TRUE VOLCANOES. 457 



collection is indebted to Herr Mollhausen, the draughtsman 

 and topographist of Lieut. Whipple's exploring expedition, 

 the third division, or that of the dioritic Toluca-trachytes, 

 also includes those of Mount Taylor, between Santa Fe del 

 Nuevo Mexico and Albuquerque, as well as those of Ciene- 

 guilla on the western slope of the Rocky Mountains, where, 

 according to the able observations of Jules Marcou, black lava- 

 streams overflow the Jura-formation." The same mixture of 

 oligoclase and hornblende which I saw in the Azteck high- 

 lands, in Anahuac proper, but not in the Cordilleras of South 

 America, are also found far to the west of the Rocky Moun- 

 tains and of Zuni, near the Mohave river, a tributary of the Rio 

 Colorado (see Marcou, Resume of a geological reconnaissance 

 from the Arkansas to California, July, 1854, pp. 46 48. See 

 also two important French treatises, Resume explicatif d* une 

 Carte Geologique des Etats-Unis, 1855, pp. 113 116, and Ex- 

 quisse d une Classification des Chames de J&Lontaqnes de T A.me- 

 rique du Nord, 1855 ; Sierra de S. Francisco et Mount Taylor, 

 p. 23). Among the trachytes cf Java, for specimer of which 

 I am indebted to my friend Dr. Junghuhn, we have likewise 

 recognised those of the third division in three volcanic dis- 

 tricts, namely, Burung-agung, Tyinas and Gurung Parang 

 (in the Batugangi district). 



Fourth division. " The leading mass contains augite with 

 oligoclase : the Peak of Teixcrilie, 78 the Mexican volcanoes 



" 8 The felspar in the trachytes of Teneriffe was first recognised in 

 1842 by Charles Deville, who visited the Canaiy Islands in the autumn 

 of that year ; see that distinguished geologist's Voyage Geologique aux 

 Antilles et aux lies de Teneriffe et de Fogo, 1848, pp. 14, 74, and 169 ; 

 also Analyse du Feldspath de Te'neriffe, in the Comptes rendus deVAcad. 

 des Sciences, t. xix, 1844, p. 46. " The labours of Messrs. Gustav Rose 

 and H. Abich," he says, " have contributed in no small degree, both 

 crystallographically and chemically, to throw light on the numerous 

 varieties of minerals which were comprised under the vague denomina- 

 tion of felspar. I have succeeded in submitting to analysis carefully 

 isolated crystals whose density in different specimens was very uni- 

 formly 2-593, 2-594, and 2'586. This is the first time that the oligo- 

 clase felspar has been indicated in volcanic regions, with the excep- 

 tion perhaps of some of the great masses of the Cordillera of the 

 Andes. It was not detected, at least with any certainty, except in the 

 ancient eruptive rocks (plutouic, granite, syenite, syenitic porphyry 

 ), but in the trachytes of the Peak of Teneriffe it plays a 

 part analogous to that of the labrador in the doleritic masses of 



