STRATIGRAPHIC CLASSIFICATION DIASTROPHIC CRITERIA 435 



striking fact is that by far the most of these orogenic movements occurred 

 in the equatorial and temperate zones. Indeed, excepting Seward Penin- 

 sula of Alaska, no folding of consequence has been reported in lands 

 bordering the Arctic. These facts doubtless are significant in their 

 bearing on the forces which have either separately or conjointly operated 

 in producing the present structural inequalities of the surface of the 

 lithosphere. 



Inland migration of Appalachian helts of folding. — During tlie Paleo- 

 zoic it is thought that in the Eastern and Southern States excessive 

 folding was confined to the Piedmont and Coastal Plain areas lying 

 between the Appalachian Valley troughs and the north middle Atlantic 

 basin and between the similar Ouachita troughs in Arkansas and Okla- 

 homa and the deeps of the Gulf of Mexico. The thrusts then affected 

 these weak border lands directly, while the broad synclinal areas to the 

 west and north of their inner margins were subjected to comparatively 

 gentle, irregular, but essentially parallel warping. It is this periodic 

 warping that caused the valley oscillations described on pages 321-329, 

 412-415, and 543-544. To a certain extent it may be responsible also for 

 the tilting of the more inland uplifts. 



As time went on the geographic zone of excessive folding and over- 

 thrusting traveled inland. Sometime about the middle of the Mesozoic 

 it had reached and passed the inner edge of the bordering lands (Taconia, 

 Appalachia, and Llano) and thereafter warping in the Appalachian and 

 other geographically corresponding synclinoria was superseded by active 

 folding and overthrusting. This supposition is based primarily on the 

 fact that while the late Ordovician shale deposits occupying an old 

 trough on the Piedmont plateau in Virginia are sharply folded and 

 metamorphosed, the similarly located Triassic deposits are but gently 

 folded and comparatively little changed. In seeking an explanation of 

 this difference the first that suggested itself was that the much longer 

 time from the present to the close of the Ordovician might sufficiently 

 account for the greater folding of the older shales. But this suggestion 

 introduced a train of others beginning with a doubt concerning the 

 prevailing view that active shrinkage movements since the beginning 

 of the Paleozoic were confined to three or four periods, namely, at the 

 close of the Pennsylvania, close of the Mesozoic and in the late Tertiary. 

 These are called the mountain-making periods of the first order, the 

 orogenic movements at the close of the Ordovician being rather generally 

 regarded as of lesser importance. (See also page 573.) 



