﻿4 MISS M. K. HESLOP AND DE. J. A. SMYTHE ON THE [Feb. I910, 



but sometimes the extinction is wavy and interrupted. The augite 

 is curved and wrapped round the felspar-laths in a peculiar way 

 (see PI. II, fig. 1), and the iron oxide occupies the spaces left by 

 the other two minerals. The general texture is shown in the 

 figure, and particular attention may be drawn to the curving of 

 the augites, since apparently the only other local dyke that shows 

 this feature to the same extent is the Collywell Dyke, although it 

 has been observed occasionally in the Tynemouth and Acklington 

 Dykes. 



The glassy base, in which the felspars are embedded, is clear 

 and colourless, as a rule, and has little or no action on polarized 

 light. There are, however, patches of green amorphous glass which 

 will be more fully dealt with later. The clear glass is crowded 

 with skeleton felspars and augites which have definite interference- 

 colours ; besides these there are innumerable black grains, which 

 are resolved under high powers into semi-translucent spherical 

 bodies of brownish hue, owing their apparent blackness to their 

 deep dark borders — caused, no doubt, by their high refractive 

 index producing considerable internal total reflection. They are 

 usually attached to some part of an elementary augite, and may, 

 indeed, be regarded as basic globulites. The skeleton augites in the 

 local dykes almost invariably have nodes at which there are little 

 accumulations of iron oxide, and in the rock here described the 

 globulites seem to unite to form small augites, the growth of which is 

 accompanied by the elimination of iron oxide at intervals along the 

 axes of the elementary crystals. From this it would seem that the 

 basic globulite contains, besides the augite ingredients, an excess of 

 iron oxide which is liberated in the process of building up the augite 

 crystals. A similar association can be traced in the ground-mass 

 generation of crystals, for the augites there are very frequently 

 studded with black cubes and grains of iron oxide. In other words, 

 we may trace an intergrowth of augite and iron oxide from the 

 globulitic stage upwards. 



The order of crystallization is clear, both in the ground-mass 

 crystals and in the elementary forms found in the glass. Felspars in 

 narrow laths crystallized out first, augites in rather massive prisms 

 followed, and iron oxide was left in the spaces formed by the 

 separation of the first two minerals. There was apparently a 

 certain amount of overlapping in these periods, for some felspar in 

 rather broad large crystals appears to be later than some of the 

 augite, and that mineral had probably not ceased to crystallize out 

 when the iron oxide was in process of formation. There was, 

 seemingly, very little free iron oxide, for no perfect crystals of it 

 have been seen in any of the slides examined, and even the skeleton 

 forms are not abundant. 



Contact-sections of the basalt with the neighbouring rocks show 

 some interesting modifications of structure. Fine, needle-like fel- 

 spars are surrounded by skeleton augite bars usually set perpen- 

 dicular to the felspars, in such wise that the whole has a fern-like 

 appearance with the felspar-lath as axis. This is the common form 

 of growth at the contacts. Often, however, the central axis is of the 



