58 PEOCEEDINaS OF THE GEOLO&ICAL SOCIETY. 



constituent particles remain. In reference to the lax use of the 

 word metamorphism, I am of course aware that any one may argue 

 in defence that a cleaved argillaceous rock is metamorphic, because 

 it has undergone a change in structure ; but in that sense almost every 

 limestone and a great many other ordinary sedimentary rocks are 

 metamorphic, for their present condition is far from identical with 

 their original one. But if the term be thus widely extended, if it 

 is made to include rocks the genesis of which, according to the ordi- 

 nary laws of reasoning, is a certaintj^, and rocks the genesis of which 

 is a matter of great uncertainty, then it ceases to have any classifi- 

 catory value, and cannot be used in any process of exact reasoning. 

 So also as regards the term " schist." I am, of course, aware 

 that, etymologically, the word means something that splits ; that, his- 

 torically, it has been applied variously. Eut so long as there is no 

 flagrant violation of history or of etymology, objections of this kind 

 are mere trilling. Cases arise in the history of all sciences when it 

 becomes necessary to fix with precision the meaning of a term pre- 

 viously used rather vaguely. So long ago as 1862 the late Prof. Jukes 

 pointed out the necessity for doing this with the word " schist," and 

 I have no hesitation in saying that until both it and " metamor- 

 phism " are properly defined, our progress will be greatly retarded. 

 Unless it be preferred to exclade the term from science and coin a new 

 one, rocks should be called metamorphic when such marked minera- 

 logical change has taken place that their original condition is a 

 subject for inductive reasoning rather than for simple observation. 

 A hard-and-fast line cannot be drawn ; but, as I pointed out last 

 year, if this difficulty is to stop us we may as well abandon most 

 branches of natural science. Even assuming (for the purposes of 

 argument) that you can discover every stage of transition between 

 a shale or a slate and a mica-schist, still there do exist in nature 

 two great groups at the opposite ends of your chain, each as common 

 as the connecting links are rare, namely, the non-metamorphic shales 

 and the metamorphic crystalline schists (to mention no others); 

 and for these two, as a necessity of clear thinking and definite 

 reasoning, we must have distinctive names*. The same precision 

 is required in using the term '^schist." If I do not know (as very 

 often I do not) whether a writer means by schist only a hard rough 

 shale, or a badly cleaved slate, or a foliated (and so truly meta- 

 morphic) rock, how can I either understand his reasoning or venture 

 to draw any inductions from it? Nay more, this ambiguous 

 terminology is often, if I mistake not, the cause of erroneous 

 reasoning. I have seen more than once in geological text-books 

 and papers not only statements and arguments with regard to general 

 principles, but also conclusions relating to metamorphism, which I 

 happened to know rested mainly, if not wholly, upon these wide and 

 loose applications of the above terms. 



It may, indeed, be alleged that the epithet " metamorphic," as 

 above defined, is too vague to be of any use ; and terms, which de- 



* To the above-named connecting forms the term " bypometamorphic," pro- 

 posed first, I think, by Dr. Callaway, may be conveniently given. 



