1864.] T. STERRY HUNT ON LITHOLOGY. 31 



spars are intermingled with some other mineral, commonly horn- 

 blende or pyroxene. 



The name of diorite is by good authorities restricted to rocks 

 whose predominant elements are triclinic feldspars with hornblende ; 

 while the names of diabase and dolerite distinguish those rocks in 

 which pyroxene takes the place of hornblende. In some anortho- 

 site rocks however, pyroxene and hornblende are intimately 

 associated, so that a passage is established from diorite to dia- 

 base. The feldspar of diorites varies in composition from albite to 

 anorthite, and is occasionally accompanied by quartz. This, 

 though most frequent with the more silicious feldspars, is some- 

 times met with in diorites which contain feldspars approaching to 

 anorthite in composition. Sometimes the two constituent minerals 

 are distinct and well crystallized, constituting a granitoid rock : 

 fine examples of this, hereafter to be described, occur in the intru- 

 sive hills of Yamaska and Mount Johnson. At other times the 

 diorite is finely granular or compact, when its color is generally 

 of a green more or less dark from the disseminated hornblende, 

 and it takes the name of greenstone. The greenstones of the 

 Huronian series are in part at least diorites, and probably indige- 

 nous ; but a great number of the so-called greenstone-traps are 

 pyroxenic, and belong to the class of diabase or dolerite. Diorite 

 not unfrequently contains a mica, which is generally brown or black 

 in color. Chlorite, magnetite, ilmenite, and sphene often occur as 

 disseminated minerals, as also carbonates of lime, magnesia, and 

 oxyd of iron. The finer-grained diorites are frequently porphy- 

 ritic from the presence of crystals of feldspar or of hornblende. 

 Occasionally this rock is concretionary in its structure, as in the 

 orbicular diorite or napoleonite of Corsica ; which contains a 

 feldspar allied to anorthite, with hornblende, and some quartz. 

 The norite from Sweden is a granular mixture of a similar kind, 

 containing also mica ; and the ophite of some writers is a diorite 

 in which hornblende greatly predominates. 



The rocks which are essentially composed of anorthic feldspar 

 and pyroxene, present still greater diversities than the diorites, and 

 have received various names based upon diff'ercnces in texture and 

 in the form of the pyroxenic element. It is here proposed to re- 

 strict the name of dolerite to such of these rocks as contain the 

 'black augitic variety of pyroxene, and to include the mixtures of 

 triclinic feldspars with all the other varieties of this species under 



