PENMAENMAWR. 
5 
quarrymen “ spar,” and much disliked, as they refuse to 
“ cut ” cleanly as the normal stone. I think it likely that 
Phillips refers to these when he says, “ In one of the sections 
examined a group of felspathic crystals, °f an inch in 
length, which do not exhibit the structure of plagioclase, is 
eiiclosed in the finely crystalline base.” Of these grey portions 
I procured a considerable quantity, as I thought the crystals 
would be sufficiently'large to permit of the separation and 
identification of the felspar. This hope was disappointed, as 
on account of the minute intergrowth of quartz and felspar, 
the veins are capital examples of micropegmatite. The 
felspar also is filled with an opaque white dust, probably due 
to decomposition. The amount of quartz present seemed so 
great that an analysis promised to be of interest, and so it 
turned out, for in addition to showing 6*6 per cent, more 
silica than the general mass of the stone, as analysed by Mr., 
Phillips, the prevailing alkali is potash instead of soda. The 
analyses are as follow :— 
I. 
II. 
Silica.. 
58-45 . 
. 65-1 
Alumina 
17-08 . 
. 12-9 
Ferrous oxide 
4-61 . 
4-7 
Ferric oxide 
0-76 . 
2-0 
Manganese oxide .. 
trace 
. trace 
Lime .. 
7-GO . 
. 4-7 
Magnesia 
5-15 . 
2-8 
Potash 
1-02 . 
. 3-9 
Soda .. 
4-25 . 
2-8 
Phosphoric acid 
trace 
W ater 
1-07 . 
. 1-9 
99-99 
100-7 
Specific gravity .. 
2-94 
2-72 
I.—By Mr. J. A. Phillips of stone from most westerly quarry. 
II.—By T. H. W. of grey vein from the same quarry. 
It would thus seem that a considerable proportion of the 
felspar is ortlioclase. The quartz exhibits occasional fluid 
cavities, with spontaneously moving bubbles. In addition to 
these two constituents the microscope shows a number of 
grains of pyroxene and nests of some mineral, apparently a 
zeolite, forming radiating fans of crystals, which seem ail to 
extinguish parallel to their iengtli, and are, therefore, in all 
probability orthorhombic. In some cases these blades are so 
mixed up with quartz as to suggest a simultaneous origin for 
the two minerals, and the probability is, therefore, that some 
at any rate of the quartz is of secondary introduction. 
Phillips queries whether, seeing that the quantity seems to 
increase with the decomposition of the rock, it may not be 
due to the progressive crystallization of dissociated silica in 
