306 COSMOS. 



El Frayle and at the limit of perpetual snow, on the volcano 

 Popocatepetl, renders the ascent so dangerous, because, when 

 it is set in motion on steep declivities, the sand-mass, rolling 

 down, threatens to overwhelm every thing. Whether this 

 lava-field of fragments (in Spanish Malpais, in Sicily Sciarra 

 viva, in Iceland Odaada-Hrauri) is due to ancient lateral erup- 

 tions of Popocatepetl, situated one above the other, or to the 

 somewhat "rounded cone of Tetlijolo (Cerro del Corazon de 

 Piedra), I can not determine. It is also geognostically re- 

 markable that, farther to the east, on the road toward the 

 small fortress Perote, the ancient Aztec Pinahuizapan, between 

 Ojo de Agua, Venta de Soto, and El Portachuelo, the vol- 

 canic formation of coarsely fibrous, white, friable perlite* 

 rises beside a limestone (Marmol de la Puebla) which is 

 probably tertiary. This perlite is very similar to that of the 

 conical hill of Zinapecuaro (between Mexico and Valladolid), 

 and contains, besides laminas of mica and lumps of immersed 

 obsidian, a glassy, bluish-gray, or sometimes red, jasper-like 

 streaking. The wide " perlite district" is here covered with 

 a finely granular sand of weathered perlite, which might be 

 taken at the first glance for granitic sand, and which, not- 

 withstanding its allied origin, is still easily distinguishable 

 from the true grayish-white pumice*-stone sand. The latter 

 is more proper to the immediate vicinity of Perote the pla- 

 teau 7460 feet in height between the two volcanic chains of 

 Popocatepetl and Orizaba, which strike north and south. 



When, on the road from Mexico to Vera Cruz, we begin 

 to descend from the heights of the non-quartzose, trachytic 

 porphyry of the Vigas toward Canoas and Jalapa, we again 

 twice pass over fields of fragments and scoriaceous lava the 

 first time between the station Parage de Garros and Canoas 

 or Tochtlacuaya, and the second between Canoas and the 

 station Casas de la Hoya. The first point is called Loma de 

 Tobias, on account of the numerous upraised basaltic blocks 

 of lava containing abundance of olivin ; the second simply El 

 Malpais. A small ridge of the same trachytic porphyry, full 

 of glassy feldspar, which forms the eastern limit of the Arena! 

 (the perlitic sand-fields), near La Cruz Blanca and Rio Frio 

 (on the western declivity of the heights of Las Vigas), sepa- 

 rates the two branches of the lava-field which have just been 



* The beautiful marble of La Puebla comes from the quarries of 

 Tecali, Totomehuacan, and Portachuelo, to the south of the high tra- 

 chytic mountain, El Pizarro. I hare also seen limestone cropping out 

 near the terrace pyramid of Cholula, on the way to La Puebla. 



