1887.] 411 [Shaler. 



life are afforded in our paleozoic sections. The most conspicuous 

 of these is that which gives us the alternations between the Medina 

 sandstone and the millstone grit. During the period of the Medina 

 sandstone a large part of the region east of the Mississippi River 

 was in the condition of a shallow sea. After it came a time of 

 considerable depression in which the seas probably became several 

 times as deep as they were in the Medina period. In the time of 

 the Oriskany sandstone a good deal of the area was returned to 

 the condition of shallow water. Then a profound subsidence oc- 

 curred bringing a large portion of the area of the Appalachian dis- 

 trict into the condition of deep sea during which the deposits of 

 the Devonian black shale or Ohio shale were formed. After a long 

 period in which this subsidence endured elevation the rocks again 

 became more and more composed of coarse sediments until finally 

 in the period of the millstone grit a large portion of the area be- 

 came dry land or shallow sea. Every important step in these 

 changes is distinctly marked by profound alterations in the char- 

 acter of the sediments. 



We thus see that changes of elevation and the consequent 

 variations of geographical relations may produce many important 

 alternations of strata. There can be no question that geological 

 science owes much to those who have developed the principle of 

 cycle in sediments. At the same time geologists must recognize 

 the fact that very important variations in the character of sedi- 

 mentary deposits are not explicable in this way. Thus, if we come 

 to the central portions of the Mississippi valley, we find a great 

 succession of limestones composed of beds varying from a frac- 

 tion of an inch to a number of feet in thickness each of which is 

 separated from contiguous beds by a thin partition of clay. Ex- 

 amining any of these beds in detail we find that it extends hori- 

 zontally for a distance of miles in every direction with more or 

 less considerable variations in thickness. It is noticeable,' how- 

 ever, that the variation in the depth of the clay parting is gener- 

 ally inconsiderable, usually indeed so slight as to escape detection. 

 There is reason to believe that some of these clay partings, not 

 over an inch in thickness, extend over many thousand square 

 miles of area. It is manifestly unreasonable to suppose that these 

 divisions can be explained on the theory of change in level. 

 They evidently need to be accounted for in other ways. The very 

 fact that they are so extensive and invariable shows clearly that 



