452 F. H. Laliee — Metamorphism and Geological Structure. 



peculiar to the deeper parts of mountain ranges,* and, such 

 being the case, often cut a country rock which has been ren- 

 dered schistose by dynamic action consequent upon folding. 

 On this account, actual field data show, not the hypothetical 

 pegmatite core with its apophyses, but, instead, scattered here 

 and there in the granite, numerous, relatively small, shapeless 

 or elongate segregations having blended contacts. Dikes with 

 sharply defined edges are less common. The small segrega- 

 tions of pegmatite, representing separate loci of the final 

 operation of the concentrated mineralizers, indicate that, within 

 layers parallel to the general external surface of the mass, con- 

 solidation is far from uniform. No doubt this is due in part 

 to the unevenness of the contact. 



Yery often a marginal pegmatite zone may be observed 

 between the main granite (or other plutonic) body and the 

 country rock. From such a marginal zone dikes of pegmatite 

 radiate outward. If the country rock is bedded or schistose, 

 they follow its structural planes. Contacts of this marginal 

 phase, both with the granite and with the country rock, are 

 often blended, but they become more distinct away from the 

 granite. Similar zones surround xenoliths. 



The marginal pegmatite with its offshoots into the country 

 rock is obviously a contact effect and cannot be a late phase in 

 the centripetal consolidation of the granite body. Crosby, 

 recognizing this discrimination, attributed it to the capacity of 

 the magma, at high temperature, to absorb water from the 

 country rock, this water later functioning as the main catalytic 

 agent in the ultimate crystallization of the marginal peg- 

 matite, f 



According to this theory, a wedge of magma, in its forward 

 advance into the country rock, would imbibe more and more 

 water, and would have its fluidity augmented. Consequently, 

 its penetrative efficiency and its tendency to freeze with peg- 

 matite texture would increase. Loss of heat or lack of supply 

 would bring the process to an end. Crystallization, if com- 

 mencing after the cessation of injection, would proceed from 

 the walls inward, and the first formed minerals would be more 

 basic than the last. Thus the dike would acquire an acid 



* Daly speaks of granites as being primarily mountain rocks (Daly, E. A., 



Abyssal Igneous Injection This Journal (4), xxii, p. 196, 1906, 



p. 214-215. See also Geikie, A., Text-book of Geology, 4th ed., Lon- 

 don, 1903, p. 724. 



f Crosby, W. O., and Fuller, M. L., loc. cit., pp. 348-352. Van Hise 

 writes: . . . "In many and perhaps most cases the water in the outlying 

 pegmatite dikes and veins has been largely derived from the sur- 

 rounding rocks." (Van Hise, C. R., A Treatise on Metamorphism, U. S. 

 G. S., Monog. xlvii, 1904, p. 728.) On the other hand, Geikie (op. cit., 

 p. 768) and Harker Cop. cit., p. 303), referring to the investigations of others, 

 state that hot intrusions expel volatile substances from the country rock. 



