6 STKATIGRAPHICAL GEOLOGY. 



generally called the Cincinnati Group, though in Tennessee it is known as the 

 Nashville Group, or more generally it is known as the Blue Limestone. The 

 Cincinnati Group, as exposed in Ohio, Indiana and Kentucky, with which I am 

 better acquainted than with any other Group of .rocks, will receive more than a 

 passing notice. The total thickness of the exposure will scarcely exceed 1,000 

 feet, the lower part of which is probably the equivalent of the upper part of the 

 Trenton Group, and the remainder belongs to the Hudson River Group. The 

 Cincinnati Group, therefore,. is of value as a technical name only so far as it ex- 

 presses the absence of the Utica slate, and points to the locality of its exposure. 



Some fossils, as Bellerophon hilobatus^ Strophomena alternata, Zygospira 

 modesta, Leptena sericea, Buthotrephis gracilis, Beyrichia chamhersi, Calymene 

 senaria, Isotelus gigas and Isotelus megistos pass entirely through the Group. 

 Trinucleus concentricus, Triarthrus hecM, Orthis multisecta, 0. emacerata, 

 Streptorhynchus hallia, Amhoiiychia bellistriata, Modiolopsis cincinnatiensis, 

 Cycloconcha mediocardinalis^ LichenoaHnus crater if or mis and Chetetes {?) 

 jamesi, are confined to the lower half of the group. Glyptoc^Hnus decadactylus, 

 G. dyeri, G. nealli, G. fornshelli, Lichenocrinus tuberculatus, Streptorhynchus 

 Jilitexta, 8. suhtenta, 8. sulcata^ 8. sinuata, 8. nutans, Orthis insculpta, 0. suh- 

 quadrata, Bhynchonella capax, R. dentata, Cypricardites haynesi, Anomalo- 

 donta gigantea, A. alata, Anodontopsis milleri, Favistella stellata, Tetradium 

 Jihratum and 8treptelasma corniculum, are found only in the upper part of the 

 Group. Some fossils occupy only a few feet in vertical range, as Orthis in- 

 sculpta, Orthis retrorsa, 0. emacerata, Glyptocrinus nealli and Streptorhynchus 

 sidcata. These facts teach us that during the deposition of the rocks, the fauna 

 of the ocean was constantl}^ changing. One form ceasing to exist at a given 

 place at one time, and another at another time ; a new species appearing at one 

 period, and another at another period. Not, however, indicating either the ex- 

 tinction or creation of a new species, because though the Orthis inscidpta has a 

 vertical range of only about three feet, it is found in the Trenton Group, in 

 New York, in much earlier strata ; and substantially the same may be said of 

 other forms. 



The Group of rocks, throughout its entire thickness, is composed of alter- 

 nate la^^ers of blue marl and limestone, of varying thickness. In some places 

 the marl is 6 or 8 feet thick, without a layer of stone. At other places, one layer 

 of stone, 4, 6 or 8 inches in thickness, follows another, with intervening layers 

 of marl, of much less thickness, for 40 or 50 feet. It is rare to fincl a layer of 

 limestone more than a foot in thickness. All the layers are broken into small, 

 irregular pieces, sufficiently large, however, for cellar and other light stone work 

 for which they are used. When the blue marl is exposed for a few years to the 

 action of the weather, it gradually loses its color, and finally presents a dull 

 gray appearance. Where the marl in the bed, only a few feet from the surface, 

 has been changed to the dull gray color, crystals of the sulphate of lime are 

 found on the layers of stone and in the marl. The silicious matter prevails over 

 the carbonate of lime in the layers of marl, while the carbonate of lime is much 

 in excess of the silicious matter in the layers of stone, due in part, at least, to 

 the fact that the stones are almost literally a mass of more or less comminuted 

 shells, corals and crinoids. There is nothing, therefore, in the general character 



