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PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [jSoV. 21, 



de Atacama, is composed of a trachyte or trachytic tuff, consisting of 

 a white or flesh-coloured felspathic base, in which are imbedded 

 plates of hexagonal brown or black mica, and numberless perfect 

 crystals of colourless quartz from -ith to |-ths of an inch in length 

 and having both ends terminated by perfect pyramids ; these quartz- 

 crystals can be extracted from their matrix, leaving a perfect mould 

 in the felspathic base, from which they had evidently been the first 

 mineral element to crystallize. 



The trachytic lavas of the other parts of Bolivia and Peru very 

 commonly show the quartz so crystallized ; and in Sections Nos. 1 and 

 2 such are also met with, but not so beautifully developed as from San 

 Bartolo southwards. Both at Atacama-Alta, Toconado, Sorras, 

 Tilopozo, and other places along the line of Dr. Philippi's route, the 

 specimens also showed characteristic trachytic tuffs and trachy- 

 dolerites. In some of the tuffs the quartz present was in the form 

 of rounded grains, as if due to attrition, or more resembling the effect 

 which igneous action would produce in rounding off the edges by 

 fusion, or by the solvent action of some fluid compound in the lava 

 acting on the crystals once formed. 



There is but little variety met with in the volcanic rocks of this 

 part of the world, those of Peru, Bolivia, and Chile being all very 

 similar in external appearance and mineral composition : the principal 

 rocks are trachytes, trachytic tuffs, trachydolerites, dolerites, and 

 felspathic lavas. 



The trachytes and trachytic tuffs are generally white, but occasion- 

 ally of a pale flesh- or fawn-colour, and are composed of a felspathic 

 base, probably only consisting of one felspar (frequently crystallized, 

 but also met with in a more amorphous form), colourless quartz (al- 

 ways crystallized, and often, as described, in perfect crystals), and black 

 or tombak-brown mica crystallized in small hexagonal plates, seldom 

 more than -j\,th of an inch across. From the smaller lateral fissures 

 the eruptions are generally composed of trachytes; but it is extremely 

 ditiicidt to draw a line between the true trachyte and the trachytic 

 tuffs formed from them, and which occasionally are met with as 

 solid and compact as the original trachytes themselves, and only to 

 be distinguished from the latter by the somewhat decomposed ap- 

 pearance of the felspar, the bronze-brown colour of the originally 

 black mica and the included fragments of pumice, &c. * ; they are, 

 however, in general much more open or porous in texture, and often 

 crumbly, so as frequently to be mistaken for white sandstones. They 

 are everywhere largely quarried and used as building- stone, being 

 durable and very easily worked ; when cut into hollow cones they 

 are used as filtering-stones for purifying water for domestic use, for 

 which purpose they are well adapted from their porous texture. I 

 have reason to suppose that these trachytes have frequently been 



* I had expected that the decomposition of the felspar would probably give 

 rise to the formation of alkaline or earthy carbonates in these trachytic tuffs, and 

 so afford a means of distinguishing them from trachytes ; in this I was disap- 

 pointed, as several trachytic tuffs from Tacna. Azapa. La Paz, &c, ou being 

 treated with acids did not effervesce at all. 



