28 ROCK-METAMORPHISM. 



CHAPTER V, 



MINERALIZED AND METHYLOSED METAMORPHIC ROCKS, 



METAMORPHIC rocks may be divided into two groups mine- 

 ralized and methylosed, differing from each other, the one in 

 having had the original substance of its members crystallized 

 into minerals of various kinds, and the other in having had the 

 same minerals altered or replaced by chemical reactions. 



The name pseudomorphosis, occasionally applied to the last 

 group, is inappropriate, as the rocks to which it has been given 

 possess no form to be imitated, and therefore no false form is 

 involved. Influenced by this consideration, we have of late 

 years employed the term methylosis in the case of metamorphic 

 rocks which have had more or less of their minerals transmuted*. 



Ophites, which we include in the methylosed group, are so 

 intimately related to the mineralized metamorphic rocks, that, 

 in treating of the origin of the latter, the same subject in regard 

 to the former forces itself on our consideration. 



Passing over the various views that have been held on the 

 origin of the mineralized metamorphics, from the remarkable one 

 held by Leibnitz in his < Protogsea' (1717) to the latest, as set 

 forth by Dr. Sterry Hunt f, we propose to consider the latter, 

 it having been contended for with a persistency and an array of 

 argumentation that have won, if not their conviction, seem- 

 ingly the favourable consideration of many geologists. 



* The term methylosis (/iera-, change, and v\rj, substance) was first pro- 

 posed in my paper <{ On a Silo-carbacid rock from Ceylon," published in the 

 * Geological Magazine,' vol. x., January 1873. The term metasomatosis (/zera- 

 o-aj/iarwens), applied to the same class of rocks by Von Lasaulx and Knop, 

 is of subsequent date, and had already been employed by myself in a memoir 

 "On the Trimerellid " (Quart, Jonrn. Geol. Soc." No. 118, p. 140, 1874), in 

 which I am associated with Mr. T. Davidson. W. K. 



t The principal views are given by Delesse in his memoirs on metamor- 

 phism in the ( Annales des Mines/ sr. 5, t. xii. 1857, &c. 



