﻿70 J. D. Dana — General terms applied to Metamorph 



ism. 



Van Hise to be a development of the grains into crystals or 

 larger crystalline grains by external additions made conform- 

 ably to the crystallographic axes of the original grains. 



2. A change from one paramorphic state to another : as from 

 augite to hornblende, from aragonite to calcite, etc. 



3. A change through chemical transformation : as that of 

 chrysolite and other species to serpentine ; a portion of a dolo- 

 mite with included or intruded silica to tremolite, etc. 



The last of these methods has been styled metasomalic (signi- 

 fying change in body) to distinguish it from change under the 

 first method, to which the word metamorphic is accordingly re- 

 stricted. But this new term is not specific as to kind of change ; 

 and it applies as well to the second method. A term express- 

 ing the chemical nature of the change is desirable, and such a 

 descriptive term is metachemic. Substituting this we may have 

 the following three descriptive terms for the three methods of 

 metamorphism : I. The Cry stallinic ; II. the Paramorphic; III. 

 the Metachemic. 



Messrs. King and Eowney, some years since, applied the term 

 methylosis (meaning change in substance), with the adjective 

 methylotic, to chemical metamorphism. But metachemic is 

 more simple and intelligible ; and priority can put in no claim 

 against a change for the better where such terms are concerned. 



By the above scheme, we have the term metamorphism as the 

 general title, instead of giving it up to the first method. Each 

 of the three methods involves change inform. Paramorphic 

 change is, eminently and specifically, change in form ; and 

 metachemic is at least as much so, though complicated with 

 change of other kinds; while change by the first method in- 

 volves the minimum amount, since, as we now know, the orig- 

 inal crystallization remains unaltered, and is only built upon 

 after its own laws. Moreover, the first and third processes, — 

 the most unlike, — are essentially one in agency ; the alkaline 

 siliceous waters that build out a quartz crystal, sufficing in 

 some cases (consecutively, if not simultaneously) to convert 

 feldspar into mica, and make other chemical transformations. 

 A common name for the whole is demanded ; and metamor- 

 phism has the necessary comprehensiveness and flexibility in 

 signification and also in its usage in other departments. 



In a case of metamorphism, two of the processes of change 

 have often gone on at once, if not the three. For example : 

 the 1st and 3d have worked together in making a tremolitic 

 dolomite; so also in making a mica schist, in which, as proved 

 by Van Hise in his excellent paper (last volume of this Journal) 

 the mica is largely or wholly derived from feldspar, while the 

 quartz grains become enlarged by the 1st method ; the 2d 

 and 3d, in the change of an augitic rock to a hornblendic, 



