TRUE VOLCANOES. 433 



augites there occur also separate blackish-green crystals of 

 uralite, of from half a line to five lines in length, with a per- 

 fect augite form and the cleavage of hornblende (see Rose, 

 Reise nach clem Ural, bd. ii., s. 353)." I brought a similar 

 fragment, with distinct uralite crystals, from the slope of the 

 Tunguragua at an elevation of 13,260 feet. Gustav Rose con- 

 siders this specimen strikingly different from the seven frag- 

 ments of trachyte from the same volcano which are contained 

 in my cabinet. It recalls to mind the formation of green- 

 slate (schistose augitic porphyry) which we have found so 

 diffused on the Asiatic side of the Ural (Ibid., s. 544). 



Fifth Division. — "A mixture of Labradorite* and au- 



say, at a height differing by only 40 feet from that given by my trig- 

 onometrical measurement at Riobamba Nuevo, in the elevated plain 

 of Tapia, in June, 1803, as the height of the summit of Chimborazo 

 — namely, 21,426 feet. This correspondence of a trigonometrical 

 measurement of the summit with one founded on the boiling point is 

 the more surprising as my trigonometrical measurement, like all 

 measurements of mountains in the Cordilleras, involves a barometrical 

 portion ; and from the want of corresponding observations on the 

 shore of the South Sea, my barometrical determination of the height 

 of the Llano de Tapia, 9484 feet, can not possess all the exactness 

 that could be desired. (For the details of my trigonometrical meas- 

 urement, see my Rcciicil d Observations Astron., vol. i., p. 72 and 74). 

 Professor Poggendorff kindly undertook to ascertain what result, un- 

 der the most probable hypotheses, a rntional mode of calculation would 

 produce. He found, reckoning under both hypotheses, that, the pre- 

 vailing temperature of the atmosphere at the sea being 81°'5 F., or 

 79 0, 7 F., and the barometer marking 29*922 inches, with the thermom- 

 eter at the freezing point, the following result is obtained by Reg- 

 nault's table : the boiling point at the summit at 171°*5 F. answers to 

 12*677 inches of the barometer at 32° temperature; the temperature 

 of the air may therefore be taken at 35 c, 3 F. =34 c, 7 F. According to 

 these data, Ohmann's tables give, for the height ascended, under the 

 first hvpothesis (81°-5),— -7328 m -2, or 24,043 feet; and under the sec- 

 ond (79°.7), = 73L4 m -5, or 23,998 English feet, showing an average of 

 777 m , or 2549 English feet more than my barometrical measurement. 

 To have corresponded with this, the boiling point should have been 

 found about 2~-25 cent, higher, if the summit of Chimborazo had act- 

 ually been reached. (Poggendorff s Annalen, bd. c, 1857, s. 479.) 



* That the trachytic rocks of ./Etna contain Labradorite was demon- 

 strated by Gustav Rose in 1833, when he exhibited to his friends the 

 rich Sicilian collections of Friedrich Hoffmann in the Berlin Minera- 

 logical cabinet. In his treatise on the minerals known by the names 

 of green-stone and green-stoue porphyry (Poggend., AnnaL, bd. 

 xxxiv., 1835, p. 29), Gustav Rose mentions the lavas of ^Etna, which 

 contain augite and Labradorite (compare Abich, in his interesting 

 treatise on the whole feldspathic family, Poggend., AnnaL, 1840, bd. 

 1., s. 347). Leopold von Buch describes the rock of iEtna as analo- 

 gous to the dolerite of the basalt formation (Poggend., AnnaL, bd. 

 xxxvii., 1836, s. 188). 



Vol. V.— T 



