composition; gabbros, pyroxenites and peridotites are not 

 uncommon, troctolites and allivalites are rare. This is 

 explained by Harker as due to the effect of differentiation 

 with mutual repulsion in a system of more than two com- 

 ponents. A pure olivine-felspar magma or a mixture of 

 felspathic and peridotitic magmas differentiates along the 

 line F O. If pyroxene be present in notable amount, this 

 differentiation line breaks almost completely, and the rocks 

 are formed adjacent to the lines F P and P O, as in the 

 Dundas series. The presence of biotite or hornblende in 

 the olivine-felspar magma has much the same effect.' 



The range of the different minerals is also of interest. 

 The presence of rhombic pyroxene may be considered as 

 partial evidence of deficiency of lime in the magma. Coinci- 

 dent with this we have the alumina, unable to enter felspar, 

 forming spinels with the ferric oxides. The association of 

 spinels and rhombic pyroxene is a marked though not a 

 constant feature of this series of rocks. The entry of lime 

 into the monoclinic pyroxene, leaving alumina free to com- 

 bine with ferrous oxide and magnesia to form pleonaste, is a 

 reversal of the normal order of chemical affinities. 2 That 

 an excess of alumina is present in the more basic members 

 is shown by the occurrence of picotite rather than chromite 

 in the almost limeless dunites. In accordance with the 

 increase of the ratio MgO : FeO as we proceed in the direc- 

 tion F PO we find hypersthene, augite, and augitic diallage 

 to be, in general, characteristic of the gabbros and pyrox- 

 enites, enstatite and diopside-diallage of the pyroxenic 

 peridotites and in the dunites the ratio reaches a maximum/ 

 Further, out of a group of fifty slides of plutonic rocks, the 



1 Harker, Geology of the Smaller Isles of Invemesshire, 1908, p. 90. 



* Quantitative Classification of Igneous Rocks, p. 190. 



3 Compare J. H. L. V~ogt, Ueber anchiputektische und anchimonomin- 



