172 



W. H. HOBBS METAMORPHISM OF BERKSHIRE SCHISTS. 



largements are visible under a lens, OAving to the unequal distribution of 

 graphite (number 3574). The commonest case of enlargement is that in 

 which there is but one secondary growth visible (figure 2, B, C, D). The 

 original growth shows crystal boundaries and is sharply outlined against 

 the enlargement, which has generally an irregular boundary and con- 

 tains more sericite, interpositions of ore, etc. The enlargement is gen- 

 erally at least of a more basic feldspar than the core, as shown by the 

 extinction angles. In the growths figured, B is evidently cut near the 

 brachypinacoid and probably near the base, as in this latter instance 

 the extinction angles are nearly the same and closely parallel to the 

 long side. It is thus probable that both feldspars are intermediate in 

 composition between oligoclase and andesine. In D is represented an 

 unusual growth, of which the core is simply twinned, though the enlarge- 

 ment is not (see plate 3, figure 2, A). In the same section is another 

 growth in which core and rim are twinned alike. I have noticed some- 



A h C if 



Figure 2.— Secondary Enlargements of Plagioclase. 



A, B, Cand D are examples of secondary enlargements of plagioelase which occur in the schist 

 of Mount Washington, near Joyceville (number 3104). / is the core in each case; //atod ///are 

 the enlargements. .4 = rounded mottled core with two enlargements; B C= examples of core 

 with crystal boundaries; D = unusual instance of twinned core surrounded by an untwinned en- 

 largement. 



what analogous cases in the growths of epidote around allanite from the 

 porphyritic granite of Ilchester, Maryland* the core of allanite being 

 twinned, though the encircling oriented epidote is a single individual. 

 In the gneiss of Warner mountain, east of Sheffield, Massachusetts, such 

 growths occur, and here the epidote has been observed twinned like the 

 allanite in one instance, and in another the allanite core is twinned and 

 the epidote untwinned. Van Hise f has figured a grain of feldspar with 

 ah enlargement, and both are twinned alike. Wolff states that in the 

 enlarged feldspars of the conglomerate schists of Hoosac and Hear moun- 

 tains, twins are sometimes common to both core and rim, but also are 

 found only in the core.J Judd || describes and figures an enlargement of 



* Am. Journ. Sci., 3d ser„ vol. xxxviii 1889, pp. 223-228, figs. 1 and 2. 



fBull. no. 8, II. S. Geol. Survey, pt. ii; also Am. Journ. S.-i. 3d ser., vol. xxvii, L884, p. 399. 



JLoc. cit., p. 182. 



HQuart. Journ. Geol. Soc, xlv, p. 186, pi. vii, figs. I and 2. 



