GEOLOGY 209 



or no intrusion takes place in the marginal overthrust portions 

 of a thin-shelled range, but granitic masses are characteristic 

 features of the cores of the deformed belts. Plutonic masses 

 are also common in thick-shelled mountains. It might per- 

 haps be added that plutonic masses in thin-shelled mountains 

 tend to be fohated, having been intruded concomitantly with 

 the folding ; whereas in thick-shelled mountains the bathohths 

 generally have cross-cutting relations and ordinary granitic 

 textures. 



These generalisations can be advantageously extended to 

 many other present and past mountain ranges all over the earth. 

 Quite similar views have been developed by Dr. J. A. Douglas 

 in his recent work on the Andes of Peru and Bolivia {Quart. 

 Journ. Geol. Soc, 76, pt. i, 1920, 1-61 ; 77, pt. 3, 1921, 246-84). 

 He shows that in the Peruvian Cordilleras great zones of over- 

 thrusting are entirely wanting, and inverted folds are the excep- 

 tion rather than the rule. He notes the complete dissimilarity 

 of these Andean structures to those of the Alps, a typical thin- 

 shelled range. His observations lead to the view that the 

 folded chains of the Andes are the results of intermittent com- 

 pression of a series of transgressive deposits laid down in a 

 geosyncline between two ancient resistant masses, represented 

 on the east by the metamorphic and plutonic rocks of the 

 Amazon region, and on the west by the crystalline rocks of the 

 coastal Cordillera. " In the Alpine type of folding vertical 

 uplift has been overshadowed by movement in a horizontal 

 direction, whereas in the Andean Cordilleras the reverse is the 

 case, and the terms ' backland ' and * foreland,' as applied to 

 the direction of movement, have no longer the same 

 significance." 



In W. Penck's elaborate monograph on the Argentinian 

 Andes (" Der vSiidrand der Puna de Atacama, N.W. Argentinien, 

 Ein Beitrag zur Kenntnis des Andinen Gebirgstypus und zu 

 der Frage der Gebirgsbildung." Abk. Sachs. Akad. Wiss., 

 Math.-Nat. Kl., 37, 1920, 420 pp.) the descriptions and the 

 profile sections corroborate the views expressed by Chamberlin 

 and Douglas as to Cordilleran tectonics. In the region described 

 open folding and enormous igneous action, both intrusive and 

 extrusive, is the rule, although close folding and overthrusting 

 do occur in small, strictly localised areas on the eastern side of 

 the main range. Penck traces the origin of the crustal move- 

 ments to the entry of magmas beneath the mountain blocks. 

 Minor folding in the upbowed regions is due to secondary 

 intrusion in the strained portions of the crust. 



H. Gerth has published a very useful elucidatory and critical 

 summary of Penck's work {Geol. Rimdschait., 12, 1922,320-40). 

 He, however, does not believe that the orogeny in general and 



