CLA SSIFICA TION OF IGNEO US RO CKS 6 1 7 



are megascopic in some cases, and microscopic in others. When 

 they are microscopic in a glassy matrix the fabric is called micro- 

 ti tic glassy. 



D. Orbicular fabrics. — There are complex fabrics so frequent 

 and so characteristic that they have formed bases for textural 

 divisions. They are : 



a) Sphenditic, allied to the microlitic fabric in some cases in 

 that it can be traced to special forms of microscopic crystalliza- 

 tion, consisting of radiating prisms. It is further allied to por- 

 phyritic fabric, in that it may be developed locally in the magma 

 and produce scattered spherulites of notable size, and having the 

 appearance of phenocrysts. 



b) Spheroidal, closely like spherulitic in some cases, where 

 there is crude radial arrangement of crystals, but quite different 

 from it in others in which there are concentric granular shells. A 

 somewhat diverse form of aggregate crystallization occasionally 

 developed in phaneric rocks. 



Heterogeneous textures produced by more or less hetero- 

 geneity in the magma are distinguished by marked variability of 

 {a) fabric, or [b') composition. 



a) Eutaxitic texture. — Variability of fabric is exhibited by 

 some banded or streaked lavas (rhyolites) in which alternate 

 layers of rock exhibit different degrees of crystallinity or dif- 

 ferent arrangements of crystals. Variability of texture on a 

 large scale is often exhibited by phaneric rocks of similar com- 

 position (granitic pegmatites). 



b) Variability of composition may not strictly belong to the 

 discussion of the texture of rock, but the two are intimately 

 blended, and the treatment of the matter in this place may be 

 justified. It results in banded textures, approaching gneissic, and 

 is exhibited in certain gabbros ; also in irregularly streaked tex- 

 tures, known in German as Sc/ilieren, which may be called 

 schlieric textures. 



In classifying rocks on a basis of textural differences we have 

 considered the fabric, or the shape and arrangement of the crys- 

 tals, as more fundamental than the crystallinity or granularity, 



