704 CROSS, IDDINGS, PIRSSON, AND WASHINGTON 



It has been suggested that this term be used when the rock is 

 granopatic phanerocrystalHne, and — 

 Graniphyric, when the groundmass is microcrystaHine, or granipatic. 



The texture called granophyre by Rosenbusch is called by us 

 graphophyre or graphiphyre, according as the groundmass is mega- 

 graphic or micrographic. 



Granularity. — The terms that may be applied to the grain of 

 equigranular rocks are applicable to the groundmass of porphyritic 

 rocks, and do not need to be repeated in this place. A distinction 

 between megascopic and microscopic sizes may be effected by using 

 the letters o and i, respectively, when the textural term is used as a 

 prefix, as in grano- and grani-. 



Fabric. — The fabric of the groundmass of a porphyry may be 

 the same as that of a rock taken as a whole, so that any term applied 

 to one may be used in describing the other. 



2. Poikilitic fabric is that kind of hiatal fabric in which the matrix 

 consists of relatively large crystals, through which are scattered 

 relatively small crystals. The rock may be called a poikilite, in 

 contrast to a porphyry. The two factors in this fabric, inclosing 

 crystals or oikocrysts and inclosed crystals, xenocrysts, may vary {a) in 

 relative proportions, and {b) in relative size; (c) the xenocrysts may 

 vary in shape^ and {d) in arrangement or distribution. 



[a) Relative proportions between oikocrysts and xenocrysts. All 

 possible proportions exist, from that in which the host constitutes 

 the slightest interstitial cement between the xenocrysts, which in 

 such a case may form the greater part of the rock, to that in which 

 the xenocrysts occupy a very small space in the oikocrysts. To 

 express the relative amount of outer and inner crystals in poikihtic 

 fabric, the following terms may be used : 



oikocrvst 7 ., , , , 



Peroikic, '- — >-, oikocrvst e.xtremelv abundant. 



xenocryst i 



oikocrvst 7 S •, i ■ 



Domoiktc, <->-, oikocrvst dominant. 



xenocryst i 3 



oikocrvst s S •, , 1 11 



Xenoikic, — <->-, oikocrvst and xenocrysts equal or nearly equal. 



xenocryst 3 5 



^ . oikocrvst ^ ^ i , . 



Doxemc, '- — <->-, xenocrysts dominant. 



xenocryst 5 7 



„ . oikocrvst i , , 1 , 



Perxemc, '- — <~, xenocrvsts extremelv abundant. 



xenocryst 7 



