DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 125 



It is surprising that many authors finding aplitic veins in 

 rocks other than granites, still remark upon them as some- 

 thing unusual ; often indeed, they do not connect them 

 with the aplites of granite. Personally, I think that aplitic 

 veins should be regarded as a normal feature of all but the 

 smallest igneous masses; and some unusual conditions of 

 cooling, or properties of the magma, must be invoked to 

 explain their absence in particular cases. 



The extension of the aplitic veins of the larger plutonic 

 rocks for a moderate distance into the surrounding country 

 rock is easily accounted for by the fact that the latter 

 must have been strongly heated, and on cooling will open 

 in cracks. Where aplites are a very prominent feature of 

 the surrounding country rock, I take it that their injection 

 is due to earth movements as above suggested. 



A few words as to the origin of pegmatite veins. Those 

 which contain no exceptional minerals, and are merely of 

 the nature of the plutonic rock, but more acid and very 

 coarsely crystalline, may have crystallised, I think, from 

 a magma very rich in water, and thus highly fluid. The 

 large size of the crystals is thus accounted for, because 

 coarseness of grain results as much from fluidity of the 

 magma as from slowness of cooling. Probably such an 

 aqueous magma would arise during the early stages of 

 crystallisation ; and it would be miscible (on the liquation 

 theory) with the general body of the magma, and with the 

 aplitic mother liquor, in the manner which will be described 

 for the Prospect mass in the next section. The very coarse 

 pegmatites, often containing rare minerals, with sometimes 

 stratified, or at least non-uniform, structure, which some- 

 times extend for great distances away from the mags, are 

 probably to be accounted for by what is generally known 

 as pneumatolytic action. This I take to be merely the 

 extrusion from the solidifying mass of dilute aqueous 



