J. Q. Goodchild — Granite Junction in Mall. 451 



however, that the uniformity referred to is by no means as general 

 as it appears to be assumed is the case. 



The sections in the Eoss of Mull suggest that as the peripheral 

 zones of a granite mass, gradually melted up, they may have furnished 

 no inconsiderable proportion of the molten rock that worked up 

 to the surface in the form of lava ; although it is doubtless true that 

 every part of the conduit contributed more or less. The peripheral 

 contributions, being at a somewhat lower temperature, and therefore 

 having a somewhat higher specific gravity than the molten rock 

 nearer the middle of the ascending current, may have been carried 

 downward for a time ; but they were destined, sooner or later, to 

 float upwai'd with the rest. 



Granite veins may thus be regarded as playing the part of capillaries 

 in the circulatory system of the larger mass ; or their function may 

 be likened to that performed by the individual leaves of a tree, 

 which elaborate products not used entirely by the leaves themselves, 

 but which are destined for distribution by means of the circulatory 

 system as nourishment for the plant as a whole. 



The mode of attack of an intrusive mass is probably influenced 

 very largely by complex inter-relations between the composition and 

 the temperature of the invader and the rock invaded. To some 

 extent also the results may vary in accordance with the degree of 

 resistance to intrusion presented by the invaded mass. Where this 

 resistance was comparatively low, as would happen in the case of 

 intrusions taking place under small superincumbent pressure, a true 

 laccolite would doubtless be formed. That is to say, the overlying 

 rock might really and actually be lifted up with little or no replace- 

 ment of the strata affected. But where the resistance to the enormous 

 intrusive force was too great to be overcome by such rupture and 

 displacement, the temperature of the peripheral zone was raised to 

 the melting point proper to the degree of pressure exerted, and the 

 invaded rock gave way by fusion instead of yielding by upheaval 

 or by fracture. 



In this way both of the recognized tj'pes of intrusion may find 

 a simple explanation. Where a laccolite really does occur, that 

 exceptional phenomenon would seem to indicate intrusion under 

 comparatively -low superincumbent pressure and usually under 

 such conditions as must obtain in the upper part of a volcanic cone.' 

 But in the case of the majority of intrusive masses the molten roclc 

 has been forced in against greater resistance ; the temperature of 

 the invaded rock has thereby been raised to its local fusing point ; 

 the melted portions have been alloyed by circulation with the general 

 magma ; and in this way the intrusive mass would, step by step, 

 replace the rock invaded, and would replace that rock by igneous 

 rock of uniform composition without displacing the rock beyond 

 in the least. 



1 It should be noted that as this upper part is that which is most readily denuded 

 away during subsidence beneath the waves, true laccolites can be but rarely preserved 

 in a fossil state. 



