OF VOLCANIC ROCKS. 25 



Fam. ?>d. Angitic 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. 



Order 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 volcanic 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 volcanic 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. Preeminently 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 volcanoes, 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 monotoiy in sceneiy. 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 andesitic 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 canons. 



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. 



Fam. 1. Horubkndic 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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