146 PROCEEDINGS OF THE AMERICAN ACADEMY. 



individuals, between the grains of broken quartz and in minute fracture 

 zones. By admitting this mode of origin for these crystallizations we 

 are at least relieved of the difficulty of explaining how the aegirite, 

 with an undoubted tendency to develop in the main at a late period 

 with or just preceding the quartz, should also have been the first se- 

 cretion from the magma. It simplifies the sequence of crystallization. 

 Future study of the porphyries, etc., associated with the granite will 

 doubtless throw further light on this question. 



The zonal structure of the pegmatites and the mineral composition 

 of each shows that there was a progressive segregation of mineral- 

 forming compounds. In each zone, however, we notice essentially the 

 same sequence of crystallization as in the granite, as if such zone had 

 to some extent at least crystallized as an individual unit. 



Following out these considerations relative to the crystallization of 

 the granite magma, etc, we may, perhaps, find a clue to the mode of 

 the formation of the pegmatite masses. The earlier compounds which 

 separated from the magma resulted in a gradual concentration of the 

 more volatile and liquid constituents and of the elements or radicals 

 which tend to accompany them. These, becoming segregated at certain 

 points, determined by unknown local factors, tended to escape by 

 rising through the surrounding material. In so doing they became 

 elongated in form by reason of the drag exerted on their margins by 

 the more viscous material enclosing them. Hence the pipe-like form. 

 This idea of movement in the pegmatitic material finds support in 

 the presence of the fluidal structure noted as characteristic of 

 parts of the dark marginal zone. This structure also suggests 

 that the band belongs perhaps as much to the granite as to the 

 pegmatite proper, as does also its texture, mineral composition, and 

 gradual merging into the granite through portions of its extent. The 

 pipe in the Ballou quarry is far below even the present surface of the 

 granite, nor is there any indication anywhere that these pegmatites 

 ever succeeded in reaching the surface. They appear to have been 

 imprisoned in the granite by its solidification. 



While the exact process of formation of the zonal structure cannot be 

 accounted for with certainty with our present knowledge of the factors 

 determining rock differentiation and crystallization, the hypothesis 

 which seems to the writers to be best in accord with the observed facts 

 is as follows : — In the case of each pipe the entire mass of pegmatitic 

 material from which they developed having segregated from the gi-anite 

 magma as above outlined, it retained by reason of its different compo- 

 sition (greater per cent of water, etc.) its liquidity and power for further 

 differentiation, etc., and perhaps retained them even after the enclosing 



