MICROSCOPIC STRUCTUEE 37 



gradual pressure exerted upon the rock masses, the final result 

 being a rock of entirely different type and structure from that 

 which originally cooled from the molten magma. The change 

 such as above described is further alluded to in the chapter on 

 metamorphism. 



This science of microscopic petrography, as it is technically 

 called, has also been productive of equally important results in 

 other lines. As an instance of this may be mentioned the dis- 

 covery that the structural features of an igneous rock are de- 

 pendent, not upon its chemical composition or geological age, 

 but upon the conditions under which it cooled, portions of the 

 same rock varying from holocrystalline granular through por- 

 phyritic to glassy forms. To this fact allusion has already been 

 made. 



The general subject of the microscopic structure of rocks of 

 various kinds will be discussed more fully in describing the 

 rocks themselves. Nevertheless, as in describing these struc- 

 tures it has become necessary to use sundry technical terms, it 

 will be well to refer to them briefly here. 



When a rock is made up wholly of crystalline matter, it is 

 spoken of as Jiolocrystalline; when, however, it shows interstitial 

 glassy or felsitie matter, it is hypocrystalline. Eocks wholly 

 without crystalline secretions are amorpJious. The glassy, or 

 felsitie matter occupying the interstices of the other constitu- 

 ents is spoken of as the base. This base, together with the 

 mierolites and smaller crystallizations of the second generation, 

 is called the ground-mass; such may be made up of mierolites — 

 small needle-like crystals imperfectly developed — when it is 

 called microlitic, or of a dense aggregate of quartzose, felds- 

 pathic and other materials, when it is known as felsitie. The 

 larger crystals developed in a glassy, felsitie, microlitic, or finely 

 granular microcrystalline ground-mass are called phenocrysts. 

 When a mineral in a rock shows good crystal outlines, having 

 been uninfluenced in its growth by the proximity of other 

 minerals, it is called idiomorpJiic : when, however, its outline is 

 due not to crystallographic forces, but to interference — to the 

 action of external forces — it is allot riomorphic. Many rocks 

 show indications of two or more periods of crystallization, in 

 each of which minerals of the same species may be developed. 

 Thus in a molten magma the augites may begin to form under 

 such conditions that for some time their growth is unimpeded 



