■^«T. 22 BICHMOND PAUNAL. ZONES — AUSTIN 5 



the hinge line. Again during a number of these periods of dis- 

 turbances, conditions appear to have developed that were extremely 

 favorable to the growth of one or more of the species then existing. 

 At several horizons where a considerable oscillation of the sea floor 

 is indicated we find one or more of the species present so overgrown 

 as to appear as veritable giants of their kind when compared with 

 specimens from other levels. In other instances the charge was 

 equally unfavorable to some species and we then see the individuals 

 greatly dwarfed, although later under better conditions they usually 

 regained their normal size. 



In the Richmond seas one species at least. Plectanibonites rugosus 

 seemed to have thrived best in the unsettled conditions that marked 

 the close of one period and the ushering in of the next. Its first 

 appearance in the group is at the beginning of the Clarksville divi- 

 sion before the change that brought that division into existence had 

 ceased to act. There in the varietal form known as P. rugosiis 

 clarksvillensis it occurs in imsual numbers and of great size. Again 

 on the heavy clay stones that marks the base of the Liberty and 

 higher up where a similar heavy laj^er marks the beginning of the 

 Whitewater, remains of countless millions of the species are to be 

 found although the shell is not especially common at other horizons. 



In preparing the following notes and geologic section, we have 

 confined our statements almost wholly to the small area lying chiefly 

 within the counties of Warren and Clinton in Ohio, east of the Little 

 Miami River, because it is the region with which we are most fa- 

 miliar, having passed our entire life within its limits and many 

 times each season visiting its exposures through a period exceeding 

 50 years. In referring to the divisions of the Richmond in the fol- 

 lowing pages, we are aware that the changes there suggested will 

 probably be called in question. Yet we can say in this connection 

 that these changes have not been made lightly, but only after a 

 careful study of the junction of these divisions extending through 

 many years, and made at practically all the exposures in this terri- 

 tory where they are to be seen and that if we desire accuracy, such 

 changes must be made. 



These rejieated examinations have convinced us that in addition to 

 the changes produced in the current life, and on the character of the 

 sedimentary deposits, by changes in sea depth, which apparently 

 were frequent and at times considerable — there were other powerful 

 forces occurring entirely outside of this region, which exerted a 

 greater influence than all others in determining the general changes 

 that took place in the Richmond seas. Apparently these outside dis- 

 turbances affected the existing life and developing strata in two dis- 

 tinct ways; first by turning loose at times floods of argillaceous 

 material which not only greatly affected life, usually modifying or 



