416 COSMOS. 



the shore of the Rio Puela, and probably dipping below the 

 mica-slate granite of a middling-sized grain, with light red- 

 dish feldspar, a small quantity of blackish-green mica, and a, 

 great deal of grayish-white quartz. There is no hornblende 

 nor is there any syenite. Thus it appears that the trachytes 

 of the volcano of Tungurahua, resembling those of Chimbo- 

 razo in their mineralogical condition, that is to say, consist- 

 ing of a mixture of oligoclase and augite, have here pene- 

 trated granite and mica-slate. Farther toward the south, 

 and a little to the east of the road leading from Riobamba 

 Nuevo to Guamote and Ticsan, in that part of the Cordille- 

 ras which recedes from the sea-shore, the rocks formerly called 

 primitive, mica-slate, and gneiss, make their appearance every 

 where, toward the foot of the colossal Altar de los Collanes, 

 the Cuvillan, and the Paramo del Hatillo. Previous to the 

 arrival of the Spaniards, even before the dominions of the 

 Incas extended so far to the north, the natives are said to 

 have worked metalliferous beds in the neighborhood of the 

 volcanoes. A little to the south of San Luis numerous dikes 

 of quartz are observed running . through the greenish clay- 

 slate. At Guamote, at the entrance to the grassy plain of 

 Tiocaxa, we found large masses of rock, consisting of quartz- 

 ites very poor in mica, of a distinct linear parallel structure, 

 running regularly at an angle of 70 degrees to the north. 

 Farther to the south, at Ticsan, not far from Alausi, the 

 Cerro Cuello de Ticsan shows large masses of sulphur im- 

 bedded in a layer of quartz, subordinate to the neighboring 

 mica-slates. So great a diffusion of quartz in the neighbor- 

 hood of trachytic volcanoes appears at .first sight somewhat 

 strange. The observations which I made, however, of the 

 overlying, or rather of the breaking forth of trachyte from 

 mica-slate and granite at the foot of the Tungurahua (a phe- 

 nomenon which is as rare in the Cordilleras as in Auvergne), 

 have been confirmed, after an interval of forty-seven years, 

 by the admirable investigations of the French geologist Se- 

 bastian Wisse at the Sangay. 



That colossal volcano, 1343 feet higher than Mont Blanc, 

 entirely destitute of lava streams (which Charles Deville de- 

 clares are also wanting in the equally active Stromboli), but 

 ejecting uninterruptedly, at least since the year 1728, a black, 

 and frequently brightly glowing rock, forms a trachytic isl- 

 and of scarcely eight geographical miles in diameter,* in the 



* Sebastian Wisse, Exploration du Volcan de Sangay, in the Comptes 

 rendus de VAcad. des Sciences, t. xxxvi., 1853, p. 721 ; comp. also above, 

 TV 239. 



