124 A. W. Jackson— Nomenclature of Crystalline Rocks, 



(— plagioclase 4- leucite), or to diallage basalt ( — augite 4- d 

 lage.) 



In this way then our nomenclature could easily adapt it 

 to the special circumstances of each ease. If it were c 

 sidered desirable, purely trivial names could be given to e 

 " sly allied ] " 



granite, 



the purely trivia! names to a minimum. 



In addition to the mineral constituents, many rocks con tain 

 more or less of the soliditi d magm -residuum that was not 

 taken up by the crystalline elements before molecular freedom 

 of motion was stopped by the final solidification of the rock. 

 This varies in quantity all the way from 100 percent (as in 

 iowii to o per cent as in granite. This evidently can 

 not be disregarded ; hut the part it should play can be better 

 discussed perhaps under the head of texture. 



Hock Tuxlure.—From the earliest times texture has been 

 -e of naming rocks, and the desirability >n 

 con! inning its use particularly for purposes of descriptive 

 petrography is evident. It is none the less so from the fact 

 that it is the expression, so far as wo yet understand it even 

 more perfectly than the minerals themselves, of the prevailing 

 physical conditions during the process of cooling and solidifica- 



Can rock-texture be used in forming a nomenclature, with- 

 out danger to our fundamental principles? Does it present 

 such forms as to be accurately and universally recog- 

 nized ? The accuracy with which Kosenbusch has defined (1. c. 

 p. 70 et seq.) da-rain important forms of macro-, and micro-tex- 

 ture renders possible an affirmative answer to these two queries. 

 I will reproduce his definitions. Ground-mass is the term ap- 

 plied to the compact portion of a rook as distinguished from 

 the large, dig jeminated crystals. It is entirely 



a macroscopic conception. Under the microscope this ground- 

 mass may be developed entirely crystalline, i. e. anisotropic; or 

 as a mixture of crystals and isotropic' material .-ailed the base ; 

 or entirely isotropic, ff entirely anisotropic, it may be either 

 micro-crystalline, i.e. made up entirely of distinct 

 granules; or cryptocrystalline, i. e. consisting of anisotropic 

 material throughout but with the crystalline individuals alto- 

 gether indistinguishable from one another. No sharp distinc- 

 tion can be drawn between these two ; they pass by insensible 

 transitions into each other. 



If the ground-mass contains isotropic materia i. this material 

 may be developed as a microfelsitic base, i. e. showing no polar- 



