CHAPTER VII. 



GROUP B. BASIC DIVISION. 



NOMENCLATURE. 



IN dealing with Group B it will be convenient for purposes of description 

 to separate it into two divisions ; the one containing the basic members 

 and the other the intermediate members of the group. The separation is, of 

 course, not sharply defined. The members of the basic division, when mainly 

 composed of crystalline constituents, have a specific gravity of about 2*9, and 

 a silica per centage of about 50. Mineralogically they are chai'acterised by the 

 abundance of ferro-magnesian constituents olivine, pyroxene, hornblende, 

 &c. The members of the intermediate division are characterised by a silica 

 percentage of about 60, a specific gravity of 2'6 or 2'7, and by the fact that 

 the ferro-magnesian are subordinate to the felspathic constituents. In this 

 chapter we deal with the members of the basic division, and with the 

 corresponding vitreous and semi vitreous rocks. 



Many names have been applied to the rocks of this division ; such as 

 gabbro, euphotide, norite, hyperstheiiite, hyperite, forellenstein or troctolite, 

 diorite, corsite, eucrite, augite-diorite, diabase, diabase-porphyrite, labradorite- 

 porphyrite (labrador- porphyr.), variolite, ophite, proterobase, epidiorite, 

 dolerite, anamesite, basalt, greenstone, melaphyre, leucophyre. The glassy 

 and half-glassy representatives of this division have received such names as 

 magma-basalt, Limburgite, pyroxenite, augitite, tachylyte, hyalomelane, 

 sideromelane, palagonite. 



In the present work we propose, as already explained, to regard rocks 

 simply as mineral aggregates. Special names, particularly those which refer 

 to the minor features, have, therefore, from our point of view, only a local 

 interest. They become a positive hindrance when any attempt is made 

 to correlate the rocks of different countries and of different geological 

 periods, for it frequently happens that one and the same mineral aggregate 

 is designated by different names in different localities. The only way, as 

 it seems to the present writer, of rescuing petrography from the confusion 

 which arises in consequence of this, is to relegate special names to a 

 subordinate position, and in every case to give instead of, or in addition 

 to the name, a concise petrographical diagnosis. As special names, 

 however, cannot be altogether abolished, it seems desirable to give a brief 



