and Guamani are
irregular. The Ecuadorian volcanoes have rarely ejected liquid lava, but
chiefly water, mud, ashes, and fragments of trachyte and porphyry.
Cotopaxi alone produces pure, foam-like pumice, and glossy,
translucent obsidian.[62] The paucity of quartz, and the absence of
basalt, are remarkable. 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
crystalline grains of glassy feldspar, and sometimes of pyroxene and
hematite. They differ from the Old World porphyries in containing no
quartz, and seldom mica.[63] 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 products have cooled under feeble pressure.

[Footnote 60: "As a general rule, whenever the mass of mountains rises
much above the limit of perpetual snow, the primitive rocks disappear,
and the summits are trachyte or trappean porphyry."--_Humboldt_. In
general, "the great Cordilleras are formed of innumerable varieties of
granites, gneiss, schists, hornblende, chloritic slates, porphyries,
etc., and these rocks alternate with each other in meridional bands,
which in the ridges frequently present the appearance of a radiated or
fan-shaped structure, and under the plains are more or less
vertical."--_Evan Hopkins, F.G.S_.]

[Footnote 61: Von Tschudi makes the incorrect statement that "throughout
the whole extent of South America there is not a single instance of the
Western Cordillera being intersected by a river." Witness the
Esmeraldas.]

[Footnote 62: It is a singular fact that true trachyte, pumice, and
obsidian are wanting in the volcanic Galapagos Islands, only 700 miles
west of Pichincha.]

[Footnote 63: As many of the crystals are partly fused, or have round
angles, the porphyries 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_.]

From the deluges of water lately thrown out have resulted deep furrows
in the sides; and from the prevalence of the east wind, which is always
met by the traveler on the crest of eit