59 



term granulitic in a different sense in his paper on the " Gabbros, dolerites 

 and basalt of the West of Scotland." It is applied by him to a rock in 

 which only one of the constituents, viz. augite, occurs in the form of 

 granules. In the granulitic dolerites and basalts the felspar gives lath- 

 shaped sections. When the term granulitic is used in the present work 

 the name of the author followed will be appended in brackets. 



The porphyritic rocks taken as a whole may be separated into more 

 or less distinct groups according to the modifications of the ground-mass. 

 Thus : 



(1) Ground-mass granitic, e.g., porphyritic granites. 



(2) Ground-mass micro-granitic, e.g., micro- granites. 



(3) Ground-mass pegmatitic or micro-pegmatitic, e.g., certain granites 



and quartz-felsites, granophyres (RosENBUscH.) 



(4) Ground-mass petrosiliceous, e.g., certain quartz-felsites. 



(5) Ground-mass microlitic, with or without glass ; e.g., many 



andesites and porphyrites. 



(6) Ground-mass crystallitic, glass always present in the unaltered 



rock ; e.g., certain basalts with globulitic base. 



(7) Ground-mass vitreous, e.g., porphyritic pitchstones and obsidians. 



Rocks of this group may be still further subdivided according 

 to the presence or absence of perlitic and spherulitic structures. 



It is not intended to imply that these different groups are sharply 

 divided from .each other. Transitional forms often occur, and it frequently 

 happens that two or more of the above types of groundmass are found in 

 one and the same rock-mass. Here as elsewhere in petrography, and, indeed, 

 in every branch of natural science, we find that the facts of nature refuse 

 to accommodate themselves to any cut and dried system of classification. 

 Such a system can only be established by ignoring some of the most 

 important facts. Any attempt, therefore, to introduce the rigidity of the 

 definitions of geometry into the terms used to express natural phenomena 

 must of necessity result in the formation of a terminology which is inadequate 

 for the purpose for which it is employed. 



The petrosiliceous texture referred to above calls for more detailed 

 description. It is especially characteristic of a large and important group of 

 rocks, containing a high percentage of silica and belonging as a rule to the 

 older geological periods. These rocks have been variously described under the 

 names quartz-porphyry, quartz-felsite, felsite-pprphyry, ortho-felsite, elvanite, 

 &c. Petrosiliceous or f elsitic matter the latter term though not entirely free 

 from objection, will be more frequently employed in the present work is 

 compact and stony in appearance, and agrees in chemical composition with a 

 mixture of quartz and orthoclase. It is mentioned in the above classification 

 as though it were distinct from the groundmass of the micro- granites and 

 granophyres, but the only difference that can be relied upon at present is a 

 negative one. In the latter case the substance is clearly recognised under 

 the microscope to be holo-crystalline and to consist of the two minerals, qviartz 

 and felspar, whereas in the former case this is impossible. Under crossed 



