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morphic, prismatic ;in<l tabular plagiocla se-phenocrjsts are often lying 

 together in groups and possess an excellent zonal structure showing 

 itself already in obliquely transmitted ordinary light by parallel 

 strongly luminous stripes and in which acid and basic /.one- frequently 

 interchange. The same twin-laws arc found as in the preceding rock; 

 the albite- lamellae sometimes lit into each other with irregular inden- 

 tations, in consequence of which through the growing together of several 

 crystals a confused polarization-image arises. The plagioclase of the 

 phenocrysts belongs to somewhat more basic mixtures than in the 

 preceding dacite; the basicity falls to that of basic labrador, but on 

 the other side approaches thai of andesine. The crystallization of the 

 basic plagioclase had already ended before that of the amphibole- 

 pbenocrysts, but for a time coincided w ith il ; the more acid plagio- 

 clase on the other hand often contains amphibole-prisms inclosed. 

 According to the dimensions the plagioclases in this rock may be 

 divided into three groups: '/ crystals with an average diameter of 

 1..") nun., generalij strongly laden with colourless and brown glass- 

 inclusions and often entirely tilled with these, with the exception of 

 a peripheric zone; h crystals, on an average twice as small and in 

 winch the brown glass-inclusions are very rare, the colourless ones 

 on the whole being much less numerous - 1% c. still smaller prismatic 

 crystals, which always show a zonal structure and a twinning after 

 the albite- and Carlsbad-law, and which gradually pass into the 

 youngest and smallest skeleton-shaped, mostly Huidally arranged 

 feldspars of the glass-base. Whilst l> lias ^till to he reckoned among 

 the real phenocrysts, c had better he placed in the second generation. 

 As appears, the boundary-line between the crystals of the L s< and 

 2 1 " 1 generation can with the feldspars not he drawn exactly, though 

 there i> no doubt about the extremes. Higher up we mentioned the 

 appearance of two kinds of glass-inclusions, which also here often 

 assume the form of negative crystals: 1. of almost colourless ones, 

 which seem to have a very light pink tinge and sometimes contain 

 numerous microlites and 2. of brownish grey one--, which are 

 strongly spherolitically devitrified. The nature of this brown glass, 

 also occurring in patches in the colourless glass-base, is not quite 

 obvious. Only it seems to he certain that the brown glass has no 

 genetic relation with the colourless. But then these two sorts of 

 glass must have existed side by side as far back as the intratelluric 

 period, as they occur by the side of each other in the plagioclase 

 and amphibole phenocrysts. Therefore the brown glass probably con- 

 tains dissolved foreign matter, or, what is perhaps more acceptable, 

 it is a product of intratelluric liquation. 



