ON ITS EXTERIOR. VOLCANOES. 305 



the yellowish-white covering of pumice which invests 

 all around. The pumice consists here of fragments of 

 coarse texture, of two or three inches diameter, in 

 which there are sometimes crystals of hornblende. 

 This coarser pumice-sand is different from the very fine- 

 grained one which on Popocatepetl itself, near the rock 

 of el Frayle and at the limit of perpetual snow, makes 

 the ascent so dangerous, because, when set in motion on 

 the steep declivity, the rolling mass of sand threatens to 

 overwhelm everything. Whether this malpais (the 

 Spanish term for these fields of lava-ruins, called in 

 Sicily sciarra viva, and in Iceland Odaada Hraun) 

 belongs to old successively superimposed lateral erup- 

 tions from Popocatepetl or to T the rounded conical 

 mountain Tetljolo (Cerro del Corazon de Piedra), I 

 cannot decide. It is a geologically important cir- 

 cumstance, that further to the east, on the route to 

 the little fortress of Perote (the old Aztec Pinahui- 

 zapan), between Ojo de Agua, Venta de Soto, and 

 el Portachuelo, the volcanic formation of coarse-tex- 

 tured white friable pearlstone ( 442 ) rises by the side of 

 a probably tertiary limestone (marmol de la Puebla). 

 This pearlstone is very similar to that of the conical 

 hill of Zinapecuaro (between Mexico and Valladolid), 

 and contains, besides laminae of mica and lumps of 

 imbedded obsidian, glassy bluish-grey, sometimes red- 

 dish, stripes resembling jasper. The extensive "pearl- 

 stone district" is here covered with fine-grained sand 

 consisting of weathered pearlstone, which at first sight 

 might be mistaken for granitic sand, and which, not- 

 withstanding their affinity in respect of origin, may 

 easily be distinguished from the greyish-white pumice 

 VOL. iv. x 



