220 



conspicuously porphyritic by the occurrence of large felspar crystals. 

 These crystals appear often clear and glassy on a fractured surface and 

 the sections thus exposed are generally longer than broad. The tabular 

 form is not very pronounced ; nevertheless, there is a tendency to this 

 form, the brachy-pinacoid being the best developed face. The prismatic 

 angles are often rounded. Sections of this felspar measuring an inch 

 in length are not uncommon. Very often the crystals are binary 

 Carlsbad twins, and at a time when this type of twinning was regarded 

 as characteristic of orthoclase this feature led to the belief that orthoclase 

 was present in the rock. The crystals referred to invariably show the 

 twin-striation of plagioclase on the basal cleavage in addition to the 

 appearance indicative of Carlsbad twinning. The writer is not aware of 

 the existence in Britain of any other plagioclase in which the co-existence 

 of twinning on the Carlsbad and albite types can be easily recognized by 

 the naked eye. Cleavage flakes of this felspar do not give absolutely 

 constant extinction, but they always indicate a felspar belonging to the 

 labradorite-bytownite series. One parallel to (010) gave an extinction of 

 28 in a minus direction, and showed in convergent light an optic axis 

 just outside the field of view. Another parallel to (100) gave an extinction 

 of 13 referred to the trace of the cleavage parallel to (010). These two 

 observations agree very well and point to a felspar of the composition 

 Ab A An 2 . The specific gravity lies between 27 and 271, and appears to 

 be generally nearer the former than the latter. The fusibility is about 

 1 of Szabo's scale, a fact which points to by townite rather than labradorite ; 

 the soda-flame, on the other hand, is stronger than in typical bytownite. 

 We may conclude from these facts that the felspar is a somewhat basic 

 ladradorite. 



This felspar often shows under the microscope a zonal structure due 

 to the arrangement of minute inclusions. One section of a large felspar 

 (lin. by in.) taken approximately parallel to the basal plane illustrates this 

 phenomenon in a very perfect manner. Two kinds of inclusions may be 

 recognized (1) extremely minute and more or less spherical cavities, (2) 

 larger inclusions, filled with opaque material and having more or less the 

 form of the section in which they lie (negative crystals). The former are 

 aggregated in narrow bands and define the zonal structure ; the latter are 

 not very numerous and are distributed without much regularity. The 

 central portion of the crystal is tolerably free from original inclusions. A 

 few negative crystals may be recognized, but the zonal structure due to the 

 minute inclusions is absent. In the external portion of the crystal the 

 zones are extremely numerous. They are crowded together in the direction 

 of the short diameter of the section, but they open out in the direction of 

 the long diameter. Not only are the spaces between the different bands of 

 inclusions wider in the latter direction, but the breadth of each individual 

 band of inclusions is also wider. This indicates that the crystal grew more 

 rapidly in the direction of the brachy- than in that of the macro- diagonal 

 Cixis r The more or less tabular form of the large porphyritic crystals is 



