412 Transactions. — Geology. 



sembling the matter in solution, since the temperature and 

 pressure required for this would not be such as would occur 

 in a dyke which reached the surface. We can hardly suppose 

 that the dyke suffered alteration while it was cooling, owing 

 to the presence of imprisoned hot water, but, if it were the 

 case, then it would explain all the facts. The water saturated 

 with mineral matter would have both a disintegrating and a 

 restoring effect. It would account for the presence of altera- 

 tion products, and the deposition round a corroded crystal 

 would proceed as the water cooled. There would be no 

 sensible alteration in the ground-mass, though some of the 

 material might be drawn from it. It is usually the case that 

 the ground-mass is more acidic than the porphyritic mineral, 

 and so the new outside layers ought to be more in accordance 

 with this than the kernel. The fact that the twinning extends 

 faintly into the periphery would not be contrary to this 

 hypothesis, as there is evidence that twinning is a structure 

 impressed on minerals as they cool. This is perhaps a 

 hazardous suggestion, but the case seems difQcult to explain. 

 It may be that these anomalous crystals were part of an old 

 lava-flow which had suffered weathering near the surface, 

 and then been buried under subsequent flows ; and that, as 

 the dyke penetrated it, parts of it had been caught up and the 

 crystals afforded nuclei for crystallization, just as a piece of 

 alum put into a solution of an alum begins to crystallize afresh. 

 All the phenomena observed would be explained by this ; but 

 it is rather hard to conceive that the occurrence should be 

 so general throughout the dyke if produced by this means 

 alone. 



Augitc. — Augite crystals occur abundantly as rounded 

 grains, but occasionally in lath-shaped forms up to Jin. in 

 length. They are of a brownish colour in ordinary light, but 

 at times they have a purplish tinge, and in these cases there 

 is very faint pleochroism. They contain inclusions of mag- 

 netite, which are usually absent from the periphery. Their 

 characteristic features are the zonal structure, which has been 

 dealt with before, and a remarkably perfect cleavage. In 

 some cases it approaches the perfection of that of diallage. 

 In the sections which show this there is only one set of 

 cleavage-planes, but in those which show two sets it is not so 

 perfect by any means, and becomes mere irregular cracks. 

 The sections which show one set of parallel cleavage-cracks 

 will be parallel to the axis of c. In estimating the extinction- 

 angles with these the maximum result recorded w^as 44°. The 

 crystals which showed this w-ere usually lath-shaped. Those 

 which showed two sets were short and idiomorphic. In one 

 of these latter the extinction was symmetrical to the cleavage- 

 cracks, and also parallel to a twin lamella running through 



