HOCK-METAMORPHISM. 



CHAPTER I. 



THE DIFFERENT KINDS OF ROCKS TREATED OF. 



THE term ophite, first employed by Vitruvius, is used in this 

 work with a wider meaning than is usual, so as to be appli- 

 cable to rocks that consist of various kinds of serpentine, a 

 number of closely related hydrous silo-magnesian minerals, and 

 some anhydrous siliceous species associated, in many instances, 

 with mineral carbonates. 



In rocks of the kind, the serpentine minerals are to be 

 regarded as essentials, irrespective of their holding a dominant, 

 or a subordinate place ; while the related minerals and the dry 

 siliceous species are to be looked upon as accessories. The 

 mineral carbonates, on account of their playing an important 

 part in a large section of ophites, must also be considered 

 essentials. 



There are certain rocks, composed of dry mineral silicates, in 

 which serpentine (and even a mineral carbonate) is known to 

 occur, but only in such small quantities as to rank no higher 

 than accessories. Cases of this kind, and others which could be 

 noticed, show the difficulties there are in drawing a sharp division 

 between any given class of rocks and the group under consi- 

 deration. 



The following arrangement of ophites and related rocks is 

 offered more for practical purposes than as being a natural one, 

 though it is not altogether devoid of the last character : 



