﻿8 J1ISS M. K. HESL0P AND DR. J. A. SMYTHE ON THE [Feb. 1 0. 10, 



The felspar is thus very closely allied to anorthite (calculated for 

 CaO, A1 2 3 , 2Si0 2 : SiD a = 43-25, Al 2 3 =36-66, CaO=20-09), 

 and the differences may be in part accounted for by the slight 

 weathering of the mineral, and by the presence of basic inclusions 

 in the crystals. 



Under the microscope these anorthite aggregates are seen to con- 

 sist of large twinned crystals irregularly intergrown, without crys- 

 talline outline, and profusely studded with small inclusions of dark 

 glass. These are generally arranged parallel to a cleavage. They 

 are usually oval in shape, but some have the Outlines of the crystal 

 or are formed by the intersection of the principal cleavages, and 

 may be seen in some sections to be of considerable length. Careful 

 examination with high powers (y^-inch objective) reveals the fact 

 that they are not homogeneous, but contain, apparently, both augite 

 and iron oxide. Between crossed nicols they often give definite 

 interference -colours, and in one or two rare cases show a radial 

 arrangement of fibres. 



There are often two sets of twinning-planes in the felspars, the 

 striae meeting at angles varying from 90° to 40° in different sections ; 

 as a rule, one set only continues after the two have met. The more 

 perfect crystals are traversed by irregular cracks, roughly parallel, 

 which seem to have no constant relation to the outlines of the 

 crystal. Some of these cracks show brilliant interference-colours — 

 those of calcite in one or two instances. 



Some of the felspars are absolutely shattered (see PI. I, fig. 1), 

 others broken and slightly faulted (see PI. II, fig. 2), but in no 

 case can it be proved that the dislocation is great — in fact, the 

 crystals seem to have burst in situ. The shattered fragments are 

 embedded in a ground-mass composed of felspar-dust, much of which 

 has decomposed into a dark granular mass. Very occasionally 

 threads of much altered basalt are seen penetrating a little way 

 into this ground-mass, and there are also circular patches of glass 

 similar to those described as occurring in the amygdaloids. Here, 

 however, the material is colourless or faintly yellow, and shows a 

 spherulitic arrangement between crossed nicols, though the inter- 

 ference colours are very faint. 



Threads of basalt, containing well-defined felspar-laths, have 

 found their way among the groups of crystals in the aggregates, 

 and now serve to cement the pieces together. In these threads the 

 augite has been entirely replaced by calcite, and there is but little 

 iron oxide. 



Where the felspars have come into contact with the basalt there 

 is a distinct zone, broadest in those crystals that are situated on the 

 outside of the aggregates ; but even the penetrating basaltic threads 

 seem to have furnished material to the felspar surfaces with which 

 they have come into contact. These zones are often very ragged, the 

 edge being indented and penetrated by the ground-mass crystals. 

 No zoning is to be seen on those fragments which are embedded in 

 the felspar-dust, and, indeed, not on any to which the basalt has not 

 had access (cf, fragments marked A & B in PI. I, fig. 1). 



