G PENMAENMAWE. 
the process of change. I think, however, that most of that 
in these grey veins must be contemporaneous with the felspar 
with which it is so intimately and intricately mingled. 
An attempt to determine the nature of this constituent 
by acting on the powdered stone with acid and separating the 
constituents was not successful, on account of the large 
amount of iron which was also dissolved out, as well as on 
account of the decomposition products of the felspar being 
attacked. 
In the Neues Jahrbuch (Beilage Band) of this year is a 
paper on some rocks of the Southern Black Forest, in which 
mention is made of certain parts where, along cracks, labra- 
dorite is changed into an intimate mixture of a more highly 
acid felspar, namely an albite, and a zeolite almost perfectly 
free from alkali. I think, however, that the appearances 
here are somewhat different, and the bulk analysis of the 
veins seems to show that this explanation will not hold good. 
The greater acidity of these veins, as compared with the 
mass of the rock, recalls certain grey or red veins in our local 
Rowley Rag, which Mr. Allport has described to this Society 
in past years. An analysis has revealed the fact that these 
also are much more (9°/°) acid than the bulk of the rock, 
and moreover, that they contain about 11% of alkalis. I 
hope, however, to make some recent observations on these 
the subject of a future paper, so will not further refer to 
them here. 
The order of consolidation, therefore, appears to be the 
usual one—first the more basic minerals and then the more 
acid, so that the magma becomes progressively more and more 
acid in the process of crystallization. This crystallization is 
accompanied by contraction, which is further increased by 
the contraction due to the cooling of the mass, so that the 
formation of cracks is quite a conceivable thing, and it would 
appear that the still fluid or viscous residuary portion of the 
mass has filled these, forming the grey veins. It has been 
ascertained that the glassy base of many rocks is much more 
acid than the total rock, as in the case of the great Cockfield 
Dyke, mentioned by Mr. Teall, in his paper on some north 
country dykes, in the “ Quarterly Journal of the Geological 
Society” for May, 1884, where the general analysis of the rock 
gave 58-1 per cent, of silica, and 4-2 per cent, of alkalis, while 
the glassy base, when as perfectly isolated as possible, gave 70*8 
silica, and 7*2 alkalis. The analysis of this dyke, as analysed 
by Mr. Stock, quoted by Mr. Teall, is strikingly similar to 
that of the rock we have under our notice to-night, except 
that it has apparently some 5 or G per cent, of alumina replaced 
by peroxide of iron. 
