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THE WONDERS OF GEOLOGY. Lect. III. 



I. Metamorphic ; or stratified crystalline rocks. 



Observations. — These rocks are presumed to be sedimentary deposits, 

 that have been metamorphosed or changed in mineral structure from 

 long exposure to a high temperature under great pressure ; by which 

 their constituent substances have passed into new combinations, and 

 all traces of any organic remains they may have contained entirely 

 obliterated. The name Hypogene, or netlier-formed rocks, (suggested 

 by Mr. Lyell,*) refers to the condition under which these rocks are 

 supposed to have originated. They are subdivided into two groups : 



1. Mica-schist system; comprising mica-slate, quartz 



rock, hornblende-schist, and crystalline limestone. 



2. Gneiss system ; layers of gneiss, quartz rock, syenite, 



&c, alternating with clay-slate, and mica-schist. 



Observations. — The mica-schist is a slaty rock, composed of mica 

 and quartz, and often passes insensibly into clay-slate. Gneiss is 

 formed of mica, quartz, and felspar, has a laminated structure, and 

 often occurs in distinct layers, or strata. 



II. Plutonic ; or amorphous crystalline rocks. 



Granitic System ; comprising Granite, Porphyry, 

 and certain Trap rocks. 



Observations. — The well known rock, termed granite, which, like 

 gneiss, is composed of mica, quartz, and felspar, is not stratified or 

 disposed in layers, but occurs either in amorphous masses protruded 

 through newer rocks, or in dikes and veins, which often traverse 

 strata of all ages. The traps, porphyries, &c, included in this system, 

 have the character of mineral masses, which have been rendered 

 crystalline by intense fusion, under great pressure, in the depths of the 

 earth. 



Volcanic Hocfts. 



1. Trap, Basalt, Tuff, Toadstone, and Obsidian, or 

 glassy lava : — products erupted from ancient volcanoes. 



* From vtto, under, and yivofxai, to be born; see "Elements of Geology," p. 20. 



