OF DENISON UNIVKRSITV. 7 



the same layers being much less. The amount of iron becomes great- 

 er as well as far more regular in the upper courses of the Clinton 

 Group at Todd's Fork, here becoming oolitic in places. Farther south 

 from this point it increases until it reaches some importance in tho 

 southern counties. 



The specimen from Huffman's (piarry marked " dark green " is 

 a fragment slioamg localized features of the ordinary limestone of that 

 locality, of small extent, usually existing only as little, often far dis- 

 tant, patches In the discussion of the chemical character of the rocks 

 here examined the analyses m.idj by Dr. T. G. Wormley, aid pub- 

 lished in the various volum-js of th j Ohio Geological Survev h 'v '- l^'-'-n 

 constandy before my eyes. 



STRATIGRAPHICAL CIROLOGV. 



Stratigraphually the C4inton Group of Ohio presents no marked 

 subdivision except that of the Beavertown marl, which occurs at Huff- 

 man's quarry, and also at Todd's Fork, but not as a marked subdvis- 

 ion. The top of the Clinton at Centreville and the Soldiers' Home 

 quarries, is composed of a few inches of clayey sediments, which pre- 

 sent a much larger number of specimens of certain species than are 

 found in the rocks below. These are: Or//iis hyhrida, O. ekgantula, 

 G.fausta, O. Daytonensis, Triplesia Ortoni, Calymenc /vj^'-i/^i/ (the lar- 

 ger forms). Rhinopom frondosa, and Homotn'pa confliicns. Hut this 

 seems to be a mere local variation. 



A comparison of the Ohio Silurian with the corresponding strata 

 of Indiana is at the present date difficult, owing to the fact that no 

 connected plan of work seems to have been followed in that state, so 

 that observations made are apt to have only very local value, and dif- 

 ferent writers apparently apply different values to the same stratigraph- 

 ical name'^. Careful paleontological observations with the exception 

 of the Waldron beds, are also lacking. Lilhological features alone, 

 unfortunately, are so often deceptive that not much reliance ran be 

 placed upon them. However the following conclusions seem fairlv 



well drawn. 



The Lower Silurian series of Indiana have been well determined. 

 In places, especially in Decatur and the more southern counties, the 

 base of the Upper Silurian is formed by a sandy rock, varying from 



