350 



C. D. WALCOTT — THE TERM HUDSON RIVER GROUP. 



thai the forms of the upper part alone of theUtica zone occur within the valley 

 of the Hudson, and that the -ivat jrraptolitic fauna of the Hudson valley is 

 largely unknown in the interior of the state. It is probable that the grapto- 

 litic fauna was prevented from spreading over the interior <>f the Btate by 

 some such harrier a- subsequently excluded the interior continental fauna of 

 this period from the valley of the Hudson. 



A- described by Professor Orton, in the sixth volume of the Geological 

 Survey of Ohio, published in 1888, the Hudson River group in southwestern 

 Ohio consists of alternating beds of limestone and shale, the latter of which 

 is commonly known as blue shale. The entire thickness of the series in 

 southwestern Ohio is aboul 750 feet. He divides the series into lower and 

 upper. The lower is known as the Cincinnati division, and the upper as the 

 Lebanon division. The Cincinnati division has a thickness of from 425 to 





HUDSON 



Lorraine 



. ---' 



UTICA 



LORRAINE 



I III. II, I, ill. 



CINCINNATI 



MAUUOKETA 





Pioi bb I.— Diagram illustrating ■'■• ' ■ 11 



.v< w York, Ohio, a < ' /■ 



• n- are arranged on this diagrai i the Bame relative scale, a description ol 



will be found in the text. I number of section" are known between western Ohio and iowa,in 

 Illinois, thai show n gradual thioning of the Hudson toward the northwest. 



150 feet, and the Lebanon division he fixes at aboul 300 feet. The divis- 

 ion- are separated on both paleontologic and Btratigraphic grounds. In 

 drilling for gas in the vicinity of Find lay, the CJtica Bhale was rael with at a 

 depth of 800 feet. Ii is a black .-hale containing one of the si character- 

 istic fossils of the Utica shale, viz., Leptobolus insignis. This bedol Bhale has 

 the normal thickness of the Utica shall- in New York : i. '., 300 feet. The 

 LJtica shale thus discovered and defined is a constant element in the deep 

 wells of northwestern Ohio. Its upper boundary is not always distinct, as 



the Hudson River -hale that overlies it - stimes graduates into it in color 



and appearance. No great falling off of black -hale appears iii the Dayton 

 well, but at Middletowa the driller reported a sharp boundary between the 

 gray -hale, 320 feel thick, and black -hale 100 feel thick: the latter 



