202 /*.'. M. l\"nidl< — Stratigraphic Relations of the 



Such variation in the thicknesses of the bine shale beds as 

 those indicated above show that they pass laterally into black 

 shales when followed some distance and vice versa. Prof. II. 

 P. Gushing has called attention to a considerable mass of inter- 

 bedded bluish gray and black shale at the base of the Cleveland 

 shale, which he finds west but not east of Cleveland, named by 

 him the Olmsted.* If, as the evidence appears to indicate, 

 the Chagrin west of Cleveland grades into and becomes 

 interbedded with the black Cleveland shale, just such beds as 

 Professor Gushing has described would result from and repre- 

 sent this grading. Beds similar to Cushing's Olmsted also 

 probably appear in the same general region near the Huron- 

 Chagrin contact, and the gradual relative thickening westward 

 of both basal and terminal zones of these gradational transi- 

 tion beds results in a series of interbedded blue and black 

 shales in the Huron River section in which it seems to be 

 impracticable to discriminate the precise limits of the Chagrin. 

 The essential fact or feature involved in the preceding dis- 

 cussion of the sediments of the Ohio group is the marked 

 lithologic differences between eastern and western sections 

 and the decrease in coarseness of the Chagrin sediments from 

 east to west or in a direction away from their source. 



This fact of the lithologic change in the character of sedi- 

 ments between eastern and western sections is not provided for 

 in Mr. Ulrich's doctrine of the persistence of lithologic units, 

 and we find him using it as an argument against the continuity 

 of the Chagrin into the Huron River section. Let us consider 

 briefly whether the variation of the lithologic features of the 

 Chagrin outlined above are consistent with the present opera- 

 tion of the agents engaged in distributing marine sediments. 

 Under ordinary conditions the two agents engaged in this work 

 are waves and currents. Very fine sediments are carried chiefly 

 by currents which may carry them great distances regardless 

 of depth. Coarse sediments are dependent for transportation 

 chiefly upon wave action, which may remove them to any dis- 

 tance from the coast whence they were derived that prevailing 

 winds and depth will permit. Depth is the sharply controlling 

 factor in the operation of this agent. Wherever the water 

 becomes too deep for waves to act effectively on the bottom, 

 there coarse sediments will cease and fine ones begin. 

 Corresponding changes in the character of the fauna may be 

 expected along this border line as the joint result of change in 

 depth and in the character of the bottom. The differences 

 then between the lithology of the Chagrin in northeastern Ohio 

 and its supposed equivalent in the Huron River section are 

 only such as we should expect to find if the sea were materially 



* This Journal. Jnne, 1912, p. 583. 



