part 3] 



METAMORPHISM IN THE MONA COMPLEX. 



345 



(and this may be the case even where it is folded), thus greatly 

 accentuating its already banded aspect. Some, however, truncate 

 that banding and foliation at various angles (fig. 5) ; while some- 

 times a conformable seam will suddenly turn round, and cut across 

 the gneissic banding at angles of 60° or even 90° (fig. 4). At the 

 time of their formation, therefore, the gneiss, already very near to 

 complete crystallization when rolled out, must have been solid. 

 Now, it is a constant feature of these pegmatites that the gneiss in 

 contact with them is much more basic than it is elsewhere, being 



Fig. 5. — Pegmatites with foliated ultrabasic encasements in basic 

 gneiss : roadside south of'P'' of Pandy Treban (x 5). 



often composed almost entirely of hornblende, with a very little 

 albite, and rather more ilmenite and sphene than usual. The 

 hornblende is the same green aluminous variety as in the body of 

 the rock. Each little pegmatitic seam, in fact, is enclosed within 

 a pair of black skins or encasements of thicknesses ranging from 

 g to half-an-inch, thus accentuating still further the banded 

 aspect of the gneiss. Moreover, where the acid seams turn round 

 and cut across the banding, they are duly accompanied by their 

 black encasements. From the respective thicknesses, it is estimated 

 that a mean of the composition of pegmatite and selvage must be 

 very nearly the average composition of the adjacent portions of 

 the gneiss. It is, therefore, evident that this dark encasement is 

 an ultrabasic residuum, left after the rock had been robbed of the 

 elements of albite. These pegmatites, accordinolv, cannot be 



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