OF VOLCANIC ROCKS. 95 
Fam. 3d. Augitic Propylite—Rocks distinguished by the accession of augite 
among the ingredients of the rocks of the foregoing family. It is present in greater 
or less quantity, sometimes predominating over the hornblende. To this family be- 
longs the propylite of Silver Mountain, which contains augite in larger proportion 
than any other known variety. 
Orper FourtH—ANDESITE. 
The history of the name ‘andesite’ has been noticed conjointly with that of 
“trachyte.” At the time when the former name was first proposed for certain rocks 
of the Andes, a few specimens of which had been brought to Europe, it was intended 
as a designation of an accidental variety, proper to that mountain range. But those 
same specimens have proved since to be the type of one of the most important groups 
of voleanic rocks, which is distinct from others in character, and has a wide distribu- 
tion. 
Mode of Geological Occurrence.—Andesite vies with basalt in regard to the 
quantity of matter ejected to the surface, and probably excels the same in this respect. 
In most of those parts of the Andes, in regard to the geology of which we possess 
reliable information, it forms the chief bulk among voleanie rocks. The same is the 
case on the southern slopes of the Carpathians, at Nangasaki in Japan, and on the 
islands of Luzon and Java. Andesite succeeded the ejection of propylite, and pre- 
pared the way to that of trachyte. Preéminently the greater part of it has, to all 
appearance, been ejected through extensive fissures ; that is, it has been produced by 
what we styled massive eruptions ; though andesitic voleanoes, too, are not of rare 
occurrence, and, including those which are extinct, appear to have been particularly 
grand in their activity. It may be supposed that many of the former craters have been 
destroyed. Andesitic mountains are characterized by monotony in scenery. They 
form continuous ranges, which are often of considerable elevation and extent, but ex- 
hibit gentle outlines in their summits as well as in their slopes. Breccias only, which 
accompany the solid rock ordinarily in vast quantities, cause local interruptions of the 
monotony by their more rugged forms. They appear in castle-shaped rocks on the 
crests of andesitie mountains, and form high walls, naked and steep, along their slopes. 
Being more liable to destruction by the erosive action of water than solid andesite, 
they frequently compose the sides of steep ravines and camions. 
Mineral Composition —Andesite is always of dark color, mostly blackish, 
though frequently reddish-brown on the weathered surface. Its mineral composition 
varies, though it is confined within more narrow limits than that of propylite. We 
may distinguish, in regard to it, two 
Subdivisions. which are connected by gradual passage in composition and, in 
a greater measure, by geological relations. 
‘am. 1. Hornblendie Andesite-—Paste of bluish-black to dark-gray color, and 
of microcrystalline texture, which passes by gradual steps into that of obsidian. In 
the former case it is frequently vesicular like trachyte. There are imbedded in it 
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