32 
THE GEOLOGIST. 
could not verify Lieut. Freyer's statement of Balani and Millepor^B being 
attached lii^h up the side of the Morro de Aiica, a perpendicular cliff at the 
water's cdire ; indeed, from the state of old Indian tumuli along the beach, and 
otlier circumstances, the author believes that no perceptible elevation has here 
taken place since the Spanish Conquest, although such an alteration of level 
has occurred in Chile. The sand-dunes of the coast, and their great mobility 
duriii"- the hot season, were noticed. Prom Mexillones to Arica the coast is 
steep and ruffged, formed of a chain of mountains, three thousand feet high, 
consisting of rocks of the Upper Oolitic age. At Arica the high land recedes, 
leaving a\ide plain formed of the debris 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 treated of as three groups, according to their 
hei^-ht 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 five hundred and fifty miles. They are mostly developed, however, 
between latitudes nineteen degrees and twenty-five degrees south. These 
salines are supposed to have originated in the evaporation of sea-water con- 
fined in them as lagoons by the original ranges of hills separating them from 
the ocean. The nitrate of soda had, in the author's opmion, resulted from the 
chemical reactions of sea-salt, carbonate of lime, and decomposed vegetable 
matter (both terrestrial and marine). The borate of lime, occui-ring with the 
nitrate, is comiected with the volcanic conditions of the district, and was pro- 
duced by fumaroles containing boracic acid. Where the highest range of 
saUnes extend beyond 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 thirteen thousand 
or fourteen thousand feet above the sea, consists of great gravel plains formed 
by the spaces between the longitudinal ranges of mountains being filled up by 
the debris of these mountains. The most western of these consists of Oolitic 
debris with volcanic tuff and scoritE ; it bears the salines above-mentioned, and 
is nearly destitute of water. The central range of plains, formed from the dis- 
integration of red sandstones and marls, with some volcanic scoriae, is well 
watered. The third range consists of plains made up of the debris of Silurian 
and granitic rocks, and is auriferous. The thickness of this accumulation of 
clays, gravel, shingle, and boulders is immense at places. At La Pas it is 
more than one thousand six hundred feet. Contemporaneous trachytic tuff 
was found also in these deposits. In fresh-water ponds on this plateau, at a 
height of fourteen thousand feet (lat. fifteen degrees south), Mr. Porbes found 
abundance of Ci/clas Chileiisis, formerly considered to be peculiar to the most 
southern and coldest part of Chile at the level of the sea (lat. forty-five degrees 
to fifty degrees south). 
The volcanic formations were next noticed. Yolcanic action has contiimed 
certainly from the pleistocene age to the present. The line of volcanic phe- 
nomena is nearly continuous north and south. Cones are frequent, some of 
them twenty-two thousand feet high and upwards ; but craters are rare. Vol- 
canic 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 one hundred 
miles of country. Besides trachyte, there are great tracts of trachydoleritic 
and felspathic lavas. On the whole, in these South American lavas silex 
abounds, and it has been the first element in the rock to crystalhze ; whereas 
apimrently in granite quartz is the last to crystallize and form the state of so- 
called " sui-fusion." Diorites (including the so-called "Andesite") occur- in 
