71 



III. OLIVINE-HORNBLENDE ROCKS. 

 (/3) Without Olivine. 



To this division belong rocks composed of one or more of the following 

 minerals : Enstatite (hypersthene), hornblende, augite (diallage) and 

 biotite.' 1 ' 



The prevailing texture amongst the rocks of Group A. is the granitic. 

 Some varieties of Tschermak's picrite are trachytic. 



Rocks of this group, or indeed of any group, are not sharply separated 

 from each other. They shade into each other by the appearance or 

 disappearance of certain minerals, and these transitions may sometimes be 

 observed in one and the same rock-mass. Thus the olivine-pyroxene rocks 

 frequently contain a certain amount of hornblende, and the olivine-hornblende 

 rocks a certain amount of pyroxene. Again, by the coming in of felspar the 

 rocks of the picrite group shade into those of Group B. The olivine-pyroxene 

 rocks graduate into the olivine-dolerites, and the olivine-hornblende rocks 

 into olivine-diorites. 



APPENDIX TO GROUP A. 



GROUP B. 



ROCKS IN WHICH PLAGIOCLASE IS THE DOMINATING FELSPATHIC CONSTITUENT. 



NEPHE'LINE AND LEUCITE ABSENT. ORTHOCLASE is FREQUENTLY PRESENT. (3) 



I. PLAGIOCLASE-OLIVINE ROCKS. 



(a) Granitic texture. Forellenstein or Troctolif.e. 



(b) Trachytic texture. (Not recognised). 



(1) The terms liornblendlte and pyroxcnile have been proposed by Professor DANA (Gr.M., 

 1881, p. 62) for two varieties of this division ; the former for rocks mainly composed of 

 hornblende, the latter for rocks mainly composed of augite. When first described by the 

 author these rocks were supposed to be of metamorphic origin ; but they are now regarded 

 as igneous. (A. J.S., Vol. XXVIII., p. 384). The term pyroxenitc has been used in different 

 senses by different writers. 



(2) The mineral serpentine may be formed by the alteration of olivine. enstatite (bronzite) 

 and non-aluminous hornblende and augite. Tho different varieties of the rock serpentine may 

 therefore be designated as olivine-serpentine, enstatite- or bronzite-serpentine, hornblende- 

 serpentine and augite-serpentine. The only typical eruptive rocks known to have yielded 

 serpentine are those rich in olivine and enstatite. 



(3) And in some cases (e.g., certain quartz-diorites) it is difficult to say whether the rocks 

 should be placed in this or the next group. 



