

'^1 



IGNEOUS ROCKS 



plete and exhaustive than it is. There are, therefore, great diver- 

 sities in the various schemes of classification which have been 

 proposed and which are still in use, and all such schemes require 

 great modifications to meet continually advancing knowledge. 



Bearing in mind the principle, already emphasized so often, that 

 geology is primarily a historical study, the most logical scheme of 

 classification is obviously one that, so far as possible, is genetic, 

 that is to say, one which expresses in brief the history and mode 

 of formation of the rocks. Other criteria, such as texture and 

 chemical and mineralogical composition, must be employed for 

 the minor subdivisions. On this genetic principle we may divide 

 all rocks into three primary classes or groups. 



A. Igneous Rocks, those which were melted and have solidified 

 by cooling. Texture glassy or crystalline. 



B. Sedimentary Rocks, those which have been laid down 

 (almost always) under water, by mechanical, chemical, and organic 

 processes. Rocks composed of more or less rounded and worn 

 fragments, seldom crystalline. 



C. Meiamorphic Rocks, those which have been profoundly 

 changed from their original sedimentary or igneous character, often 

 with the formation of new mineral compounds in them. Texture 

 fragmental or crystalline. 



IGNEOUS ROCKS 



We have every reason to believe that the igneous rocks were the 

 first to be formed, and arose, in the first instance, from the cooling 

 of the surface of the molten globe. In later ages and at the 

 present time, the igneous rocks have a much more deep-seated 

 origin and have either forced their way to the surface, or have 

 cooled and solidified at varying depths beneath it. The igneous 

 rocks being thus the primitive ones, all the others have been 

 derived, either directly or indirectly, from them. The products of 

 the chemical disintegration or mechanical abrasion of the igneous 

 rocks have furnished the materials out of which the sedimentary 

 rocks were formed, at least in the first instance. 



