330 MR. ALFRED HARKER ON THE GABBRO [Aug. 1 894, 



plugged the channel would have consolidated without the conditions 

 necessary for the kind of differentiation described. Indeed we may 

 take- it as a general rule that the duct of a volcano is characterized by 

 very considerable everse, but an absence of inverse metamorphism. 1 

 In the present case the form of the intrusive body and the 

 manner in which the volcanic rocks follow their strike undisturbed 

 through the heart of the gabbro also indicate unmistakably the 

 nature of the intrusion. 



Again, the alleged passage from the gabbro to the granophyre is 

 seen to have no real existence, at least so far as concerns the 

 phenomena in the former rock. That modification of the gabbro 

 which in some petrographical features approaches the acid rock, 

 forms the heart of the gabbro mass, while that in actual contact with 

 the granophyre is of a highly basic variety. I propose to consider 

 the question again from the side of the granophyre, but it is quite 

 clear that the two rocks represent two distinct and successive 

 intrusions. This is, of course, quite consistent with the possibility 

 of the two magmas having been derived from different portions of 

 one deep-seated reservoir. 



Finally, I cannot accept Prof. Sollas's suggestion 2 that the 

 micropegmatite of the quartz-gabbro is due to an injection of 

 solid gabbro by the granophyre magma. There are certainly 

 veins of granophyre penetrating the gabbro, and locally numerous, 

 but these are never on the microscopic scale described by 

 Prof. Sollas at Barnavarve, and the orderly disposition of the 

 various types of gabbro in the Carrock Fell intrusion would be 

 unintelligible on the injection hypothesis. I shall have to speak 

 later of the possibility of intermediate rocks originating by the 

 admixture of acid with basic, in a somewhat different manner, 

 but I do not believe that the idea is capable of any very wide 

 extension. 



These conclusions seem to follow fairly from the phenomena of 

 differentiation described, though they can be fortified by other 

 considerations. I wish to point out, however, that these phenomena 

 may sometimes lead to conclusions which could not otherwise be 

 reached ; so that a careful survey of an igneous mass by chemical 

 tests, or simply by specific gravities, maj^ give definite information 

 regarding the field-geology of the district. Thus, the separation of 

 the areas which we have distinguished as gabbro and diabase comes 

 out distinctly in this way, the zone of basic rock which bounds the 

 gabbro area on the side of the granophyre being evidently prolonged 

 westward by Pound Knott. When thus divided, the relative age 

 of the two rocks can be decided. It might, of course, be conjectured 

 plausibly that, since veins of granophyre are abundant in the gabbro 

 and wanting in the diabase, the one rock is older and the other 



1 These terms, due to Morlot, besides having priority, seem to be more 

 pointed and less clumsy than their equivalents ' exomorphic and endomorphic,' 

 or ' exogenous and endogenous,' used by some German and French writers. 



2 Greol. Mag. 1893, pp. 551, 552. [Subsequently elaborated in Trans. Roy. 

 Irish Acad. vol. xxx. (1894) pp. 477-512.] 



