June, 1916] The Or dovician- Silurian Boundary 333 



Maysville and culminated at the close of the Richmond, and 

 the temporary lowering of barriers would be expected to let 

 in a few forms. But it is significant that only a few new genera 

 and only one species are common to the Richmond and the 

 Upper Medina-Clinton, and the great invasion does not come 

 in until after the Richmond-Upper Medina break. That 

 break must have ended in the broader letting down of faunal 

 barriers. 



It may be said that H. catenularia is absent from the 

 Richmond of the Cincinnati area, but it should occasion no 

 surprise should it be found here. 



3. When two series of strata are separated over broad 

 areas by both a distinct physical break and a radical faunal 

 difference, it should detract little if any from the value of that 

 break as a division plane, should there somewhere be dis- 

 covered a series of strata filling in the break. 



For it is inconceivable that there should be erosion over 

 the whole earth at once. Somewhere there must have been 

 bodies of water, and in these must have accumulated the 

 record of strata and of fossils which is represented elsewhere by 

 the physical break. So, unless removed by subsequent denuda- 

 tion, there must exist somewhere a complete physical and 

 faunal record of each break. 



Therefore the filling in of such lost intervals is but to be 

 expected, and it is not the presence of such transition strata 

 which determines the value of the break, but it is the amount 

 of sedimentation and the horizontal distribution of the transition 

 strata, the amount of erosion and the degree of faunal break 

 over broad areas, which determine the value. 



4. If the Richmond-Upper Medina boundary, so distinct 

 in'Ohio, Indiana and Kentucky, be impracticable because the 

 Richmond-Albion boundary is occasionally obscure, and if it 

 thereby be degraded to a minor position, then the same thing 

 certainly should apply to the Maysville-Richmond break, which 

 may show a decided physical break in the Appalachians or 

 elsewhere, but shows either the most obscure kind of a break 

 in Ohio, Indiana and Kentucky, or else none at all. 



It is to be expected in the shallows about the ocean 

 margins that every little movement of the shore line will be 

 strongly marked in the accumulating sediments. Here every 

 little oscillation will produce a physical break, and the results of 



