﻿Vol, 66,^ DYKE AT CE00KDENE (XOETHTJMBEELANd). 15 



The porphyritic felspars are much smaller than those of the 

 Tynemouth Dyke, and they occur in groups of two or three inter- 

 grown or in single crystals. They are always surrounded by a 

 zone which, as a rule, furnishes the crystal faces. 



The green patches are very peculiar, and imitate on a' small scale 

 the unique structure found in the dyke at St. Oswald's Chapel, 

 near Hexham. There is no gradation from the normal clear glass 

 with its innumerable microlites, to the green, almost amorphous 

 type ; but patches of the latter are embedded, like any crystal, in 

 the normal glass. They are singularly free from skeleton forms 

 and globulites, but frequently enclose felspar laths and grains of 

 augite. In some cases they are almost rounded, and have circular 

 cracks. 



The amygdaloids are usually darker at the circumference than 

 at the centre, and between crossed nicols the latter remains 

 isotropic, or is only faintly illuminated, while the circumference 

 shows bright interference-colours and a spherulitic arrangement 

 of fibres. The same gradation of colour is found in the irregular 

 patches as in the am} T gdaloids. 



A curious variation of texture occurs in this rock for no 

 apparent reason. There is no evidence of chilling, but the ground- 

 mass suddenly changes to a minute intergrowth of felspar, augite 

 and iron oxide, with apparently no glassy base. Crystals of felspar 

 and augite are embedded in this crystalline base, in which also 

 there are rounded grains and small crystals of olivine. Here, as 

 elsewhere, the gas-cavities are filled with green glass, and are 

 surrounded taugentially by the very small felspar laths which 

 characterize this part of the dyke. In one case, small laths pene- 

 trating a sphere of glass are all directed towards the centre at almost 

 equal distances, forming felspar radii in a glassy sphere. 



This close-grained variety of rock seems to be characterized by 

 the predominance of small, well-defined crystals of iron oxide, of the 

 same order of magnitude as the small felspars and augites. This 

 local variation is remarkable, as the bulk of the iron oxide is still 

 in the globulitic stage ; but, for some reason, the conditions seem 

 to have been so modified in this locality that all the minerals had 

 equal facilities for crystallizing out. There is a distinct wavy line 

 of junction between this and the ordinary type of basalt, and the 

 transition from one to the other is perfectly abrupt. The occurrence 

 is strongly suggestive of the patches of a micropegmatitic inter- 

 growth of felspar and augite found in the Cleveland Dyke. These 

 patches are very small and usually rounded ; they are embedded in 

 the normal basalt, and the transition from one form to the other is 

 abrupt as in the case just described, but on a very much smaller scale. 

 It seems probable that the phenomenon is due to the local preva- 

 lence of conditions favourable to the formation of an eutectic, with 

 the resulting simultaneous crystallization of all the constituents. 



No equivalent structures have been found in any of the other 

 dykes dealt with in this paper. 



