270 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



for the Coastal plain, if we imagine the direction of the drainage and 



position of the sea to be reversed, i. e., on the west instead of on the 

 east. 



At or near the end of the Carboniferous period, when the accumula- 

 tions of sediments must have transformed the deep Appalachian 

 trough into shallow marshes or estuaries, the vast thickness of strata 

 were folded into the remarkably regular series of wrinkles which give 

 to this mountain range at present its peenliar character. A cross-sec- 

 tion of the Appalachians is nowhere symmetrical. On the contrary, 

 it presents only one side or half of a symmetrical range, for if is com- 

 posed of anticlinal and synclinal folds, all more or less overturned 

 toward the west and also becoming steadily more and more abrupt 

 toward the east. Along the section cat through the Appalachians by 

 the Potomac River for instance, which the line of the railway follows 

 very closely, one finds on the west side, between Cumberland and Graf- 

 ton (in Garrett Comity, Md., and in West Virginia), only the low, 

 flat folds of upper Paleozoic strata, inclosing the nearly horizontal 

 coal beds. Further east these gently sloping basins are replaced by 

 others whose sides are steeper and which also display older rocks. It 

 is then observed that each anticlinal has its western side more steeply 

 inclined than its eastern, and still farther east both sides may dip 

 toward the ocean so as to make the fold overturned. Last of all, the 

 fold may become too sharp for the strength of the materials to stand, 

 when the flexure 1 becomes a thrust with the same general dip and strike. 

 This latter may be seen in the isolated sandstone mountain, " Sugarloaf," 

 40 miles west of Washing-ton. The perfect regularity, with which 

 the folds become more and more abrupt toward the east, is at some 

 points interrupted. On this section it is notably the case at Cumber- 

 land and Hancock, where a much sharper fold than the surrounding 

 Structure would lead one to expect discloses much lower horizons than 

 are to be seen in the adjoining ridges (see the two sections beyond, figs. 

 Sand 6). Nevertheless, the regularity is so great as to have led so 

 good a geologist as Rogers to the idea that the wave-like folds of the 

 Appalachian system had been actually produced by undulations in a 

 flexible crust due to horizontal pulsations or waves in the earth's liquid 

 interior. 10 



Eruptive rocks in the gentler folds of the Appalachian system are 

 noticeably absent. Along this section they are only to be found at all 

 in the eastern portion, where the folds become abrupt or are replaced 

 by faults and thrusts. Thus near Harpers Perry and W'everton some 

 ancient dikes occur, and the Line Ridge sandstone (Lower Cambrian) is 

 underlain somewhat farther north by large masses of ancient quartz- 

 porphyries and rhyolites. .More basic greenstones also occur in this 

 region.* 



Am. .lour. Soi., .'id scries. Vol. xi.i\ . Dec lsirj, p. 482. 



