F. H. Lahee — Metamorphism and Geological Structure. 451 



Let us consider, by way of illustration, an ideally regular 

 mass of granitic magma which has been injected into a relatively 

 cool rock formation. We may take it for granted that such 

 an intrusion cannot lose heat uniformly throughout its volume. 

 The marginal regions must cool more rapidly than the interior. 

 Hence, ceteris paribus, the processes of cooling and of con- 

 solidation must progress inward from the contact with the 

 country rock. In other words, these processes must be cen- 

 tripetal* 



tin til the magma has quite hardened as a granite, there must 

 be a constantly diminishing residuum which, as appears from 

 the study of many examples, is known to have a tendency to 

 increase in acidity. Since the mineralizers do not, except to a 

 slight degree, enter into the reactions which they assist, their 

 relative quantity in the diminishing remainder of the freezing 

 magma must necessarily grow. Thus we have a liquid residuum 

 in which the proportion of acid constituents and of catalyzers 

 is steadily increasing, f 



Without the catalytic agents such minerals as albite, ortho- 

 clase, and quartz would be so viscous that they could not crys- 

 tallize. With these agents present, however, the molecular 

 mobility of the remaining acid minerals is so augmented that 

 crystallization is not only possible, but is even assisted to such 

 a degree that the resulting crystals may be much larger than 

 the average grain of the main body of the rock. In this way 

 pegmatite may originate as a late product in the consolidation 

 of the magma.J 



In the ideal case, then, according to this explanation, we 

 might expect to find a mass of pegmatite in the heart of the 

 intrusive body. If the tension due to cooling had developed 

 joints, dikes of pegmatite might extend out as apophyses from 

 the inner pegmatite segregation into the normal granite. The 

 first type would have blended contacts ; theoretically, the 

 second type might possess sharp contacts where the granite had 

 already hardened, at some distance from the central segrega- 

 tion. When derived in this way, pegmatites may be called 

 residual. 



Here it is necessary to observe that an intrusive body of 

 ideal regularity of shape, such as we have been considering, is 

 not natural. The great granite masses, being usually of the 

 batholithic variety, are highly irregular. Moreover, they are 



* Compare Crosby, W. 0., and Fuller, M. L., The Origin of Pegmatites, 

 Tech. Quart., ix, p. 325, 1896, p. 351. 



f See, for example, Van Hise, C. K., op. cit., p. 728. 



X The pegmatite mass found in the Quincy quarry, Mass.. and recently 

 described by Warren and Palache, is probably of this type. (Warren, C.H., 

 and Palache, C, The Pegmatites of the Kiebeckite-iEgirite Granite of 

 Quincy, Mass., IT. S. A., Proc. Am. Acad., xlvii, 125, 1911, p. 146.) 



