XII 



The Crust of the Earth 



231 



FIG. 46. Production of thrust-planes. 

 The strata represented are layers of 

 clay and sand separated by cloth ; 

 they were laid down horizontally, 

 and ridged into the position shown 

 by a thrust acting in the direction 

 of the arrow. 



synclines, cracked, and allowed one part to be lifted up and 

 thrust bodily over the other, in certain cases for a distance 

 of ten miles or more. The 

 consequent crushing, faulting, 

 and folding produced a very 

 confused arrangement of the 

 rocks, and extensive meta- 

 morphism. The structure of 

 the region was extremely 

 puzzling until Messrs. Peach 

 and Home traced out the 

 thrust-planes along which the 

 sliding movement took place. 

 Figure 46 represents the pro- 

 duction of thrust-planes, A, 

 in a series of experiments on mountain structure recently 

 carried out by Mr. H. M. Cadell. 4 



303. Mountains of Elevation. When lateral com 

 pression of the Earth's crust takes place the strata pucker 

 up along the line where they are weakest, and are thrown 

 into a series of anticlines and synclines growing sharper 

 and higher toward the central line. The rocks in the 

 interior of the mass and those occupying the hollows of the 

 synclines are necessarily compressed, heated, and altered, 

 while those on the outer curve of the anticlines are stretched 

 and split in the process. A mountain range is formed in 

 this way, with anticlines as ridges and synclines as longi- 

 tudinal valleys between them, the slopes of the surface 

 corresponding to the dip of the strata. The true mountain 

 ranges of the world are all of this character, the Alps, 

 Himalayas, and Andes being, typical examples, and it is 

 significant that all such ranges are situated near the edge 

 of great depressions, the subsidence of which probably 

 accounts for their uplifting. Rocks of recent sedimentary 

 origin always form the first gentle undulations on the slope 

 of a mountain range, but toward the main ridge the strata 

 are of greater age and more contorted, while in the centre 

 there are masses of schistose or igneous rocks, probably pro- 

 duced either by the rolling and compression of the uplifted 



