(iKuiiPs OF mi:tam()i;imii<' rocks. 73 



(4) Plitlianites or schistose rocks which h;ive Ijeeu subjected to a process 

 of silicification. 



There is seldom any doubt about tlie macroscopical determination of 

 the third and fourth of these groups ; in a large proportion of cases also, the 

 granular rocks can readily be disting-nished from the altered sandstones 

 with the naked eye or the loupe, but this is by no means alwnys possible. 

 Many rocks which to the naked eye appear to be mereU^ considerably 

 altered but perfecth^ recognizable sandstones turn out, upon microscopical 

 examination, to be holocrystalline and to have lost entirely the character- 

 istic clastic structure. 



The granular rocks are separable, under the microscope, into several 

 varieties, according to their mineralogical composition ; but it is seldom 

 possible to distinguish these varieties niacroscopically. In dealing with 

 eruptive rocks the eye soon accustoms itself to the perception of verv 

 minute differences of apjjcarance which represent or are associated with 

 microscopical peculiarities, Tlie metamorphic rocks are pliysically and 

 chemically much more heterogeneous than eruptives, and it is only in 

 extreme cases that the habitus is characteristic of the precise mineralogical 

 composition. 



As will appear in the sequel, the altered sandstones and the granular 

 rocks form a series which is in reality unbroken. Tiie ])rocesses of altera- 

 tion can be studied in rocks retaining as clenrlv as possible evidences of 

 their clastic character. The same processes can bo traced through series in 

 which the clastic elements grnduall}^ disappear and in the extreme members 

 of wdiich a holocrystalline mass of authigenetic minerals is ])resented. In 

 the altered sandstones various transformations begin sinmltaneousl}-, and, 

 according to the physical and chemical conditions under which the meta- 

 morphism occurred, one or other of these changes may predominate in the 

 fully altered rock. In this way types are produced so distinct that were 

 these alone submitted to examination little analogy would be perceived 

 between them ; but they are, in fact, connected with one another, as well 

 as with the unaltered sedimentary rocks, by very gradual transitions. 



In describing the various types more or less repetition is unavoidable. 

 For the sake of brevity it seems expedient to begin the discussion of the 



