304 ' REY. J. F, BLAKE ON THE 
are more or less rounded so as to obscure their regular crystalline 
form, and they are surrounded by a radiate spherulization, which 
has therefore formed rapidly; the remainder is composed of inter- 
mediate-sized crystals, and has therefore crystallized more slowly. 
This would seem to suggest that the quartz centres were either 
derived from some preexisting rock, or formed under peculiar cir- 
cumstances in the magma before its extrusion as a lava, and that 
they have acted as starting-points for crystallization on the cooling 
of the mass ; but when their influence ceased, the rest became crys- 
talline more gradually. In the other form the porphyritic crystals 
are chiefly felspar, but they do not form centres, the spherulites 
being scattered in the ground-mass and smaller in size; there is 
also quartz developed as a pseudomorph, as better seen in other 
rocks. Some of these are remarkable for the abundance of pyrites, 
and all contain minute green radiate crystals. A very distinct 
rock, however, bounds this group in the Church-School quarries, in 
which the ground-mass consists of a multitude of small quartz and 
felspar crystals arranged in an almost graphic manner, but with a 
few large porphyritic crystals and indications of a few spherulites, 
This would seem an indication of intrusive character. It is remark- 
able that a very similar rock (in which, however, the spherulites 
are more numerous and the minute crystals forced into more regular 
situations) occurs as a dark-looking rock, resembling a basic lava, 
at Penmaen Melyn. Another mass of the same group, but flinty 
and compact in appearance, is found north of the Llanhowel rock. 
This shows none of the spicular crystals, but the ground-mass is 
minutely and irregularly crystalline, many individuals being recog- 
nizable as quartz, a considerable proportion of which may be of clastic 
origin ; and here and there radiate crystallization has taken place ; 
but besides this there are numerous very round blebs of perfectly 
clear quartz, the origin of which is obscure.. They seem to be pre- 
existing and not to have developed in the rock, but they are so 
round as to be quite unlike water-worn grains. The idea which 
suggests itself on a comparison of this with the masses nearer the 
central granite is, that the quartz grains which are there so large, 
and often retain their crystalline form, have here been subjected to 
more complete melting and have nearly disappeared, for they are 
very like what would be produced by such a process. They would 
then be too feeble to set up much of the spherulitic structure, if 
such were originated by the quartz. In none of the rocks which are 
remote from the granite, so far as | have examined them, are any 
porphyritic crystals found. 
The felsitic mass which bounds the Solva valley, 8.W. of Llan- 
howel, is more like the non-spherulitic portion at the Church-School 
quarry. Itis composed of a network of fine crystals, of which those 
of quartz are irregular; but the other mineral, possibly mica, is so 
finely divided into fibres that it gives gorgeous colours in polarized 
light. Some of this occupies the place of an old felspar crystal. 
On the ashy rocks nothing need be said in proof of their nature ; 
but their yariety is astonishing, 
