Studies for Students. 



GENETIC RELATIONSHIPS AMONG IGNEOUS ROCKS. 



It is desirable that the student of igneous rocks should 

 appreciate the fundamental relationships existing between various 

 kinds of igneous or eruptive rocks so far as they are understood 

 at the present time, in order that he may form a proper idea not 

 only of what an igneous rock actually is, but also of the uses 

 and limitations of the terms by which they are designated. So 

 it has been thought desirable to present, in an elementary form, 

 some of the data and opinions bearing upon the genesis of differ- 

 ent kinds of rock magmas. 



It can be shown that all eruptive rock masses, whether 

 emanating from volcanic vents at the surface of the earth or 

 found enclosed within such vents, or confined to fissures not 

 immediately connected with actual volcanoes, with the exception 

 of certain infrequent occurrences of sandstones, which have 

 been forced, while in a loose and incoherent state, into cracks — 

 it can be shown that all ordinary eruptive masses were in a com- 

 pletely molten or fused condition before solidifying into the 

 rocks they now are, and hence the terms eruptive and igneous 

 are practically synonymous. 



The igneous mass or molten magma, as we know by observa- 

 tions at active volcanoes, may obtain a liquidity comparable 

 to that of water, 1 which, of course, would obtain for different 

 temperatures in the case of magmas having different chemical 

 compositions ; the less silicious magmas reaching this liquidity 

 at a somewhat lower temperature than the more silicious ones. 

 During the process of cooling, magmas become gradually more 



1 James D. Dana: Characteristics of Volcanoes, etc. New York, 1891, p. 143. 



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