A. W. Jackson— Nomenclature of Crystalline Bocks. 125 



nation and no glass, but consisting entirely of granules, scales 

 and thread-like forms: or as a glassy base, i. e. a glass with 

 products of devitrification; or as a pure glass without any of 

 these devitrification products. 



I would not pretend to base any distinction in nomenclature 

 upon differences between these three latter forms of ground- 

 mass. They are exceedingly interesting and important as 

 showing at precisely what stage molecular freedom of motion 

 was arrested in the cooling magma; or if one will, the precise 

 stage of subsequent molecular rearrangement in the " older" 

 rocks. But the distinction cannot invariably be sharply drawn 

 between them. 



One can however at once distinguish sharply between the 

 'presence or absence of isotropic material in the ground-mass, 

 and upon this a distinction in nomenclature can safely be based. 

 Bearing in mind the terms thus defined, I will illustrate the 

 use that can safely be made of rock-texture, by applying it to 

 the series of rocks having obsidian at one extreme and granite 

 at the other, including thus the liparites and quartz porphyries. 



If the rock is to the unaided eye distinctly and completely 

 granular, it would be "granite." If some of the constil I its 

 were developed in larger crystals than the rest it could be indi- 

 cated by the term "porphyritic granite." If the size of the 

 granules diminished until 'the rock became compact and the 

 texture were micro-crystalline or crypto-crystalline, it could be 

 called with Rosen busch " microgranite," and if porphyritic, 

 ''porphyritic microgranite."' If the ground-mass contained 

 isotropic material it would be des tsb between 



two cases; first, where the crystalline portion is in excess (the 

 " quartz porphyries"), second, where the isotropic portion is in 

 excess ("the obsidian porphyries.") I would call the first kind 

 " quartz porphyry" whether the base were micro fel si tic. glassy 

 or pure glass. Or the trivial name "liparite" could be used, 

 with the understanding however that the name should always 

 indicate the presence of base in the ground-mass with dissemi- 

 nated crystals in excess. The "obsidian porphyries" could be 

 distinguished, if desirable, as " microlitie obsidian porphyry," 

 "felsitic obsidian, porphyry" and "glassy obsidian porphyry" 

 according as the base showed a microlitie differentiation (as in 

 the "younger" pitchstones), a felsitic devitrification (as in the 

 "older"' pitchstones) or remained in a purely glassy state. 

 Finally when no recognizable crystal! tie minerals were present, 

 the term "obsidian" could be used. and. if desirable, we could 

 sil.-o distingui i microli obsidian (trachyte pitchstone), 

 " felsitic obsidian" (felsite pitchstone) and "glassy obsidian."' 



Subjoined is a table of the terms I have thus constructed, 



