46 HUDSON RIVER GROUP. 



§ 92. The seas swarmed with animal life and fucoidal organisms, and the rocks 

 are composed almost wholly of their remains. It is literally a graveyard of inver- 

 tebrate life. The Brachiopoda and Bryozoa reached in this age the stage of their 

 greatest varietal development, and possibly the highest state of their existence. 



§ 93. As the exposure in Ohio, Indiana, and Kentucky is very large and quite 

 characteristic of it in other places, it may be fit and proper to further define it. 

 To go from the Ohio River, at Cincinnati, west 51 miles to Osgood, Indiana, or 

 north to Dayton, or north-east to Xenia, Ohio, one will pass across the upturned 

 edges of this Group, and reach the Niagara. The rocks dip westerly and northerly 

 at the rate of about ten feet in a mile. The hills at Cincinnati expose about 

 400 feet in thickness, constituting the lower half of the Group; and the upper 

 half, or about 400 feet, occurs between the top of these hills and the bordering 

 Niagara Group, about 50 miles distant to the north and west. The area of its 

 exposure in Ohio is all of Hamilton, Butler, Warren, Clermont, and Brown 

 Counties, and part of eight counties that border upon these. The exposure 

 in Indiana is about half as great, reaching as far north as Richmond, and border- 

 ing the Ohio nearly half-way from Madison to Jeffersonville. The exposure in 

 Kentucky is greater than in Ohio, for it surrounds the Trenton Group in that 

 State. Throughout the whole area it is composed of alternate layers of calcareous 

 clay and limestone of varying thickness. In some places calcareous clay is 6 or 8 

 feet thick, without a layer of stone. At other places one layer of stone, 4, 6", 8, 

 or 10 inches in thickness, follows another, with intervening layers of calcareous 

 clay of much less thickness, for 40 or 50 feet. It is rare to find a layer of lime- 

 stone more than a foot in thickness. All the layers are broken into small, irregu- 

 lar pieces of suitable size for cellar and other light stone-work, for which they are 

 used. The blue calcareous clay exposed to the action of the weather for a few years 

 loses its color and becomes of a dull gray hue. The sulphuret of iron occurs in 

 the blue rocks, but instead of this we find iron oxide and sulphate of lime in the 

 gray. The silicious matter prevails over the carbonate of lime in the layers of 

 calcareous clay, while the carbonate of lime is much in excess of the silicious matter 

 in the stone, due, in part at least, to the fact that the stones are a mass of more or 

 less comminuted shells, corals, and criuoids. There is nothing in the general 

 character and appearance of the rocks and calcareous clays to indicate the changes 

 which the fossils undergo ; that is, the changes are not to be attributed to sur- 

 rounding conditions without the aid of that law of animal evolution which the 

 science of palaeontology teaches us has taken place in all past geological ages. 



§ 94. Some fossils, as Calymene callicephala, Asaphus megistus, A. gigas, Beyrichia 

 chambersi, Leptcena sericea, Bellerophon bilobatus, Zygospira modesta, Strophomena alter- 

 nata, and Orthis testudinaria, pass from the extreme lower part to the extreme upper 

 part of the Group ; and all of them save Beyrichia chambersi are known from lower 

 rocks, and Leptama sericea occurs in higher ones. Streptorhynchus hallianum has 

 a limited range in the lower part, S. planoconvexum and S. sinuatum a limited range 

 below the middle of the Group, S. nutans and S. sulcatum in the middle of the upper 

 half of the Group, and S. subtentum and S. filitextum in the upper part. Lielieiwcri- 

 nus crateriformis, L. dyeri, and L. pattersoni are confined to the lower half, and L. tuber- 

 citlatus and L. affinis to the upper part. Acidaspis crossotus occurs in the lower part, 

 A. anchoralis and A. cincinnatiensis in the middle part, while A. onealli occurs in the 



