﻿6 MISS M. K. HESL0P AND DE. J. A. SMYTHE ON THE [Feb. 1 9 10, 



developed in the Collywell Dyke, bub it is, nevertheless, quite 

 distinct here. 



In this dyke, as in those at Collywell, Tynemouth, and Morpeth, 

 there are calcite amygdaloids. These have usually a semi-lunar 

 residue of glassy material in which skeleton crystals of felspar and 

 augite and grains of iron oxide have separated out, while calcite 

 occupies the rest of the cavity. In one or two cases, however, 

 cavities have been observed, occupied either partly or entirely by 

 green material. In these there is usually a gradation, from deep 

 green at the borders, to pale green or colourless material at the 

 centre. This is emphasized between crossed nicols, when the outer 

 edge is seen to show distinct doubly-refracting fibres, the colours 

 of which diminish in intensity towards the centre of the cavity, 

 which is almost invariably isotropic in such cases. There can be 

 no doubt that the substance occupying these cavities is glass, in 

 which devitrification has begun around the margin and is travelling 

 inwards. Reference will be made to a similar occurrence in the 

 description of the Morpeth Dyke. 



Weathering of the Basalt. 



It has already been mentioned that the basalt in contact with 

 the breccia of the fissure is weathered to a depth of about 9 inches 

 into a brownish sandy-looking rock, which in the field bears little 

 or no resemblance to the original basalt. When examined micro- 

 scopically, however, the structure of the basalt is quite apparent : 

 the felspars are clear, and evidently but little decomposed ; a good 

 deal of brownish oxide of iron has separated out ; and the augite is 

 almost entirely replaced by calcite. A small amount of rutile can 

 also be detected. Analysis of this rock (dried at 110° C.) yielded 

 the following results : — 



Si0 2 38-06 



A1 2 3 16-38 



Ti0 0-96 



Fe^ 6-97 



MnO 084 



CaO 2042 



MgO 0-87 



K 2 0-56 



NaoO 1-95 



C0 2 1037 



HoO 2-31 



S0 3 0-13 



Total 99-82 



If the carbon dioxide be taken as combined with calcium oxide, 

 then the rock contains 25*57 per cent, of calcium carbonate, all, or 

 nearly all, of which has come in by infiltration and replaced some 

 constituents of the basalt. A better idea of the changes wrought 

 by weathering may be obtained by fixing one's attention on the 

 essential constituents (acidic and basic oxides) of the original 

 and weathered rocks. Calculating these in percentages from the 

 analytical data, we get : — 



