GEOLOGY. 85 



melted matter thrown out by volcanoes: these are composed 

 almos't entirely of feldspar and augite. Some lavas are por- 

 phrytic, and contain imperfect crystals, "derived from some 

 older rocks, in which the crystals pre-existed, but were not 

 melted, as being more infusible in their nature." 



When lava is cooled in the open air, it is light, porous and 

 spongy, and floats on water, as is the case with pumice stone ; 

 but when cooled under great pressure, at considerable depths 

 below the surface, solid rock is the result 



There are several varieties of lava, varying in composition, 

 and also of different colors, as gray, whitish, greenish and dark: 

 fragments of granite and other rocks, several metals and 

 gases, water, sulphur, mud, glass, and various salts and acids, 

 are ejected from the craters of active volcanoes. 

 GNEISS. 



Gneiss is composed of quartz, feldspar and mica, and some 

 specimens contain hornblende. This rock is essentially the 

 same as granite, except it is stratified. The laminated struc- 

 ture becomes obscure where the gneiss passes into granite: 

 its stratification is remarkably regular in some specimens, and 

 in others tortuous and irregular. This rock is said to be very 

 extensive in the United States, particularly in New England. 

 QUARTZ. 



This rock is composed either of an aggregate of fine grains 

 or crystals compacted together, or of a solid homogeneous 

 mass of quartz, sometimes containing feldspar, mica, horn- 

 blende, talc or clay slate. " In these compound varieties, [says 

 Hitchcock,] the stratification is remarkably regular; but in 

 pure granular quartz, it is often difficult to discover the planes 

 of stratification." 



It is alternated or interstratified with all the primary rocks, 

 in which case its structure is regular. Some quartz is capable 

 of sustaining a powerful heat without cracking or other 

 change, hence it makes an excellent fire stone. 



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