700 CROSS, IDDINGS, PIRSSON, AND WASHINGTON 



equigranular rocks may be related to one another in two ways. They 

 may be in juxtaposition, or they may interpenetrate one another. 



(a) Consertal. — Irregularly shaped crystals in juxtaposition yield 

 cross-sections that are closely fitted together or conserted. The 

 fabric may be called consertal. This fabric is well developed in 

 some metamorphic rocks where adjacent crystals interdigitate with 

 serrated outline. 



(b) Graphic. — Mutually interpenetrating crystals produce irregu- 

 larly shaped forms, which in some cases yield straight-edged patterns 

 as in graphic granite, where quartz and feldspar are intergrown. 



II. Inequigranular rocks, composed of crystals of more than 

 one size, possess different fabrics according to the manner in which 

 the sizes of the crystals vary from one another. The sizes may 

 vary gradually from one extreme to the other; or there may be 

 marked contrasts in size among the crystals. For these two ideas we 

 may use the terms: 



(A) Seriate — where the sizes of the crystals vary gradually or in 

 a continuous series. 



(B) Hiatal — where the sizes are not in a continuous series, but 

 in a broken series with hiatuses, or where two or more sizes are 

 markedly different from one another. 



The most familiar variety of hiatal fabric is that porphyritic 

 fabric in which there is a marked contrast between the sizes of the 

 crystals forming the groundmass, and those of the phenocrysts. 

 Another hiatal fabric which is the antithesis of the porphyritic is the 

 poikilitic. In this there is also a marked contrast between the sizes 

 of the crystals forming the matrix and of those scattered through 

 it, but the crystals of the matrix are the larger. The small crystals 

 scattered through the matrix may be called xenocrysts ( |eVo<? = 

 stranger). The large ones may be called oikocrysts (ot/co? = house). 



Omitting for the present a discussion of seriate fabrics, which 

 are intermediate, as it were, between equigranular fabrics and 

 hiatal fabrics, let us consider the various expressions of hiatal fabric, 

 commencing with the commoner kinds, the porphyritic. 



(B) Hiatal fabric. — (i) Porphyritic fabric may be defined as one in 

 which a groundmass of glass or crystals carries scattered through it 

 crystals noticeably larger than those composing the groundmass. 



