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 western part of the Andes. In this sea, in which 
normal sediments were being deposited, immense eruptions of basic 
voleanic 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 diorites 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 typical. 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. 
Tn the sixth zone granite 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, with 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 found as yet, and they accordingly 
belong to the series of Mesozoic sediments. 
AGE OF THE CoRDILLERAS AND DEVELOPMENT OF 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 
