TEXTURE OF IGNEOUS ROCKS 693 



3. Fabric — the shape and arrangement of the crystalHne and 

 non-crystaUine parts. 



It is proposed to consider further each of these factors in the 

 general constitution of texture separately, in order to determine 

 what characters in each need special recognition and specific des- 

 scriptive terms. 



I. Crystallinity 



The degree of crystalhzation attained by an igneous rock is 

 measured by the relative amounts of crystallized and glassy portions. 

 All degrees of crystallinity are known to exist, from perfectly glassy 

 rocks, through those consisting of glass with more or less crystals, 

 to completely crystalline rocks. The size of the crystals is not 

 involved in the crystahinity, but is a factor in the granularity. 



The idea of complete crystallinity, as the condition most commonly 

 observed, has a definite term in holocrystalline, while the general 

 idea of partly crystalline is expressed by hypocrystalline. The 

 general idea of glassiness is expressed by the terms vitreous and 

 hyaline; and partly glassy is expressed by hypohyaline. The term 

 subvitreous is commonly employed in describing the luster of some 

 minerals, but has not been appHed to rock texture. Completely 

 glassy is expressed by holohyaline. There is a term signifying that 

 a rock is evidently crystalhne to the unaided eye — phanerocrys- 

 talline — and one that signifies that its crystalHne character can 

 not be recognized by the unaided eye — aphanitic. 



It would be proper to use the term phanerohy aline for rocks whose 

 glassiness is evident to the unaided eye ; and the term phaneric should 

 be apphed to those rock textures that are apparent megascopically, 

 whether crystalhne or glassy; just as the term aphanitic is used for 

 textures that cannot be discerned by the naked eye, whether crystal- 

 Hne or glassy. But there are no terms to indicate any intermediate 

 degrees of crystalHnity. Such terms would permit more definite 

 descriptions which become more and more necessary with the intro- 

 duction of quantitative standards. 



Definitely determined proportions between glassy and crystalHne 

 parts should be expressed in exact mathematical terms, but it may 

 frequently happen that approximate proportions only are known 

 or are considered sufficient for the purpose of the description. In 



