124 The Andes and the Amazon. 



pure, foam-like piimice, and glossy, ti'anslucent obsidian,* 

 The paucity of quartz, and the absence of basalt, are re- 

 markable. Some of the porphyroids are conglomerate, but 

 the majority are true porphyries, having a homogeneous 

 base. Dr. T. Sterry Hunt calls them porphyroid trachytes. 

 They have a black, rarely reddish, vitreous, or impalpable 

 base, approaching obsidian, with a specific gravity of 2.59 

 in pure specimens, and holding crystals or cr^'stalline grains 

 of glassy feldspar, and sometimes of pyroxene and hematite. 

 They differ from the Old World porphyries in containing 

 no quartz, and seldom mica.f D'Orbigny considers the 

 porphyries of the Andes to have been ejected at the close 

 of the cretaceous period, and formed the first relief of the 

 Cordillera. The prevalence of trachyte shows that the pro- 

 ducts have cooled under feeble pressure. 



From the deluges of water lately thrown out have result- 

 ed deep furrows in the sides ; and from the prevalence of 

 the east wind, which is always met by the traveler on the 

 crest of either Cordillera, there is a greater accumulation of 

 ashes, and less snow on the west slope. Cotopaxi is a fine 

 example of this. In Pichincha, Altar, and Euminagua, 

 however, the western wall is lowest, apparently broken 

 down.:]; There is no synchronism in the eruptions of Coto- 

 paxi and Pichincha. These volcanoes must have independ- 

 ent reservoirs, for the former is 3000 feet higher than the 

 latter, and only thirty miles distant. The reputed erup- 

 tions of Pichincha are dated 1534, 1539, 1566, 1575,1588, 

 and 1660 ; that of 1534 resting on the assertions of Checa, 



* It is a singular fact that true trachyte, pumice, and obsidian are wanting 

 in the -volcanic Galapagos Islands, only 700 miles west of Pichincha. 



t As many of the crystals are partly fused, or have round angles, the por- 

 phyries were probably formed by the melting of a crystalline rock, the base 

 becoming fused into a homogeneous material, while the less fusible crystals 

 remain imbedded. — Dr. Hunt. 



X In the Galapagos volcanoes the south wall is lowest, while the craters in 

 Mexico and Sandwich Islands are lowest on the northeast. 



