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This we have already done. Can we give any further precision to the 

 term diorite ? 



Now it will not be denied that we want a general term for the 

 plutonic rocks of intermediate composition ; that is for the hypogene 

 representatives of the andesites. Of the terms available diorite seems 

 the most suitable, and so far as possible we shall employ it in that sensa. 

 It must be remembered, however, that it is also used for basic rocks 

 containing original hornblende. When these are spoken of they will 

 always be referred to as basic diorites ; or, if they contain olivine, as 

 olivine-diorites. They appear to be unimportant when compared with 

 the other representatives of the basic group ; and it is a question, after all, 

 whether they might not be more suitably named hornblende-gabbros and 

 hornblende-dolerites. 



Leaving out of account, then, the basic plagioclase-hornblende rocks 

 we may roughly define diorite as the plutonic representative of the andesitic 

 magma ; just as we defined gabbro as the plutonic representative of the 

 basic magma. The different varieties of diorite may be more accurately 

 defined by reference to the nature of the ferro-magnesian constituent. 

 The best marked types are the following : 



AuGiTE-DiORiTE, e.g., certain rocks occurring near Carrock Fell 

 already referred to as quartz-gabbro. 



ENSTATTTE-DIORITE, e.g., rock extensively quarried near Penmaenmawr. 



HoRNBLENDE-DiORiTE, or Diorite proper, e.g., intrusive rock of 

 Glen Tilt (in part). 



MicA-DiORiTE, e.g., certain rocks occurring at the Lizard. 



As quartz is probably always present in the plutonic rocks of inter- 

 mediate composition, it seems scarcely desirable to separate the rocks 

 under consideration into quartz-diorites and quartz-free diorites except 

 in the case of the horn blende- bearing diorites. Thus, the quartz-free 

 augite-diorites are gabbros, and the quartz-free enstatite-diorites are norites. 

 Mica-plagioclase rocks of normal basic composition do not appear to have 

 been recognized. The mica-traps form an exceptional group of rocks 

 and will be considered by themselves. 



One important argument in favour of the above scheme is that it 

 brings the nomenclature of the plutonic rocks into direct relation with 

 that of the volcanic rocks. Augite- diorite corresponds to augite-andesite, 

 enstatite-diorite to enstatite-andesite, hornblende-diorite to hornblende- 

 andesite, and mica-diorite to mica-andesite. In considering this relation 

 it must always be remembered that, the quartz-free andesites have their 

 plutonic representative in quartz-bearing rocks. In the andesite the 

 silica is present in the interstitial matter ; in the holocrystalline rock it 

 separates out as quartz. 



Tonalite. This name was applied by VON RATH in 1864 to the rock 

 which forms the mass of Monte Adamello, south of Tonale, in the eastern 

 Alps. It consists of plagioclase, quartz, magnesia-mica, and hornblende. 

 Orthoclase is present only in very small quantity. The felspar was 

 analysed and found to lie between labradorite and andesine (oxygen-ratio 



