Vol. 70.] THROUGH THE AXJDES OF PERU AND BOLIVIA. 11) 



cones as Sajama, the Payachatas, and possibly Tacora, it is hardly 

 possible to doubt their existence without proof to the contrary, 

 since farther north in Peru the well-known Misti (of almost 

 identical shape) possesses a finely-developed crater. 



Along the line of the Arica-La Paz section, no one of these 

 cones is at present in a state of activity, though minor phenomena 

 associated with waning volcanic action are constantly observed. 

 On the sides of Mount Taapaca, for instance, near the village of 

 Putre, hot springs still issue from the snow-covered ground, the 

 water, which is nearly at boiling-point, being conducted into baths 

 and used for curative purposes. 



The whole coastal region, as has been already mentioned, 

 consists of a rainless sandy desert formed by the disintegration 

 of volcanic products, and fresh specimens of the underlying rocks 

 <?an onlv be obtained from the cuttings made during: the construction 

 of the railway, or in the immediate vicinity of the volcanic cones. 

 The enormous amount of volcanic material which has been spread 

 over the' surrounding country may be realized from the fact that, 

 between the highest visible outcrop of Mesozoic strata in the 

 Llutah Valley and the summit of Mount Taapaca, there is a 

 difference of level amounting to over 11,000 feet. 



It was found impossible, therefore, to study these vast accumu- 

 lations of lava and tuff in great detail, and no attempt will be made 

 here to deal with them otherwise than in general outline. From 

 an examination of a number of microscopic sections, it appears 

 that they can be resolved into three main groups, characterized by 

 "their dominant ferromagnesian mineral. Succeeding one another 

 in the following order, they seem to obey a law of increasing 

 basicity with decreasing age : — ■ 



(a) Acid rhyolites and rhyolitic tuffs with bio tit e. 

 (8) Trachytes or trachy-andesites with hornblende. 

 (y) Andesites or basalts with pyroxenes. 



It must be pointed out, however, that some of the rocks of the 

 first group occasionally include xenoliths of more basic character, 

 derived from earlier-formed lavas not exposed at the surface. 



(a) The rhyolites and rhyolitic tuffs are chiefly charac- 

 teristic of the coastal region ; it is frequently impossible to say 

 whether they have been poured out as true lavas, or represent 

 pyroclastic deposits. 



They have a rough trachytic texture, and are usually pale in 

 colour, with a range varying from purple and pink to grey and 

 white. They are characterized by an abundance of small quartz- 

 crystals with bipyramidal terminations — in fact, it is scarcely 

 possible to pick up a handful of the desert sand without noticing 

 this feature. Clear glassy porphyritic crystals of felspar, both 

 sanidine and plagioclase, usually occur accompanying the quartz, 

 and biotite is the onlv common ferromagnesian element. The 

 ground-mass is frequently vitreous or cryptocrystalline, and often 

 -exhibits microspherulitic structure. 



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