PLTJTOXrC EOCKS OF GARABAL HILL AND MEALL BREAC. 117 



alkali-molecules play an important part. Moreover, the microscopic 

 examination of thin sections has proved that iron ores precede olivine, 

 that olivine precedes the other ferro-magnesian constituents, and 

 that the ferro-magnesian constituents as a whole precede felspar and 

 quartz in the individual rocks. Novr, if we suppose that the area 

 represents a vast subterranean reservoir which has become differen- 

 tiated during the process of consolidation,^ we see at once that there 

 is a natural connexion between the two classes of facts above referred 

 to. The peridotites are the oldest rocks, because the minerals of 

 which they are composed are the first to form in a consolidating 

 plutonic magma, such as the one with which we are at present con- 

 cerned. Iron ores and olivine are the first minerals to form. Their 

 development at any point must leave the surrounding magma poorer 

 in certain molecular groups. Diffusion will tend to restore homo- 

 geneit}^ and so the first-formed crystals may increase in size. 

 Other causes, such as Soret's principle, or the subsidence of the 

 first-formed crystals by gravity, may also operate and assist in pro- 

 ducing heterogeneity in the original magma. As the process of 

 consohdation progresses, other minerals make their appearance, and 

 rocks of a more complicated composition are the result. The last 

 rock of which we have any record is composed essentially of quartz 

 and orthoclase. It occurs in the form of contemporaneous veins, 

 and probably represents the mother-liquor remaining after the other 

 constituents have separated out. If we take the alkali as the basis 

 for calculating the relative proportions of quartz and felspar in the 

 eurite-veins, we have the following result from analysis YII. : — 



Felspar 58-34 



Quartz 41-66 



100-00 



This leaves, however, about 3 per cent, of alumina unaccounted 

 for, so that in all probability the alkali has been somewhat reduced 

 by alteration. It has been suggested that the micro -pegmatite of 

 plutonic rocks is an eutectic compound in Guthrie's sense,^ and 

 that the composition is 



Felspar 62-05 



Quartz 37-95 



100-00 



The facts here described tend rather to support than to negative 

 this -view. 



In conclusion, it may be interesting to call attention to certain 

 speculations which are suggested by a consideration of the facts 

 we have been describing. Progressive consolidation of one reservoir 



^ [The essential point of the hypothesis is that the various rocks a.© the result 

 of the differentiation of an originally homogeneous magma. The precise locahty 

 •where the differentiation took place is more or less uncertain. That it was not 

 always the place where the rocks are now found is proved by the phenomena of 

 veins and inclusions.— J. J. H. T., Feb. 27th, 1892.] 



2 ' Brit. Petrogr.' (1888) p. 402. 



