NOTES. civil 



and lastly, the rock of the Esterel Mountains, in the Depart, du Var (Elie de 

 Beaumont, Explic. de la Carte ge'ol. de France, t. i. p. 473). 



C 602 ) p. 432. The felspar in the trachytes of TenerifFe was first recognised 

 in 1842, by Charles Deville, who visited the Canary Islands in the autumn of 

 that year. See that distinguished geologist's Voyage geologique aux Antilles 

 et aux lies de Te'ne'riffe et de Fogo, 1848, p. 14, 74, and 169 ; and his Analyse 

 du Feldspath de Te'ne'riffe in the Comptes rendus de 1'Acad. des Sc. t. xix. 1844, 

 p. 46. He says, " Les travaux de Messrs. Gustav Eose et H. Abich n'ont pas 

 peu contribue sous le double point de vue crystallographique et chimique, a re'- 

 pandre du jour sur les nombreuses variete's de mine'raux qui etaient comprises 

 sous la vague denomination de feldspath. J'ai pu soumettre a 1'analyse des 

 cristaux isoles avec soin et dont la densite en divers echantillons etait tres-uni- 

 forme'ment, 2*593, 2'594, et 2'586. C'est la premiere fois que le feldspath 

 oligoclase a etc' indique dans les terrains volcaniques, a 1'exception peut-Stre de 

 quelques-unes des grandes masses de la Cordillere des Andes. II n'avait e'te 

 signale', au moins d'une maniere certaine, que dans les roches e'ruptives auciennes 

 (plutoniques, granites, sye'nites, porphyres sydnitiques ....); mais dans les 

 trachytes du Pic de TenerifFe il joue un role analogue a celui du labrador dans 

 les masses dole'ritiques de 1'Etna." Compare also Eammelsberg, in the Zeit- 

 schrift der deutschen geologischen Gesellschaft, Bd. v. 1853, S. 691, and the 

 4th Supplement to Handworterbuch der chemischen Mineralogie, S. 245. 



C 603 ) p. 432. The first determination of the height of the great volcano of 

 Mexico, Popocatepetl, is, so far as I am aware, the trigonometric measurement 

 made by me on the 24th of January 1804, in the Llano de Tetimba. The 

 summit was found to be 1536 toises above the Llano, and as the latter is, ac- 

 cording to barometrical determination, 1234 toises above the coast of Vera Cruz, 

 we have the absolute height of the volcano 2770 toises, or 17,713 feet. The 

 barometric measurements which followed my trigonometrical determination, led 

 to the conjecture that the height of the volcano was still greater than that stated 

 by me in the Essai sur la Geographic des Plantes, 1807, p. 148, and in the 

 Essai politique sur la Nouv. Espagne, t. i. 1825, p. 185. William Glennie, 

 who, on the 20th of April 1827, was the first to reach the margin of the crater, 

 found the height, according to his own calculation (Gazeta del Sol, publ. en 

 Mexico, No. 1432), 17,884 English feet (2796 toises) ; or, according to a correc- 

 tion by Oberbergrath Burkart, who has done so much for American hypsometry, 

 and by comparison with an almost simultaneous barometric observation at Vera 

 Cruz, even 18,011 feet. A barometric measurement by Samuel Birkbeck (10th 

 Nov. 1827), calculated according to Oltmann's tables, again gave, however, 

 17,854 feet, and the measurement of Alexandre Doignon (Gumprecht, Zeitschrift 



