728 A TREATISE ON METAMORPHISM. 



ing rocks which have been long affected by the batholiths become highly 

 heated as a result of direct conduction and in consequence of convection 

 through water. The occluded water which undoubtedly exists in the parent 

 mass, at the moment of crystallization is in large measure expelled. As 

 crystallization continues the residual magma gradually becomes more 

 watery. Thus a considerable part of the water, the presence of which is 

 evidenced by the character of the pegmatites, is derived from the magma 

 itself. Doubtless the pegmatite dikes in immediate connection with the 

 great parent masses of igneous material, which closely resemble ordinary " 

 igneous dikes, largely and perhaps predominantly derive their water from 

 the igneous rock itself. 



As the pegmatites close to the central mass solidify, a large portion of 

 the water is expelled and travels outward to help form the peg'matitic rock 

 having a more distinctive vein character; but in many and perhaps most 

 cases the water in the outlying pegmatite dikes and veins, which more 

 and more assume the character of ordinary veins, has been largely derived 

 from the surrounding rocks. We thus have completely explained the 

 variations from the true igneous dikes by imperceptible gradations to 

 materials which have the unmistakable characters of aqueous mineral veins. 



This injection-cementation theory of pegmatization gives an adequate 

 explanation of the extreme coarseness of crystallization which the pegma- 

 tites frequently exhibit. The rocks in which the pegmatites form, both the 

 parent crystallized igneous mass and the surrounding rocks, are highly 

 heated, and hence during the closing pegmatitic stages, even if the masses 

 be small, the temperature falls so slowly that there is ample time for the 

 formation of very coarse crystals. Also it is well known that there is no 

 limit to the size to which the crystals may grow from water solutions. The 

 separation of the crystalline material in many pegmatite veins is more 

 nearly analogous to crystallization from a solution than to the solidifica- 

 tion of a solvent (see p. 113), and this analogy, so far as it exists, is 

 favorable to very coarse crystals. 



FUSION AND ABSORPTION. 



In connection with metamorphism in the deep-seated zone where igne- 

 ous rocks are among the agents, the question of the fusion of the intruded 

 rocks should be considered. It has been held by Mallet and by others 

 that heat produced by mass-mechanical action is a sufficient cause to pro- 



