6l2 CROSS, 1DDINGS, P/RSSON, WASHINGTON 



Various textural modifications of igneous rocks may be dis- 

 tinguished from one another, but it is understood that in this 

 respect, as in all others, there are all gradations between different 

 textures. Moreover, all known textures are not developed 

 within every known magma unit, and some textures are specially 

 frequent in rocks of particular compositions. 



I. Crystallinity. — The degree of crystallization attained by 

 an igneous rock is measured by the relative amounts of crystal- 

 lized and glassy portions. Perfectly glassy rocks are very 

 uncommon, since most obsidians abound in microscopic crystals, 

 while many lavas that are almost completely crystallized contain 

 small amounts of glass. Most rocks are holocrystalline, and all 

 gradations between the extremes exist. 



Distinctions that have been based on crystallinity are of two 

 sorts : one an absolute distinction based on the known absence 

 or presence of glass base or matrix, resulting in {a) holocrystal- 

 line, {ti) liypocrystallitie, or partly crystalline, and (c) holohy aline, or 

 completely glassy, rocks. The other sort of distinctions are less 

 definite, being based on the megascopic appearance of the rock, 

 but they are of great practical value. They are [a) phanero- 

 crystalline, (fr) aphanitic, and (<r) vitreous. 



Pha?ierocrystalline rocks are generally holocrystalline. 



Aphanitic rocks are in some instances microscopically holo- 

 crystalline, in others hypocrystalline. Because in some cases it is 

 not possible without microscopical study to determine the 

 presence or absence of glass matrix, it is extremely useful to refer 

 a rock to this textural division. It is discussed again in the 

 next phase of the subject. 



Vitreous rocks are hypohyaline or holohyaline. 



II. Granularity is the quality based on the absolute size of 

 the crystals, which, when all igneous rocks are considered, range 

 from microscopic sizes to megascopic ones measured in feet. 

 The quality derived from the relative sizes of the crystals in one 

 rock may be considered as a phase either of granularity or of 

 fabric. 



Distinctions based on differences in the absolute size of the 



