794 



DYNAMICAL GEOLOGY. 



for planes of bedding. This horizontal jointing characterizes, as Whit- 

 ney states, the lofty granite domes of the Yosemite region ; but here 

 it is slightly curving, corresponding with the curvature of the domes. 

 A brittle, semi-crystalline limestone is often broken into small rectan- 

 gular fragments in the joint-making process ; and quartzyte has often 

 its bedding shaken out of it, and a nearly vertical bedding, or rather 

 jointing, substituted. Owing to the differences in rocks, one bed may 

 be full of them, and another, above or below, without them. 



The schistose structure, or foliation, of mica schist, gneiss, and other related rocks, 

 may sometimes correspond to slaty cleavage ; but, in general, it is due to the original 

 bedding of the materials. This is proved in numberless instances by the alternations 

 in these rocks of finer and coarser layers; that is, the gneiss often contains thin lay- 

 Fig. 1150 A. ers °f m i ca schist, or mica 



schist thin layers of gneiss, 

 in many alternations, or more 

 and less feldspathic layers of 

 these rocks alternate with one 

 another, or garnetiferous and 

 non-garnetiferous layers, and 

 so on. The annexed figure 

 illustrates a common case of 

 the kind (from Derby, Conn.), 

 Gneiss of Derby, Conn. j n w hich some of "the layers 



of gneiss are porphyritic. No such alternation in material could be occasioned by any 

 joint-making process. 



4. Distortion and Fracture of Fossils and Crystals. — These con- 

 sequences of the compression and stretching, direct and oblique, of 

 rocks, are exemplified by figures on page 99. So crystals in crys- 

 talline rocks, as well as their veins, bear evidence of movements after 

 they took their shape and size. 



5. Solidification. Metamorphism. Igneous Ejections. — These 

 common results of mountain-making movements have already been 

 considered. 



(2.) Preparation for the Event of Mountain-making. 

 1. Material for a Mountain Range Slow in Accumulation. — 

 A mountain range of the common type, like that to which the Appa- 

 lachians belong, is made out of the sedimentary formations of a long 

 preceding era ; beds that were laid down conformably, and in succes- 

 sion, until they had reached the needed thickness; beds spreading over 

 a region tens of thousands of square miles in area. An individual 

 mountain range is a very large object in nature. The region over 

 which sedimentary formations were in progress in order to make, 

 finally, the Appalachian range, reached from New York to Alabama, 

 and had a breadth of 100 to 200 miles; and the pile of horizontal 

 beds along the middle was 40,000 feet in depth. The pile for the 



