'. Jackson — Nomenclature of Crystalline Rocks 





(Granite. 

 .. -Porphyritic g 



Porphyritic micrograr 

 Quartz porphyry, 



vu^u^^.ji™^, -- j vitrophynin part). 



[ glassy, 01 isidian porphyry. 



i microlitic, Trachytie pitehstone. 



Obsidian \ felsitic, Felsitic pitchstone. 



(glassy. ....Obsidian. 



In these names which I have thus suggested, the much ab- 

 used term ''porphyry" obtains a perfectly definite i 

 indicates always the presence of determinable crystalline con- 

 stituents and of a base, which is of course always isotropic. 

 The term "porphyritic" would be used only in the macroscopic 

 sense. These names are as much as possible descriptive; trivial 

 names could be substituted throughout but, as I have said be- 

 fore, I think such names should never be coined where a con- 

 venient descriptive one will answer the same purpose. It will 

 "be observed that following the principles previously laid down 

 every possible textural aggregate in the series has received a 

 name, that each has received but one name, that, consequently, 

 the same name is never applied to aggregates that are object- 

 ively distinct, and lii i!l\ tl at they are based upon di>' 

 about which there can be no differences of opinion. 



In a similar manner these textural distinctions could be ap- 

 plied to every similar rock-series and perfect uniformity and 



i chapter only the most general 

 if the pi nc pie of a st,ri« tl\ mim rah ideal 

 epted, the exact pari which th 

 essential rocK-consutuents would play therein would still re- 

 main to be determined. The discussion of this question is 

 premature, however, until the more fundamental proposition is 

 recognized. 



It seems to me clear that a rock-nomenclature founded upon 



purely chemical or geological or upon mixed chemical or geo- 



ther with each other, or with mineralogical 



principles, necessarily must lead, as it certainly has led, to much 



