Vol. 50.] OF CAEEOCK FELL. 327 



those described above at Carrock Fell. Brogger points out that 

 in his area rocks genetically connected may differ widely in 

 mineralogical as well as in chemical composition. Thus, in his 

 olivine-gabbro-diabase, the ferromagnesian minerals are pyroxene, 

 olivine, and biotite ; in the camptonite, which is an offshoot of it, 

 brown hornblende largely predominates. He justly concludes that 

 differentiation has " taken place in a liquid magma, even before 

 crystallization of any importance had begun." In the Carrock Fell 

 gabbro the case is quite different. Here the different varieties of 

 the rock consist invariably of the same minerals, only in different 

 relative amounts. 1 I have already remarked that the felspar is of 

 the same variety in almost the whole of the rocks examined. If 

 the differentiation of the magma had been completed prior to any 

 crystallization, we should expect different parts of the magma to 

 have given birth to different varieties of felspar. The complete 

 want of olivine, even in the ultrabasic varieties of the rock, is 

 another fact that would be difficult to explain on the hypothesis 

 that the magma was differentiated first and then crystallized ; and 

 indeed such a sequence of events seems to be quite inconsistent 

 with the phenomena that I have described. On the other hand, to 

 suppose that the differentiation was brought about by a migration 

 of minerals already crystallized out would raise obvious difficulties. 

 No cause can be imagined to produce such a movement of crystals 

 to the margin of the reservoir. The only alternative is to suppose 

 that the differentiation took place by diffusion in a fluid magma, 

 but not as a process distinct from and quite anterior to crystallization. 

 It was, as I believe, effected in a quasi-saturated magma concurrently 

 with the crystallization of the earlier-formed minerals. 



The remarks just made apply, as stated, to the particular case 

 under discussion, but it seems probable that many of the examples 

 of differentiation recorded by Vogt and others will fall under the 

 same head. The characteristic of all is that the several constituents 

 are concentrated in a definite order, which is identical with the order 

 in which they crystallize out from the magma (Rosenbusch's " order 

 of decreasing basicity "). The concentration is greatest for the 

 minerals belonging to the earliest stage of crystallization, viz. 

 apatite, ilmenite, magnetite, etc. The minerals of the second stage, 

 the ferromagnesian silicates, are less strongly concentrated. In 

 such cases, when differentiation apparently " has been determined 

 by, and is dependent on, the laws of crystallization in a magma," 

 it seems reasonable to seek the cause of differentiation in the 

 crystallization itself. 



The conditions introduced by this simple hypothesis have no 

 analogy with those of a dilute solution ; and, though we may 

 conveniently employ the terminology of solutions in speaking of it, 

 it does not follow that we need frame any precise theory of the nature 

 of an igneous rock- magma. The process of differentiation is brought 



1 Quartz and orthoclase are wanting altogether in the most basic varieties, 

 but these, being the very latest products of consolidation, do not enter into the 

 argument. 



