IOWA ACADEMY OF SCIENCES. 149 



Correlation by comi^arison of the stages of physiographic 

 development is highly important, and fertile of exact results 

 in the later deposits, but it cannot be extended directly to the 

 older formations, though the principle is of first importance. 



CORRELATION OF PROVINCES OF DISSIMILAR HISTORY. 



Since sedimentation goes on most actively along the borders 

 of the great land masses of the globe, it is mainly a function of 

 continental growth and decline. Its most important relation is 

 with the shore-line, for the latter marks the boundary along 

 which the process goes on. On the one side materials are being 

 continually prepared to be carried away; on the other they are 

 being deposited. To rising or sinking of the land with refer- 

 ence to the sea, or to the continual advance or retreat of the 

 shore-line, are to be ascribed all the widespread changes in 

 the character of the deposits thrown down in any particular 

 place, and it is the variations in level, chiefly, that give rise to 

 the intricate succession of lithologically different layers. 



The immediate causes for the changes between the relations 

 of the land and sea areas are to be sought in orogenic and 

 epeirogenic movements. As the two kinds of movements can- 

 not be readily separated practically, and as it is of small 

 advantage to separate them theoretically, the results produced 

 may be all regarded as arising from the one cause, from moun- 

 tain-making forces. 



The greatest and most abrupt changes in sedimentation, and 

 consequently in lithological, stratigraphical and f aunal, and in 

 fact all characters, are those connected directly with diastatic 

 changes, producing depression of some land areas below sea 

 level, and the uprising of other districts above the level at 

 which they once stood, forming those great surface features 

 called mountains. Geological chronology, therefore, is believed 

 to find a rational basis in the same processes that are involved 

 in the genesis of mountain systems, and it is proposed to mark 

 off the leading subdivisions of geological time, and strati- 

 graphical succession, in accordance with the cycles of oro- 

 graphic development, calling the classification a systematic 

 arrangement by mountains, and the principle orotaxis By the 

 term mountain is meant, not alone those geographic features 

 which at the present time are so conspicuous on the surface of 

 the earth, but also all of those structures which have in the 

 past been prominent characters in the surface relief, and 



