TEXTURE OF IGNEOUS ROCKS 



loS 



(b) The sizes of the xenocrysts as compared with the matrix 

 crystal in which they occur may be described as relatively large, 

 medium^ and small or fine. The relative sizes are best expressed in 

 terms of the average diameters of the inner crystals and host. 



Fig. 4 



Fig. 5 



Fig. 6 



Fig. 7 



Relatively large xenocrysts — those whose average diameter is greater 

 than one-eighth the average diameter of the host (Fig. 4.) 



Relatively medium-sized xenocrysts — having average diameters be- 

 tween one-eighth and one-twelfth that of the host. (Figs. 

 5, 6.) 



Relatively small xenocrysts — having average diameters less than 

 one-twelfth that of the host. (Fig. 7.) 

 Poikilitic fabric may be megascopic or microscopic, when its 



granularity may be described with reference to the size of the oiko- 



crysts in the terms applied to equigranular rocks as coarse, medium^ 



ox fine grained. 



(c) The shapes of the xenocrysts lead to modifications of poikiHtic 

 fabric, as in porphyritic fabric. The general shapes may be — 

 Equant. 



Tabular. 

 Prismatic. . 



All the xenocrysts may have similar shapes ; they may be equiform. 

 Or they may have different shapes; they may be multiform. 



(d) The arrangement of the xenocrysts further modifies the fabric. 

 They may be — 



(i) Scattered more or less uniformly through the oikocryst; or 

 (2) Grouped together in various ways. 



Those xenocrysts that are tabular or prismatic may stand in all 

 possible positions, or they may be parallel or nearly parallel to one 

 another, sub-parallel, or their positions may be diverse. 



Ophitic fabric is a particular case of poikilitic in which the xeno- 



