614 CROSS, IDDINGS, PIRSSON, WASHINGTON 



and fine-grain, which are useful when megascopically applied, it 

 is possible to describe the average measured diameter of micro- 

 scopic crystals. But there appears to be no demand for system- 

 atic subdivisions based on the magnitude of these minute 

 textures. 



Textural distinctions based on the relative sizes of the crys- 

 tals in one rock recognize ( I ) evenly granular or non- porphyritic 

 rocks, and (2) porphyritic rocks. These distinctions cannot be 

 sharply drawn, because, as already said, it almost never happens 

 that all crystals in a rock are the same size. Moreover, there is 

 a great variety of ways in which differences of size in crystals 

 may exhibit themselves. 



The difference of size which constitutes porphyritic texture 

 may be defined as that in which the size of some crystals is so 

 much larger than that of a sufficient number to form a matrix 

 for the first, that the larger ones appear notably distinct from 

 the matrix or groundmass. There are porphyritic rocks with 

 phaneric (phanerocrystalline), aphanitic, or glassy, ground- 

 masses, the phenocrysts being megascopically notable. And there 

 are microporphyritic aphanitic or glassy rocks, in which the 

 phenocrysts are not megascopically notable, although they may 

 be visible. 



Granular and porphyritic textures may also be considered as 

 modifications of the fabric. 



III. Fabric. — The arrangement of the crystalline and non- 

 crystalline parts of a rock, or its fabric, as we propose to call it, 

 is dependent on ( 1 ) the shapes of the crystals, and (2) their 

 positions with respect to one another and to the glass base when 

 present. 



When considered with reference to all the minerals in the 

 rock the fabric of most rocks is so complex that no simple 

 expression is applicable to it, and nearly all the formal relations 

 which it is customary to distinguish as different rock textures 

 frequently exist in one rock. However, it usually happens that 

 certain features dominate in each case while others may be quite 

 insignificant though present. Hence we may again disregard 



