260 STRUCTURE COMMON TO ALL ROCKS. 



the Wahsatch (Plateau region) during Juro - Triassic .time, is little 

 known. But during the Cretaceous the region of the Wahsatch was the 

 western marginal bottom of the great interior Cretaceous sea (see map, 

 Fig. 755, p. 470), receiving abundant sediments from the great land-mass 

 of the Basin and Sierra region. This greatly increased the enormous 

 thickness of sediments already accumulated along this line in earlier 

 times. At the end of the Cretaceous the sediments yielded, and the 

 Wahsatch was born. It is necessary, however, to say that both the 

 Sierra and Wahsatch underwent very great changes of form produced 

 by a different process and at a much later period. We shall speak of 

 this later. 



5. Alps. — Mr. Judd has recently shown that the region of the 

 Alps, during the whole Mesozoic and Early Tertiary, was a marginal 

 sea-bottom, receiving sediments until a thickness was attained not less 

 than that of the Appalachian strata. At the end of the Eocene these 

 enormously thick sediments were crushed together with complicated 

 foldings and swollen upward to form these mountains and afterward 

 sculptured to their present forms. 



The same may be said of the Himalayas and nearly all other mount- 

 ains. We may, therefore, confidently generalize, and say that the places 

 now occupied by mountain-ranges have been, previous to their forma- 

 tion, places of great sedimentation, and therefore usually marginal 

 ocean-bottoms. In some cases, however, the deposits in interior seas or 

 mediterraneans have yielded in a similar way, giving rise to more ir- 

 regular ranges or groups of mountains. 



It is easy to see now why mountain-ranges so often form the bor- 

 ders of continents, and that continents consist essentially of interior, 

 basins with coast-chain rims. The view of formation of mountains, 

 above presented, necessitates this as a general form, while it prepares 

 us for exceptions in case of mountains formed from mediterranean 

 sediments. We see also why in the case of parallel marginal ranges of 

 the same system, such as the Sierra and Coast Ranges, these should be 

 formed successively seaward. 



Why Thick Sediments should be Lines of Yielding.— Admitting, 

 then, that mountains are formed by the squeezing together of lines of 

 very thick sediments, the question still occurs, Why does the yielding 

 take place along these lines in preference to any others ? This is a capi- 

 tal point in the theory of mountain formation. The answer is as fol- 

 lows : We have already seen (p. 222) that accumulation of sediments 

 causes the isogeotherm to rise and the interior heat of the earth to 

 invade the lower portion of the sediments with their included waters. 

 Now this invasion of heat in its turn causes hydrothermal softening or 

 even fusion, not only of the sediments, but also of the sea-floor on 

 which they rest. Thus a line of thick sediments becomes a line of 



