GEOLOGY OF PERU ADAMS. 423 



7. Zone of slates (Silurian) and granites. 



Comparing these zones with those enumerated by Steinmann, later 

 to be mentioned, it will be seen that there are no granitic rocks in 

 the coast and that the coast range which extends from Arica south- 

 ward into Chile is not comj^arable with the coast range at Mollendo. 

 In fact, there is a gap between the two just north of Arica. In other 

 respects the zones are quite comparable excepting for the difference 

 due to the structure of the Titicaca basin. The rocks which Forbes 

 called " Permian " or " Triassic " are now called " Cretaceous " by 

 Steinmann, and above are included with the Mesozoic. 



SECTION THROUGH THE DEPARTMENT OF ANCACHS, BY RAIMONDI (1873). 



It should be remembered in considering this region that the Cor- 

 dillera Occidental divides into two branches, the western known as 

 the " Cordillera Xegra " and the eastern or principal one, the " Cor- 

 dillera Blanca." Raimondi made no section, but from his writings 

 one may recognize the following zones : 



1. Granites and syenites of the coast. 



2. Mesozoic sediments with porphyries and diorites. The sedi- 

 mentaries are rare in the coast but are found more abundantly inland. 



3. The diorites are seen in the Cordillera Xegi'a and the Cordillera 

 Blanca up to the limit of snow, but not in the crest of the range or 

 axis. The eruption of the diorites posterior to the Jurassic removed 

 and lifted some formations of the Cretaceous and introduced metallic 

 veins. 



4. Trachytes anterior to the present, there now being no volcanoes. 

 These rocks are present in the Cordillera Blanca and to some extent 

 in the Cordillera Negra but not forming peaks in the latter. Rai- 

 mondi thinks the eruption of the trachytes occurred at a time when 

 the two Cordilleras formed one mass and that they have since been 

 separated by erosion. 



5. In the valley of the Maranon are found older sediments, talcose 

 slates with quartz veins which are referred to the Silurian. A small 

 area of similar rocks was also noted at Pallasca on the western slope 

 of the Cordillera Nevada. 



SECTION OF ECUADOR, BY WOLF (1892). 



Reviewing the geologj^ of Ecuador as outlined by Wolf and co- 

 ordinating the data in such a way as to compare it with the sections 

 already given of Peru we find the following more or less distinct 

 zones : 



1. The Tertiary and Quaternary formations of the coast of marine 

 origin. 



88292— SM 1908 28 



