be stated that evidences of marine life are absent in the 

 Black shale except at the base. 



The base of the Black shale is often decidedly earthy and is often also 

 phosphatic. It is well known that the base of Black shale is in many parts 

 of southern Tennessee sufficiently phosphatic to be worked as a phos- 

 phate rock. One of the theories of the accumulation of the phosphatic 



