The Types of Plutonic Rock. 



The various types of plutonic rocks found at Dundas may 

 now be briefly described. Their range in mineralogical 

 composition lias been already discussed. It is noticeable 

 that in certain types, particularly the peridotites and basic 

 gabbros, considerable variation exists in the proportions 

 between the constituent minerals, so that two sections of 

 may fall, if considered alone, into differ- 

 > of rocks. This is due to the tendency of the 

 pyroxene to form aggregates. In normal gabbros the 

 tendency is less marked. 



Anorthosites— These rocks are composed almost entirely 

 of basic labradorite with a little diallage, occasionally with 

 spinel intergrowths. Such specimens as have been studied 

 (they are not very common) have been much altered by 

 schillerisation of felspar and diallage, and usually by the 

 alteration of the latter to chlorite, biotite and smaragdite. 

 By increase of pyroxene these pass into gabbros. The 

 following is the chemical composition of a gabbro rich in 

 basic felspar (A). With it is the analysis (B) of a gabbro 

 from Oberbeerback in the Odenwald. 1 



Si0 2 48^8*6 47-97 Orthoclase Ml 1-67 



Magnetite 

 Ilmenite 

 Apatite 



Eosenbusch, Elemente der GesteiDlehre, 1901, p. 155, No. 13. 



