IOWA ACADEMY OP SCIENCES. 
149 
Correlation by comparison 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 OE PROVINCES OP 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 retreab 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 faunal, 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 geoesis 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 
