ANl^IVEESAEY ADDEESS OF THE PEESIDENT. 59 



fine more precisely the nature of the change, have heen suggested. 

 Messrs. King and Eowney, for example, have proposed to divide 

 metamorphic rocks into two groups, mineralized and methylosed; 

 the one denoting those in which the original constituents have 

 formed, by crystallization, various definite minerals ; the other those 

 in which the constituents have been changed by chemical reactions. 

 For the latter process the terms metasomatosis and paramorjpliosis 

 have also been proposed by certain other authors. Further, Mr. 

 Kinahan has proposed to apply the term paroptesis to the effects 

 produced by contact with intrusive masses of igneous rocks, and 

 metapepsis to the effects of " regional metamorphism." I doubt, 

 however, whether the more high-sounding paroptesis is better than 

 the plaiuer " contact-metamorphism," and it is at least not more 

 accurate ; neither do I think metapepsis any particular gain. I 

 believe that if it were necessary to distinguish the various modifi- 

 cations comprehended in the general term metamorphism, the 

 following would be the most accurate : — 



(1) Metastasis (change of order), denoting changes rather of a para- 

 morphic character, such, for example, as the crystallization of a 

 limestone, the devitrification of a glassy rock. (2) Mctacrasis (re- 

 combination), denoting changes like the conversion of a mud into a 

 mass of quartz with mica and other silicates. (3) Metliylosis (change 

 of substance) denoting change rather of a pseudomorphic character. 

 Under these terms, I believe all the principal changes which we 

 have to consider would be included; but I must confess to no 

 great love for augmenting the technical terminology of science. A 

 compound word of foreign origin is no doubt sometimes a gain, by 

 serving as a brief symbol to express a complex idea ; but there is 

 always a danger lest it should be used to mask ignorance rather 

 than to facilitate reasoning. 



Minor, but important requirements in our nomenclature are : — 

 The definite recognition of groups where the changes are mineral, 

 rather than structural ; namely of metamorphic rocks, in which the new 

 constituents have not, as in the case of the foliated rocks, a definite 

 orientation. Obvious examples are quartzites and crystalline lime- 

 stones, which, indeed, like argillites *, may be sometimes more 

 properly reckoned among the hypometamorphic. AVith these cases, 

 however, we are familiar ; but it is less clearly understood or admitted 

 that every igneous rock has, or may have, its metamorphosed repre- 

 sentative. Thus luxulyanite and greisen result from the alteration 

 of granite; diabases and certain hornblendic rocks from that of 

 dolerite ; serpentine (such as the Lizard) from that of a peridotite. 

 Many of the compact felstones also have resulted from the devitri- 



^ This term is itself not of uniform application. It ^youkl best be used for 

 the much-indurated argillaceous rocks which, were they cleaved, would be 

 called slates. The more siliceous varieties are sometimes called hdllejlmtas. 

 This name, though useful for field-purposes, as an admission of incomplete 

 knowledge, is worthless for classification, many so-called hdllefllntas being 

 merely compact felstones or old rhyolites. As the etymology of the word 

 promises, it has often been the means of leading geologists astray. 



