﻿and to the porphyritic structure of Rocks. 71 



whenever iron oxide or other ingredient is at the same time 

 set free, as has often happened, this indicating that a change in 

 constitution accompanied the paramorphic ; the 1st and 2d, in the 

 metamorphism of a common limestone to a crystalline limestone 

 (calcite), supposing such a limestone to be (as it usually is) partly 

 aragonite ; and so on. In the derivation of a hornblendic 

 rock from a sedimentary rock, the metamorphism would ordi- 

 narily be metachemic ; but if from an augitic rock, para- 

 morphic in part if not wholly.* 



, B. Porphyritic Structure. 



Petrography has such names as felsite-porphyry, diabase-por- 

 phyry, quartz-porphyry, augite-porphyry, etc. ; the first two, and 

 others like them, implying that the rocks named are porphyritic 

 with crystals of some kind of feldspar; the third, that some kind 

 of rock is porphyritic with crystals of quartz ; the fourth that 

 some kind of rock is porphyritic with crystals of augite. It has 

 also the name orthophyre, for a porphyritic rock having ortho- 

 clase in crystals ; augitophyre, as an equivalent of augite-por- 

 phyry ; and so on. 



It is now generally admitted that the porphyritic character is 

 of trivial value in petrography ; that it marks off only varieties, 

 and therefore cannot, in any right scientific system, be the name 

 of a group of rocks. For the designation of varieties its recog- 

 nition is important. Moreover, for this purpose, the precise 

 feldspar or other mineral that constitutes the crystals should 

 be indicated in the qualifying term. The following words, 

 based in plan on such terms as orthophyre, augitophyre and the 

 like, may well be applied to porphyritic varieties, whatever 

 the kind of rock : 



Orthophyric, when the crystals are Orthoclase. 



Oligophyric, " " Oligoclase. 



Labradophyric, " " Labradorite. 



Anorthophyric, " " Anorthite. 



Leucitophyric, " " Leucite. 



Nephelophyric, " " Nephelite. 



Augitophyric, " " Augite. 



Chrysophyric, " " Chrysolite (Olivine.) 



Quartzophyric, " " Quartz. 



Granite, gneiss, mica schist, syenyte, felsyte, trachyte, rhyo- 

 lyte, obsidian, are often orthophyric ; felsyte, rhyolyte, dioryte, 



* The terms metapepsis and paroptesis have been proposed by Mr. Kinahan of 

 Ireland for, respectively. " regional " and " contact " metamorphism. Contact is 

 an objectionable word ; but local is good ; for it includes changes due to heated 

 emanations and other conditions where there are no contacts. Regional and local 

 describe the conditions well, and have been used for a dozen years at least in this 

 country ; and why we need go to the Greek for two words is not clear. 



