Mr. D. Forbes on the Geology of Bolivia and Southern Peru. 155 
of red sandstones and marls, with some volcanic scoriz, is well 
watered. ‘The third range consists of plains made up of the débris 
of Silurian and granitic rocks, and is auriferous. The thickness 
of this accumulation of clays, gravel, shingle, and boulders is, 
at places, immense. At La Paz it is more than 1600 feet. Con- 
temporaneous trachytic tuff was found also in these deposits. In 
freshwater ponds on this plateau, at a height of 14,000 feet (lat. 
15° 8.), Mr. Forbes found abundance of Cyclas Chilensis, formerly 
considered to be peculiar to the most southern and coldest part of 
Chile at the level of the sea (lat. 45° to 50° S.). 
_ The volcanic formations were next noticed. Volcanic action has 
continued certainly from the pleistocene age to the present. The. 
line of volcanic phenomena is nearly continuous N. and S$. Cones 
are frequent, some of them 22,000 feet high and upwards; but 
craters are rare. Volcanic matter, both in ancient times and at 
present, has in a great part been erupted from lateral vents, often of 
great longitudinal extent; recent trachytic lavas from such orifices 
have covered in some cases more than 100 miles of country. Be- 
sides trachyte, there are great tracts of trachydoleritic and felspathic 
lavas. On the whole, in these South American lavas silex abounds, 
and it las been the first element in the rock to crystallize; whereas 
apparently in granite quartz is the last to crystallize and form the 
state of so-called ‘‘ surfusion.” Diorites (including the so-called 
«« Andesite”) occur in force along two parallel N. and S. lines of 
eruption in this region, reaching through Chile, Bolivia, and Peru, 
for more than 40 degrees of latitude. These diorites, and more espe- 
cially the rocks which they traverse, are metalliferous; and the 
author looks upon the greater part of the copper, silver, iron, and 
other metallic veins of these countries as directly occasioned by the 
appearance of this rock. | 
Shales and argillaceous limestones, with clay-stones, porphyry- 
tuffs, and porphyries, form the mass of the Upper Oolite formation 
of Bolivia, equivalent to Darwin’s Cretaceo-Oolitic Series of Chile, 
At Cobija these are traversed in all directions by metallic veins, 
chiefly copper, and which, as before mentioned, appear to emanate 
from the diorite. : 
Red and variegated maris and sandstones, with gypsum and cu- 
riferous and yellow sandstones and conglomerates, come next in 
order; they have a thickness of 6000 feet, and are much folded and 
dislocated. ‘These are considered by the author to resemble closely 
the Permian rocks of Russia. Fossil wood is not uncommon in some 
of these strata, which extend for at least 500 miles N. and 8. 
Carboniferous strata occur chiefly as a small, contorted, basin- 
shaped series of limestones, sandstones, and shales, with abundant 
characteristic fossils. Cle 
The quartzites which are generally supposed to represent the De- 
vonian formation in Bolivia, but which the author is rather disposed 
to group as Upper Silurian, are really not of very great thickness, 
but are very much folded, and perhaps are about 5000 feet thick. ~ 
The Silurian rocks (perhaps 15,000 feet thick) are well developed 
over an area of from 80,000 to 100,000 miles of mountain coun- 
bd * 
