PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 535 



tion at the same time, or entirely similar in degree of activity and in 

 character of effects produced, on different continents. They varied also 

 very greatly in intensity and character on different parts of the same 

 continent. Often one part was in course of submergence while another 

 remained emerged or was being elevated. The broader of such diverse 

 movements are indicated more especially as affecting the continents in a 

 north-south direction, but tilting, usually more limited in scope, oc- 

 curred frequently also in other directions. The alternating Arctic and 

 Gulf of Mexico invasions which, as partly described in preceding chap- 

 ters of this work, occurred during the middle Ordovician, Silurian, and 

 Devonian, are good examples of the broader differential movements. 



Again, in certain parts of the continents, as for instance in the lands 

 bounding the Arctic sea and in the broad median flats of North America, 

 movements in either horizontal or vertical directions were relatively 

 slight. Indeed the former were so broadly disseminated that their local 

 manifestations are scarcely determinable. The vertical movements of 

 these regions were similarly wide in their operations, but in this case, 

 though the maximum vertical displacement was perhaps never very 

 great, their ultimate effects on the size and pattern of the continental 

 seas were manifestly of great and patent consequence. 



Though all parts of the crust were affected, nearly the wliole of the 

 conspicuous orogenic results of the movements are confined to tracts 

 adjacent to the borders of the continents; and in these the north and 

 south trending orogenic wrinkles increase in development from the poles 

 toward the equator. On the whole, further, the results of the east-west 

 movements are greater on the west borders than on the east. There 

 were also north-south movements of equal and possibly greater magni- 

 tude. These resulted in excessive folding and mountain building. The 

 resultant ranges have a generally east and west trend, and were best 

 developed in belts bordering the Gulf of Mexico in the western hemis- 

 phere and the Mediterranean and Indian Ocean in the eastern. Tliese 

 north-south movements differed from the east-west, in that while they 

 caused land elevation in the belts mentioned, concomitant subsidence 

 occurred in the Polar regions. 



CATEGORICAL REVIEW OF PRINCIPLES OF CORRELATION BY DIA8TR0PHIG 



MOVEMENTS 



General statement. — The principles discussed under this heading are 

 inferred from physical phenomena and criteria indicating vertical and 

 horizontal movements of or within the shell of the lithosphere and con- 

 sequent shif tings of the strandline. The following categorical state- 



