6 STRATIGKAPHICAL 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 Kentuckj-, 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 Bellerox)hon bilohatus, Strophomena alternata, Zygospiro 

 modesta, Leptena sericea, Buthotrephis gracilis, Beyrichin ckambersi, Calymene 

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

 Trinmleits conceniricus, Triarthrus becki, Orthis multisecta, 0. etiiacerata, 

 Slreptorhynchti.-i hallia, Ambonychia bellistriata, Modiolopsis cinciimatiensis, 

 Cycloconcha mediocardinalis, Lichenocrinus crater if or mis and Chetetes (?) 

 jamesi, are confined to the lower half of the group. Glyptocrinus decadactyhis, 

 G. dyeri, G. nealli, Q. fornshelli, Lichenocrinus tubercidatus, Strejitorhyiichi's 

 fllitexta, S. subteuta, S. sulcata, S. sinuata, 8. nutans, Orthis insculpta, 0. suh- 

 quadrata, Rhynchonella capar, B. dentata, Cypricardites haynesi, Anonialo- 

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

 Jibratum and Streptelasma cornicuhim, are found only in the upper part of the 

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

 sciilpta, Orthis retrorsa, 0. emacerata, Glyptocrinus nealli and Streptorhynchiis 

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

 of the ocean was constantly 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 cither the ex- 

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

 vertical range of onl}- 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 la3'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 find a layer of 

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

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

 for which they are used. When the blue marl is exposed for a few j'ears to the 

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

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

 has been changed to the dull graj' 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 



