446 COSMOS. 



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 imbedded in a layer of quartz, 

 subordinate to the neighbouring mica-slate. So great a diffu- 

 sion of quartz in the neighbourhood 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 

 loot of the Tungurahua (a phenomenon as rare in the Cor- 

 dilleras as frequent in Auvergne) have been confirmed, after 

 an interval of 47 years, by the admirable investigations of 

 the French Geologist Sebastian 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 trachy tic- 

 island of scarcely 8 geographical miles in diameter 62 in the 

 midst of beds of granite and gneiss. A totally opposite 

 condition of stratification is exhibited in the volcanic district 



62 S^bastien Wisse, Exploration du Volcan de Sangay, in the Comptes 

 rendus de I'Acad. des Sciences, t. xxxvi, 1853, p. 721 ; comp. also above, 

 p. 251. 



According to Bo ussingault, the ejected fragments of trachy te brought 

 home by Wisse and collected on the upper descent of the cone (the 

 traveller reached an elevation of 960 feet below the summit, which is 

 itself 485 feet in diameter), consist of a black, pitch-like fundamental 

 mass, in which are imbedded crystals of glassy (?) felspar. It is a very 

 remarkable phenomenon, and one which up to the present time seems 

 to stand alone in the history of volcanic ejections that, along with these 

 large black pieces of trachyte, small sharp-edged fragments of pure 

 quartz are thrown out. According to a letter from my friend Boussin- 

 gault, dated January 1851, these fragments are no longer than 4 cubic 

 centimetres in bulk. No quartz is found disseminated in the trachytic 

 mass itself. All the volcanic trachytes which I have examined in the 

 Cordilleras of South America and Mexico, and even the trachytic por- 

 phyries in which the rich silver veins of Real del Monte, Moran and 

 Regla are contained, to the north of the elevated valley of Mexico, are 

 entirely destitute of quartz. Notwithstanding this seeming antagonism, 

 however, of quartz and trachyte in still-active volcanoes, I am by no 

 means inclined to deny the volcanic origin of the " trachytes et porphy- 

 res meulieres (mill-stone trachytes)" to which Beudant first drew atten- 

 tion. The mode, however, in which these are formed, being erupted 

 frcrn fissures, is entirely different from the formation of the conical and 

 dome-like trachyte structures. 



