8.] 



THE PHYSICS OF THE EARTH. 



235 



" The inward dip and consequent inversion traceable towards the center 

 of a mountain chain lead up to the fan-shaped structure (p. 678) where 

 the oldest rocks of a series occupy the center and overlie younger masses, 

 which plunge steeply under them. Classical examples of this structure occur 

 in the Alps (Mont Blanc, Fig. 258, St. Gothard), where crystalline rocks 

 such as granite, gneiss, and schists, the oldest masses of the chain, have 

 been ridged up into the central and highest peaks. Along these tracts, 

 denudation has been of course enormous, for the appearance of the granitic 

 rocks at the surface has been brought out, not necessarily by actual extru- 

 sion into the air, but more probably by prolonged erosion, which in these 

 higher regions, where many forms of sub-aerial waste reach their most 

 vigorous phase, has removed the vast overreaching cover of younger rocks 

 under which the crystalline nucleus doubtless lay buried." 



Fig. 13. Fan-shaped Structure, Central Alps. 



Again on page 1372, we read: 



" A mountain chain may be the result of one movement, but probably 

 in most cases is due to a long succession of such movements. Formed on 

 a line of weakness in the crust, it has again and again given relief from the 

 strain of compression by undergoing fresh crumpling and upheaval. Suc- 

 cessive stages of uplift are usually not difficult to trace. The chief guide 

 is supplied by unconformability. . . . 



" In most great mountain chains, however, the rocks have been so 

 intensely crumpled, dislocated, and inverted, that much labor may be re- 

 quired before their true relations can be determined. 



" The Alps offer an instructive example of a great mountain system 

 formed by repeated movements during a long succession of geological 

 periods. The central portions of the chain consist of gneiss, schists, granite, 

 and other crystalline rocks, partly referable to the pre-Cambrian series, but 

 some of which (Schistes lustres, Biindnerschiefer) include metamorphosed 

 Palfeozoic, Secondary, and in some places, perhaps, even older Tertiary de- 

 posits (pp. 802, 1099). It would appear that the first outlines of the Alps 

 were traced out even in pre-Cambrian times, and that after submergence, 

 and the deposit of Palaeozoic formations along their flanks, if not over 

 most of their site, they were reelevated into land. From the relations of the 



