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152 



and LEHMANN (1) evidently point to the conclusion that the twin lamellatiou 

 is at any rate in part a secondary structure. Professor JUDD has 

 argued in favour of the secondary origin of the lamellation in the 

 felspars of the Tertiary gabbros of the West of Scotland. (2) LOSSEN 

 has studied the metamorphic phenomena exhibited by the diabases 

 (dolerites) of the Hartz in great detail and with the most important 

 results, so far as the subject now under discussion is concerned. The 

 granular diabase of this region is a plagioclase-augite rock with ophitic 

 structure an ophitic dolerite as we should term it. In the district where 

 regional metamorphism has operated with the greatest intensity (S.E. Hartz), 

 the original lath-shaped plagioclase, proved by 0. SCHILLING to be a typical 

 labradorite (Ab, An 2 ), may often be seen to pass into an aggregate of 

 colourless and for the most part simple (untwinned) grains. This mosaic-like 

 aggregate (albite-mosaic of LOSSEN) in the rock, is sometimes seen to be 

 continuous with a corresponding aggregate which occurs in the form of veins, 

 and in which the grains are sufficiently large to be examined by optical and 

 chemical methods. In such cases they have been found to be albite, and the 

 general conclusion has been reached that this felspar is constantly formed in 

 connection with the metamorphosis of labradorite. That the plagioclase- 

 mosaic is really a secondary formation and not a product of the consolidation 

 of the igneous magma is proved by the fact that it is entirely absent from 

 the normal ophitic do-lerites of the Hartz and all other districts, and occurs 

 only in regions which give independent evidence of having been profoundly 

 affected by metamorphic agencies. Epidote and calcite are frequently found 

 associated with the secondary felspar-mosaic, and the original lime of the 

 labradorite is thus accounted for. The individual grains of the aggregates 

 are as a rule perfectly colourless and devoid of twinning. Hence a great 

 difficulty arises in distinguishing between quartz and felspar, as both minerals 

 agree approximately so far as refraction and double-refraction are concerned. 

 When the grains are sufficiently large convergent light may be used, and 

 their biaxial or uniaxial character determined. 



If then we combine the researches of LOSSEN, KLOOS and CATHREIN we 

 seem justified in concluding that a basic felspar may be replaced by albite 

 and epidote, zoisite or a zeolite of the scolecite type. It seems also 

 probable that minerals of the scapolite group may arise in consequence 

 of the metamorphism of basic plagioclase. We are ignorant at present as to 

 the precise conditions under which the different kinds of replacement may be 

 effected, but the mosaic structure is certainly often connected with regional 

 metamorphism. 



We have now to consider British illustrations of the phenomena above 

 referred to. The gradual passage of glassy labradorite into opaque white 

 saussurite is well illustrated in many of the Lizard gabbros. The commence- 

 ment of the change is indicated by the development of chalky-white spots in 

 the glassy felspar substance. These increase in abundance until all traces of 

 the original felspar have disappeared. Great difficulty is experienced when 



(1) Die Entstehung dor altkystallinischen Schiefergesteine, p. 196, and Taf. C. fig. 4. 

 (-0 Q.J.G.S. Vol. XLL, p. 366. 



