STRUCTURE OF IGNEOUS ROCKS 



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The main divisions are: Igneous rocks sometimes 

 called eruptive which have been brought up from below 

 in a molten condition from which they have cooled and 

 solidified. They usually have two or more essential 

 minerals, and are massive, crystalline, glassy, or, in 

 certain altered forms, colloidal in structure. Aqueous 

 rocks have been formed mainly through the agency of 

 water, as (a) chemical precipitates, or as (b) sedimentary 

 deposits. They are usually fragmental, but may be 

 crystalline or colloidal, but never glassy. They have a 

 laminated or bedded structure, and usually have 

 many constituent minerals. vEolian rocks are formed 

 from wind-drifted 

 material. They 

 are fragmental in 

 character and ir- 

 regularly bedded 

 in structure. Met- 

 amorphic rocks 

 embrace those of 

 any of the fore- 

 going divisions 

 which have been 

 changed from 

 their original con- 

 dition through the 

 agencies of dyna- 

 mic and chemical 

 forces so that they exhibit new properties. They may 

 have one or many constituent minerals, and in structure 

 they are usually crystalline and bedded or foliated. 



Fig. 3. Photomicrograph of basalt (trap) rock. 

 (Lord.) 



