GEOLOGY OF PERU ADAMS. 425 



In order to make clear the aspects under which the porphyries pre- 

 sent themselves, the following explanation is offered: From the 

 close of the Triassic, during Jurassic and Cretaceous time, a shallow 

 sea with a gradually sinking bottom occupied the region which to-day 

 constitutes the w^estern part of the Andes. In this sea, in which 

 normal sediments Avere being deposited, immense eruptions of basic 

 volcanic rocks occurred, taking the forms of flows, conglomerates, 

 breccias, sandstones, and stratified tuffs. 



The third, or diorite, zone is found on the western slope of the 

 Cordillera Occidental. The cliorites are clearly younger than the 

 Cretaceous sedimentaries, since they have cut and metamorphosed 

 them. The normal diorite contains dikes and masses of a darker, 

 more basic, and finer-grained diorite. The Mesozoic rocks which 

 occupied this zone have nearly all disappeared. 



The fourth zone includes the crest and eastern slope of the Cordil- 

 lera Occidental. Here the porphyritic facies in the Mesozoic rocks 

 is tj'pical. The formations, Jurassic and Cretaceous, are strongly 

 folded, and the inclination of the beds is more frequently to the west 

 than to the east. In this region andesitic eruptions abound (for the 

 most part quartzitic) and extend eastward into the next zone. The 

 mineral deposits of the region are related to these andesites. 



The fifth zone in the calcareous formations gradually replaces the 

 porphyritic facies until it becomes a great limestone formation, 

 which, from the fossils, is shown to be of Jurassic and Cretaceous age. 



In the sixth zone gTanite and slate are found. Although no fossils 

 have been found in the slates, they are considered to be Silurian be- 

 cause of their resemblance to the known zone of Silurian in southern 

 Peru and Bolivia. 



Below the Mesozoic sediments there is a series of dark siliceous 

 slates and sandstones, wdth some conglomerates, which are believed 

 to be Paleozoic and especially Carboniferous, the existence of Car- 

 boniferous in the region being proven by finding a few character- 

 istic fossils. Inasmuch as the Permian is not present in the Cordil- 

 lera of Peru, the red sandstones and shales, with salt and gypsum, 

 which overly the Silurian quartzites and slates, are referred to the 

 lower Lias, no fossils having been fomid as yet, and they accordingly 

 belong to the series of Mesozoic sediments. 



Age or the Cordilleras and Development or the South American 



Continent. 



In the atlas accompanying d'Orbigny's monograph there is a map 

 of South America showing the general distribution of the geologic 

 formations according to his ideas. The map is very conventional and 

 is of little value to-day. The most noticeable error as regards Peru is 



