154 _ Geological Society :— 
GEOLOGICAL SOCIETY. 
[Continued from vol. xx. p. 486.] 
November 21, 1860.—L. Horner, Esq., President, in the Chair. 
The following communication was read :— 
“On the Geology of Bolivia and Southern Peru.” By D. 
Forbes, Esq., F,R.S., F.G.S. With Notes on the Fossils by Prof. 
Huxley, F.R.S., Sec.G.S. . and J. W. Salter, Esq., F.G.S. 
After some observations on the previous researches by others, and 
on the general features of the region, the author proceeded to de- 
scribe the Post-tertiary formations of the maritime district. ‘These 
beds, containing existing species of shells, occur at various heights 
up to 40 feet above the sea-level. Guano deposits are frequent 
along the coast, and deposits of salt also in raised beaches a little 
above the sea. ‘The author could not verify Lieut. Freyer’s state- 
ment of Balani and Millepore being attached high up the side of the 
Morro de Arica, a perpendicular cliff at the water's edge ; indeed, 
from the state of old Indian tumuli along the beach, and other cir- 
cumstances, the author believes that no perceptible elevation has. 
here taken place since the Spanish Conquest, although such an alter- 
ation of level has occurred in Chile. The sand-dunes of the coast, 
and their great mobility during the hot season, were noticed. From 
Mexillones to Arica the coast is steep and rugged, formed of a chain 
of mountains, 3000 feet high, consisting of rocks of the Upper Oolitic 
age. At Arica the high land recedes, leaving a wide plain formed 
of the débris of the neighbouring mountains; and in the middle of 
this area was observed stratified volcanic tuff contemporaneous with 
the formation of the gravel. 
The saline formations were next treated of as three groups, ac- 
cording to their height above the sea-level, and were shown to be 
much more extensive than generally supposed, extending over the 
rainless regions of this coast for more than 550 miles. They are 
mostly developed, however, between latitudes 19° and 25° South. 
These salines are supposed to have originated in the evaporation of 
sea-water confined in them as lagoons by the longitudinal ranges of 
hills separating them from the ocean. ‘The nitrate of soda had, in 
the author’s opinion, resulted from the chemical reactions of sea-salt, 
carbonate of lime, and decomposing vegetable matter (both terrestrial 
and marine). The borate of lime, occurring with the nitrate, is 
connected with the yolcanic conditions of the district, and was pro- 
duced by fumaroles containing boracic acid. Where the highest 
range of salines extend bey ond the rainless region, they are much 
modified in the rainy season, and generally take the form of salt 
plains encircling salt lakes or swamps. 
The great Bolivian plateau, having an average elevation of 13,000 
or 14,000 feet above the sea, consists of great gravel plains formed 
by the spaces between the longitudinal ranges of mountains being 
filled up by the débris of these mountains. The most western 
of these consists of Oolitic débris with volcanic tuff and scorie; 
it bears the salines above-mentioned, and is nearly destitute of 
water. The central range of plains, formed from the disintegration, 
