Devonian Shales of Northern Ohio. 203 



deeper in the latter than in the former region. If, as the 

 evidence seems to indicate, the sea deepened rather rapidly 

 west of Cleveland during the Ohio epoch, wave action 

 would have become in that region powerless to remove coarse 

 sediments beyond the latitude of Lorain. No one who is thor- 

 oughly familiar with the Devonian stratigraphy of New York 

 and the northern Allegheny region would ever think of using 

 difference in coarseness of materials, or difference in thickness 

 of beds of supposed synchronous age, as an argument against 

 their simultaneous origin in different areas of the same sea. 

 Numerous examples of such changes could be given from this 

 region, but one will suffice. The first Hamilton section west 

 of Harrisburg shows nearly 800 feet of eoarse sandstones with 

 typical Hamilton shale and fossils above and below, the whole 

 having a thickness of about 1000 feet. Eighty miles to the 

 westward at Altoona the Hamilton formation is 500 feet thick, 

 composed of dark olive-gray shales without a trace of sandstone 

 in it. 



The relations of the different types of sediments in the Ohio 

 shale group are graphically shown in the accompanying east- 

 west cross section figure 3. The observations of the writer all 

 indicate conformable relations between the different shales of 

 the Ohio group and between the base of the latter and the Olen- 

 tangy. The positive evidence which can be adduced for the 

 conformable relations of these beds includes interbedding and 

 intergrading of the Cleveland and Chagrin types of sediments. 

 We may here profitably use Mr. Ulrich's testimony as to the 

 validity of this class of evidence for conformable relations. Mr. 

 Ulrich has, in his paper (see p. 188) referred to the writer's 

 earlier allusion* to the unconformity at the top of the Chat- 

 tanooga shale in Tennessee. He quotes from the folio as evidence 

 of local conformity at this horizon, that locally "the black shale 

 seems to pass very gradually into overlying green shale which 

 constitutes the base of the full Tullahom a section." Mr. Ulrich'sf 

 claim, then, is that whatever the columnar section and the 

 structure sections show, and whatever the text may say about 

 these graphic representations of the Tullahoma- Chattanooga 

 unconformity, the above reference to intergrading sediments 

 is nevertheless sufficient evidence of conformable relations where 

 they exist. Now the present interest in Mr. Ulrich's insistence 

 upon intergrading of formations as evidence of conformability 



* This Journal, vol. xxxiii, p. 121. 



f Mr. Ulrich has also stated that the authors of the Columbia folio did 

 not attach high importance to the Chattanooga- Tullahoma unconformity. 

 It must be observed, however, that they made it the boundary between 

 the Devonian and Carboniferous systems. Just how they could have given 

 it more importance is not easy to see. 



