312 



J. V. Elsden — Origin of Pegmatite Veins. 



other examples of similar contortions, as also in the district around 

 Solvesburg, from which locality, at Siritorp, I noted the appearance 

 represented in Fig. 4. 



The explanation demanded by these phenomena seems to be that 

 the granite magma was traversed by streaks and bands of different 

 composition from it. Now a streaky or banded magma can only 

 occur in one of two ways. It may be due to the unequal mixing of 

 the magma in the first instance, forming the so-called constilutions- 

 schlieren, or mischungsscJdteren of Eeyer ; or it may be caused by an 

 originally homogeneous magma becoming streaky in the final stages 

 of its existence in a molten state. With regard to the first of these 

 causes, we are reminded of the phenomena observed in the process 

 of glass manufacture, and of the difficulty experienced in obtaining 

 glass free from striation or wreath. It would, however, be very 

 difficult, on such an assumption, to explain why the strise are so 

 uniform in mineralogical character ; so that we are driven to a con- 

 sideration of the second hypothesis. 





Fig. 4. 



-Pegmatite vein in grauite at Siritorp, near Solvesburg. 

 4 feet. 



Scale, 1 inch 



An examination of a number of thin slices prepared from the 

 above-mentioned rocks throws light upon the order of the different 

 consolidation phases. The normal rock has a simple constitution. 

 Biotite and sphene are the chief products of the earliest crystallisa- 

 tion ; while felspar, chiefly microcline, and quartz seem to have 

 struggled together for the mastery in the final stages. Accessory 

 minerals, such as occasional hornblende, some magnetite, apatite, 

 and a little zircon do not occur in sufficient quantity to affect the 

 character of the rock, and the same may be said of certain pneumato- 

 lytic minerals, of which chalcopyrite is the most conspicuous 

 example. Dark basic secretions are abundant, and are distinguished, 

 like the pegmatites, by evidences of considerable movement during 

 consolidation. They are frequently drawn out into irregular elon- 

 gated forms, and at Hasteryd thej' may be seen pressed out into 

 lenticles along the foliation planes. These dark patches are typical 

 quartz diorites, containing an abundance of well-formed brown 



