﻿10 MISS Mi K. SESLOP AND DR. J. A. SMYTHE ON THE [Feb. I9IO, 



round them in continuous strands, and thus it is often impossible to 

 fix the limits of individual crystals. The extinction gives little 

 help in this matter, for it creeps over the field in a most erratic way. 

 Very often the felspar-laths serve as axes for the augite crystals ; 

 at times they seem to have been swept into their present position 

 and kept there by an overwhelming mass of augite. In such cases 

 the arrangement is nearly always fan-shaped, and the augite mass 

 seems to have a point of origin from which it spreads out laterally, 

 carrying the felspars with it. 



Grains of iron oxide frequently fringe these joint masses of 

 felspar and augite, and the glass which appears between them 

 is darkened by the presence of that mineral in a more or less 

 unindividualized form. Elementary growths of felspar and augite, 

 the latter strongly predominating, are also common in the glass. 



At the top of the dyke there is a comparative scarcity of felspar, 

 the few laths that occur being embedded in an apparently almost 

 amorphous glass in which, however, very small augites and grains 

 of iron oxide are found. There are the usual modifications at the 

 contacts ; the felspars are clear, and the augites occur in the same 

 beautiful skeleton forms as those found in the Crookdene Dyke. The 

 acanthus-like twists shown in fig. 1 (p. 5) are also common in 

 the better-crystallized parts near the contact, while the more 

 elementary stages show felspars surrounded by light-coloured areas, 

 mostly isotropic, except where they are penetrated from without 

 by little bars of augite. As these have been entirely replaced by 

 calcite, they show brilliant interference-colours between crossed 

 nicols, and are readily distinguished from the somewhat turbid 

 ground-mass in which they are embedded. The felspars are quite 

 fresh ; the pale areas surrounding them are usually colourless, but 

 sometimes stained yellow, and, though it seems probable that they 

 consist of felspathic material, there is little positive evidence in 

 support of this view, except the concentration of skeleton augites 

 and basic globulites in the narrow spaces between the pale areas. 

 These may, perhaps, be conveniently termed felspar aureoles. 



The modification described above is very rare, but it is perhaps 

 worthy of mention as showing the influence of physical conditions 

 on the mode of crystallization : for, if we compare this case with 

 any from the more central portions of the dyke, we see that 

 the augite is concentrated near and wrapped round the felspars in 

 the latter ; while, in the former, the basic constituents seem to be 

 repelled from the felspar laths. The order of crystallization is the 

 same in both cases, and it seems highly probable that the felspars 

 were formed before the basalt reached its present position, so that, 

 being the only solid bodies present, they acted as nuclei for later 

 crystallizations. It would be difficult to say exactly why in one 

 case they seem to have attracted, and in the other case to have 

 repelled the basic elements, but it is probable that the repulsion of 

 these from the felspars at the contacts is a viscosity effect. 

 It is hoped that future investigations may throw some light on 

 this subject. 



