THE CARBONIFEROUS PERIODS 479 



In the former, it is metamorphosed to anthracite, while in the latter 

 it is bituminous. The effect of lateral pressure on competent strata 

 (p. 257), such as quartzites and limestones, and on incompetent, such 

 as shales, is well shown. Where the former were thick the strata 

 were either thrown into great folds or when broken were thrust 

 over the adjacent rocks. The effect on shales is in marked contrast, 

 for they were crumpled into minute folds and crushed. 



Not only were the sediments of the Appalachian trough folded, but 

 the rocks of the continent of Appalachia were also deformed as they 

 had indeed been a number of times before. As a result, those por- 

 tions of this old land which are at present exposed at the surface 

 are extremely complex. 



Although the Permian was the period during which the principal 

 folding occurred, some deformation had previously taken place. In 

 the Ordovician folding occurred, and in the Middle Devonian moun- 

 tains were formed in Maine, New Brunswick, and Nova Scotia. " The 

 Appalachian revolution began in the Middle Devonian, the first 

 mountain bulwarks being thrown upon the eastern side of the Appala- 

 chian system and to the north. " 1 



Age of the Deformation. — The time at which the Appalachian def- 

 ormation took place is known from the usual evidence. The young- 

 est rocks which are infolded are Pennsylvanian. Upon the upturned 

 edges of these the Triassic rocks rest unconformably in certain 

 places. Since the Permian strata are absent in eastern Pennsylvania, 

 it is probable that the deformation took place during the latter and 

 that it continued into the Triassic, since the oldest rocks of that period 

 seem to be everywhere lacking. 



Other Continents. — The western half of Europe was part of a 

 large continent that extended from Russia far into the Atlantic. 

 In the southern part of this continent lakes and swampy depressions . 

 existed in the Lower Permian, in which rank vegetation grew and large 

 and small amphibians and primitive reptiles dwelt. These swampy 

 areas were drained by an elevation in the Upper Permian which con- 

 verted them into broad plains separated by hills and mountains. 

 The climatic effect of these changes was marked, some portions of 

 the region becoming very humid and others, from which the moist 

 winds were shut off by the mountains, becoming arid. Volcanic 

 activity was prevalent during a portion of the period. The epoch 



1 Barrell, J., — The Upper Devonian Delta of the Appalachian Geosyncline: Am. Jour. Sci. 

 Vol. 37, 1914, PP- 225-253. 



CLELAND GEOL. — 3 1 



