458 F. H. Lahee — Metamorjpliism and Geological Structure. 



lens-shaped masses.* (See fig. 2.) Some of the dikes are 

 straight ; others have elbow-bends ; and still others appear to 

 be highly contorted (see figs. 2-12). 



As regards contact phenomena, the margins of these dikes, 

 although ordinarily pretty sharply defined, may display bend- 

 ing in a narrow zone. The edges are often jagged or otherwise 

 irregular, in a way to suggest that the schists were torn, and 

 not broken, at the time of irruption. Elongate, bent, shredded 

 inclusions, as frequent here as in the granite, generally trend 

 parallel to the walls. Within the smaller dikes, not more than 

 a few inches in thickness, the central zone may consist of 

 nearly pure quartz, or of quartz and muscovite, and the feld- 

 spars with a little quartz and muscovite are then limited to the 

 marginal portions. Biotite and garnet were also observed to 

 occur more particularly near the contact. 



Fig. 2. 



Fig. 4. 



Fig. 3. 



Fig. 2. Lenses of pegmatite in schist. (Seen on the Bonnet.) 

 Fig. 3. Dikes of coarse granite cutting schist. (Tower Hill.) 

 Fig. 4. Dike, largely of quartz with a little feldspar, cutting across 

 schistosity. (The Bonnet.) 



Many of these apophyses show a progressive increase in 

 acidity away from their point of origin, and this modification 

 may go so far that they are composed of quartz only near their 

 tapering extremities. In other words, the pegmatite dikes 

 may grade into quartz veins. 



The country rock may exhibit evidences of exomorphic 

 alteration in the recrystallization of its constituents, in the 

 formation of knot-like bunches of quartz up to one-half an inch 

 across, or in the introduction of such minerals as feldspar and 

 especially muscovite and sericite.f It may possess a charac- 

 teristic greasy lustre consequent upon such changes. 



* Similar successions of lenticular masses are described by E. S. Bastin. 

 (Quartz and Feldspar Deposits of Maine, U. S. G. S., Bull. 315, p. 383; 

 1906, p. 384.) 



f Sericite is no doubt the result of dynamic metamorphism, an effect of the 

 intrusion of the pegmatites. 



