376 COSMOS. 



analysis of the tufas of Chatham Island.(*) This island, the 

 most easterly of the whole group, and whose situation is fixed 

 by careful astronomical observations by Captain Beechey, is, 

 according to my determination of the longitude of the city of 

 Quito (78° AV 8^^), and according to Acosta's Mapa de la 

 Nueva Granada of 1849, 536 geographical miles distant from 

 the Punta de S. Francisco. 



IX. Mexico. 



The six Mexican volcanoes, Tuxtla,* Orizaba, Popocate- 

 petl,* Toluca, Jorullo,* and Colima,* four of which have been 

 in a state of igneous activity within the historical era, were 

 enumerated in a former place,! and described in their geog- 

 nostically remarkable relative position. According to recent 

 investigations by Gustav Eose, the formation of Chimborazo 

 is repeated in the rock of Popocatepetl, or great volcano of 

 Mexico. This rock also consists of oligoclase and augite. 

 Even in the almost black beds of trachyte, resembling pitch- 

 stone, the oliglocase is recognizable in very small acute-an- 

 gled crystals. To this same Chimborazo and TenerifFe forma- 

 tion belongs the volcano of Colima, which lies far to the west, 

 near the shore of the South Sea. I have not myself seen this 

 volcano, but we are indebted to Herr PieschelJ (since the 



(*) Bunsen, in Leonhard's Jahrh.filr Mineralogic, 1851, s. 856 ; also in 

 Poga;end., ylnna/ew der Pftysik, bd. Ixxxiii., s. 223. 



t^See above, p. 279-281. 



t See Pieschel, Ueher die Vulkane von Mexico, in the Zeitschrift fur 

 allgem. Erdkunde, bd. vi., 1856, s. 86 and 489-532. The assertion 

 there made (p. 86), "that never mortal has ascended the steep summit 

 of the Pico del Fraile," that is to say, the highest peak of the volcano 

 of Toluca, has been confuted by my barometrical measurement made 

 upon that very summit (which is, by-the-way, scarcely 10 feet in wid^h) 

 on the 29th of September, 1803, and published first in 1807, and again 

 recently by Dr. Gumprecht in the same volume of the journal alaove 

 referred to (p. 489). The doubt raised on this point was the more 

 singular, as it was from this very summit of the Pico del Fraile, whose 

 tower-like sides are certainly not very easy to climb, and at a height 

 scarcely 600 feet less tlian that of Mont Blanc, that I struck off the 

 masses of trachyte which are hollowed out by the lightning, and which 

 are glazed on the inside like vitreous tubes. An essay was inserted 

 so early as 1819 by Gilbert, in volume Ix. of his Annates der Physik, 

 (s. 261), on the specimens placed by me in the Berlin Museum, as well 

 as in several Parisian collections (see also Annates de Chimie et de Phy- 

 sique, t. xix., 1822, p. 298). In some places the lightning has bored 

 such regular cylindrical tubes (as much as three inches in length), that 

 they can be looked through from end to end, and in those cases the 

 rock surrounding the openings is likewise vitrified. I have also brought 

 with me pieces of trachyte in my collections, in which the whole sur- 



