822 



DYNAMICAL GEOLOGY. 



of the Triassico-Jurassic sandstone (p. 403) were areas of subsidence, 

 or sinking troughs, and sedimentary accumulations in progress in each 

 trough ; and the geosynclinal, in each case, ended in catastrophe, as 

 exhibited in upturned or displaced rocks, and in lines of extensive 

 igneous ejections. The progress was that of a synclinorium, although 

 no true mountain range was made. 



Whether the fusion of the liquid rocks of volcanoes and of igneous 

 ejections is to be included among the effects of the lateral pressure 

 which has made mountains, or not, is doubtful. It seems hardly 

 possible that the great lines of igneous eruption and of lofty volcanic 

 cones over the globe could have originated from fusion of material in 

 the supercrust. A series of fractures and outflows, but a thousand 

 miles long, like that of the Mesozoic era on the Atlantic border of 

 North America, is long compared with the probable thickness of the 

 earth's crust. It is not possible that the fusion of just the same kind 

 of rock could have been produced by friction within the variously 

 composed beds of the thin supercrust along so extended a line, es- 

 pecially from such feeble movements as are indicated by the slightly 

 upturned Mesozoic beds. But the small upturnings and fracturing of 

 these beds were a consequence of fracturings of the earth's crust 

 beneath ; and, as Prof. E. W. Hilgard has said, there may have 

 been, in such cases, friction enough (along these deep fissures) to 

 cause an extensive fusion of the lower portion of the crust where the 

 temperature was near the melting point; and uniformity of constitu- 

 tion in the ejected material would be a natural consequence of such 

 conditions. This method is but a step removed from that which de- 

 rives the material from liquid rock below. 



The strongest argument in favor of friction in the supercrust as a source of volcanic 

 heat and fusion (or of the part not received from lower depths in the crust) is derived 

 from the resemblance of many trachytes to ordinary granites, and of dolerytes to the 

 Archsean norytes or hypersthenytes. But the material of the granites and norytes 

 was derived*briginally from the fused material of the globe; and trachytes and dol- 

 erytes have flowed out from the largest craters of volcanic lines, as well as from local 

 vents. It is hence possible that all trachyte and doleryte may have had the deeper 

 source, while not impossible that part may be a result of fusion through friction in the 

 supercrust. The fact that granitic material has been put into the pasty state by the 

 heat of metamorphism is strong reason for admitting that the freer fusion needed to 

 make trachyte may have come from like conditions. 



The final shaping of the range is carried on afterward through 

 erosion, which usually carves valleys out of anticlinals and ridges out 

 of synclinals (p. 652) ; and the shallower synclinals of greatest breadth 

 and depth, other things being equal, often become ultimately the 

 highest of the mountain ridges, because more material is embraced in 

 them. Some examples of this exist in Graylock and Mount Wash- 

 ington, and other high summits of the Taconic Range of western 



