CX1V NOTES. 



de Pe'rote, near the eastern declivity of the great plateau of Mexico, stands al- 

 most alone, but yet its great mass belongs to an important line of heights which, 

 forming the margin of the declivity, extends from Cruz Blanca and Rio Frio 

 towards las Vigas (N. lat. 19 37' 37"), passing the Cofre de Pe'rote (N. lat. 

 19 29', W. long. 97 08'), west of Xicochimalco and Achilchotla, to the Peak 

 of Orizaba (N. lat. 19 2', W. long. 97 14') in a north and south direction 

 parallel to the chain (Popocatepetl-Iztaccihuatl) which divides the valley of the 

 Mexican lakes from the plain of la Puebla. (For the bases of these determina- 

 tions, see my Recueil d'Observ. Astron. vol. ii. p. 529532, and 547, as well as 

 my Analyse de I 1 Atlas du Mexique, and Essai Pol. sur la Nouv. Espagne, t. i. 

 p. 55 60.) As the Cofre has been elevated so as to rise abruptly in a pumice- 

 covered space many miles in breadth, it appeared to me exceedingly interesting 

 that when I ascended the mountain in the winter, on the 7th Feb. 1804, on the 

 summit the thermometer sank to 28 0> 4 ; the covering of pumice, of which I 

 measured the height and thickness barometrically at several points, both in 

 ascending and descending, was found by me to rise above 780 feet. The lower 

 limit of the pumice in the plain between Pe'rote and Rio Frio is 7590 feet above 

 the level of the sea, the upper limit on the northern declivity, 8370 feet ; and 

 from thence through the Pinahuast, the Alto de los Caxones (12,495 feet high), 

 where I was able to determine the latitude by the sun's meridian altitude, to the 

 summit itself, there was no trace of pumice to be seen. When the mountain 

 was elevated it must have carried with it a portion of the pumice-covering of 

 the great Arenal, on which it had perhaps been made a smooth flat surface by 

 water. When on the spot, in February 1804, I made a drawing of this pumice- 

 covered zone in my journal. It is the same important phenomenon as that de- 

 scribed in 1 834 by Von Buch at Vesuvius, where horizontal strata of pumice- 

 tufa have been carried up to a greater height (1900 or 2000 feet), about the 

 Hermitage del Salvatore. (PoggendorflTs Annalen, Bd. xxxvii. S. 175179.) 

 The surface of the dioritic-trachyte rock at the Cofre was not concealed from 

 observation by snow at the part where I found the pumice highest up on the 

 mountain. In Mexico, in lat. 19 and 19|, the limit of perpetual snow is on 

 an average about 14,760 feet high ; and the summit of the mountain, at the 

 foot of the small house-like cubical rock where I set up the instruments, is 

 13,416 feet above the sea. By measurement of angles the cube of rock is 134 

 feet high, making the absolute summit, which could not be reached on account 

 of this last vertical precipice, 13,550 feet. I found only some patches of spora- 

 dically fallen snow, of which the lower limit was 12,150 feet above the sea, fully 

 700 or 800 feet below the upper limit of forest trees, being fine trees of Pinus 

 occidentals interspersed with Cupressus sabinoides and Arbutus madrono. The 





