59 2 Summary of the Structure paet u. 



strata have been abruptly broken and upturned. When 

 viewing the Cordillera, before having read Mr. Hopkins's 

 profound ' Researches on Physical Geology,' the convic- 

 tion was impressed on me, that the angular dislocations, 

 however violent, were quite subordinate in importance 

 to the great upward movement in mass, and that they 

 had been caused by the edges of the wide fissures, 

 which necessarily resulted from the tension of the 

 elevated area, having yielded to the inward rush of 

 fluidified rock, and having thus been upturned. 



The ridges formed by the angularly upheaved strata 

 are seldom of great length : in the central parts of the 

 Cordillera they are generally parallel to each other, 

 and run in north and south lines; but towards the 

 flanks they often extend more or less obliquely. The 

 angular displacement has been much more violent in 

 the central than in the exterior main lines ; but it has 

 likewise been violent in some of the rumor lines on the 

 extreme flanks. The violence has been very unequal 

 on the same short lines ; the crust having apparently 

 tended to yield on certain points along the lines of 

 fissures. These points, I have endeavoured to show, 

 were probably first foci of eruption, and afterwards of 

 injected masses of porphyry and andesite. 1 The close 

 similarity of the andesitic granites and porphyries, 

 throughout Chile, Tierra del Fuego, and even in Peru, 

 is very remarkable. The prevalence of feldspar cleav- 

 ing like aibite, is common not only to the andesites, 

 but (as I infer from the high authority of Prof. G-. 

 Rose, as well as from my own measurements) to the 

 various clay-stone and green-stone porphyries, and to 



1 Sir It. Murchison, and his companions state (' Geolog. Proa' 

 vol. iii. p. 717), that no true granite appears in the higher Ural 

 Mountains ; but that syenitic greenstone — a rock closely analogous 

 to our andesite —is by far the most abundant of the intrusive masses. 



