73 



III. PLAGIOCLASE-HORXBLENDE ROCKS. 



(a) Without quartz. 



(a) Granitic texture. Diorite. (1) 



(ft) Trachytic texture. Hornblende- andesiie.^ 



(ft) With quartz. 



(a) Granitic texture. Quartz- diorite. (S} 



(ft) Trachytic texture. Hornblende -andesite and Quartz- 

 liornblende-andesite, Dacite (in part}. 



IV. PLAGIOCLASE-BIOTITE ROCKS. 



(<i) Without quartz. 



(a) Granitic texture. Mica-diorite. 



(ft) Trachytic texture. Mica-andesite. 

 (ft) With quartz. 



(a) Granitic texture. Quartz- mica-diorite. 



(ft) Trachytic texture. Quartz-mica-andesite, Dacite (in part). 



The plagioclase-pyroxene, plagioclase-hornblende and plagioclase-biotite 

 rocks are not sharply separated from each other. Two or more of the ferro- 

 magnesian minerals, augite, enstatite, hornblende and biotite, usually co-exist 

 in the same rock. These compound rocks can best be designated by terms 

 made up in each case of the names of the constituent minerals, the textural 

 characteristics being indicated by adjectives, or by concluding the term with 

 such words as diorite or andesite, the former of which always implies a 

 granitic and the latter a trachytic texture. This terminology is cumbersome, 

 but it has the great merit of being descriptive. Moreover it is elastic, and 

 this is an absolutely essential qualification for any terminology which deals 

 with objects so closely connected by intermediate forms as rocks. A rigid 

 terminology is only useful where types are rigidly defined. 



APPENDIX TO GROUP B. 



The changes which rocks of this group undergo when subjected to the 

 different agents of metamorphism are very complicated and, in many cases, 

 little understood. It is impossible, therefore, to formulate any definite system 

 of terminology. In the present work the terms diabase, melaphyre, and 

 porphyrite, in so far as they are used at all, will be employed merely to 

 designate varieties of dolerite, basalt and andesite due to alteration by surface 



(1) When the felspar is anorthite we have the variety known as Corsite. 



(2) The hornblende-andesites and their altered representatives, the hornblende-porphyrites, 

 form a well characterised group of rock?. As they contain an excess of silica in the ground- 

 mass they must, however, be regarded as the porphyritic equivalents of the quartz-diorites. 



(3) This includes the Tonalite of Vom, Rath and the Banatite of Gotta. 



