32 



THE GEOLOGIST. 



could not verify Lieut. Freyer's statement of Balani and Millepora 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 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 

 during the hot season, were noticed. From Mexillones to Arica the coast is 

 steep and rugged, 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 wide 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 

 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 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 opinion, resulted from the 

 chemical reactions of sea-salt, carbonate of lime, and decomposed vegetable 

 matter (both terrestrial and marine). The borate of lime, occurring with the 

 nitrate, is connected with the volcanic conditions of the district, and was pro- 

 duced by fumaroles containing boracic acid. Where the highest range of 

 salines 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 scoriae ; 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. 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. forty-five degrees 

 to fifty degrees south). 



The volcanic formations were next noticed. Volcanic action has continued 

 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 fron] such orifices have covered in some cases more than one hundred 

 mili-sol' country. Besides trachyte, there are great tracts of trachydoleritic 

 and felspathic lavas. On the whole, in these South American lavas silex 

 abounds, and it lias 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." Dioritcs (including the so-called "Andesite") occur in 



