56 SECTIONAL ADDRESSES. 



calcic rocks, and that the separation was the result of differentiation under 

 stress. In this he is supported by a great number of penologists, and 

 so far as the evidence of the Tertiary province is concerned, no other 

 explanation appears possible. 



If we consider the course differentiation has followed to produce the 

 various rock-types met with amongst the Tertiary plutonic rocks we find 

 that it depends chiefly on three factors which are, the separation of olivine 

 and lime felspar and the production of a quartz-rich and alkaline residuum. 



The formation of a magma supersaturated as regards silica from one of 

 basaltic composition was for long regarded as improbable. Now, however, 

 on theoretical and experimental grounds there appears no difficulty in 

 developing quartz from a hydrous basic magma. Whatever the explana- 

 tion, however, no one studying the intrusive rocks of Mull and Ardna- 

 murchan in the field could for a moment doubt the consanguinity of the 

 abundantly represented quartz-gabbros and quartz-dolerites and their more 

 basic associates the normal gabbros, eucrites and peridotites. 



It appears that the differentiation of the plateau basalt magma has 

 been responsible for the production of two magma-types, which as differen- 

 tiation progressed converged in the direction of increasing acidity and 

 alkalinity. The representative rock-members of each magma are linked 

 together by common compositional characters that are certainly of genetic 

 significance. 



In Skye, Mull and Ardnamurchan we find that the basic major intrusions 

 constitute a group of rocks that on a chemical basis of comparison are richer 

 in alumina, lime and magnesia than corresponding members of the normal 

 magma series. These more basic rocks suggest that their origin lay in a 

 basaltic magma enriched by the addition of the constituents of lime-felspar 

 and olivine, either singly or together. It follows that the magma supplying 

 the enrichments will be correspondingly poorer in olivine and the basic 

 plagioclases, and proportionately richer in alkali-felspar and quartz. 



The differentiation characteristic of the normal magma series is the 

 rapid fall in the percentage of lime, magnesia and iron, with a more or less 

 constant percentage of alumina, and a rise in alkalies, more especially of 

 potash. Such variations point to the continued separation of non- 

 aluminous ferromagnesian minerals as being the dominant factor in the 

 differentiation. The general increase in the alkalies and the relative 

 concentration of potash is undoubtedly due merely to the separation of 

 soda-lime felspar. Such a differentiated magma would account for the 

 non-porphyritic central types of lava in Mull, the intermediate and sub-acid 

 intrusive rocks, and the great group of granites, granophyres and felsites. 

 All these rocks can be derived from the plateau-basalt magma by the 

 normal process of the crystallization and abstraction of one or more solid 

 phases, and the separation may be produced either by the gravitational 

 settling of the crystals, or the removal of the liquid residuum as the result 

 of stress. 



The other magma-type is responsible for the gabbros and related 

 rock-types. It appears to be a magma of plateau basalt composition 

 enriched by the addition of olivine and lime-felspar. Its character, as 

 reflected in the gabbros of Skye, Mull and Ardnamurchan, and carried on 

 into the. tonalites and monzonites of the last-named centre, is the generally 



