286 HH, HICKS ON A NEW GROUP OF 
British Isles. They were marked on the Geological Survey Maps as 
intrusive felstones ; but a very cursory examination proved that they 
were not of that nature, and that they were in reality bedded sedi- 
mentary rocks which had undergone metamorphic change. ‘The first 
area which I propose to describe lies about five miles to the N.W. of 
Haverfordwest. It has a general E.N.E. strike, is narrow towards 
the west, and widens out eastward and there throws out two arms 
which at their extremities are distant about two miles. The 
actual width exposed at its greatest breadth is rather over a mile; — 
and as the beds appear everywhere nearly vertical and not repeated, 
we have probably a vertical thickness of strata shown of not much 
less than a mile in extent. The ridge formed by these rocks is for 
the most part barren and wild-looking, and the highest point, called 
Plumstone mountain, attains to a height of nearly 600 feet. Lower 
Cambrian rocks rest upon it alone the N.W. edge for about two 
thirds of the distance, and faulted Lingula-flags along the remainder. 
The southern and eastern sides are also in contact with faulted 
Lingula-flags for the most part, though at some places Lower Silurian 
rocks have been brought down against it. Roch Castle, an old 
fortress, and a very conspicuous object on the road-side between 
Haverfordwest and St. David’s, stands almost on the western ex- 
tremity of the ridge ; and as the rocks are very well exposed at this 
point, the spot is a very favourable one for their examination. In 
colour they are mostly of a dull yellowish-grey tint, though some of 
the beds are light and very quartzose in appearance, others dark 
and cherty-looking. The lighter ones also are frequently banded in” 
deeper tints. Some, again, appear at first sight asif porphyritic from 
grains of quartz ; but these, it will be seen, are identical in character 
with the others when examined under the microscope. ‘The texture 
usually is hard and flinty, and they break frequently with an im- 
perfect conchoidal fracture. Others have a more horny fracture. 
I have had several microscopical sections made of these rocks; and 
Mr. T. Davies, F.G.8., of the British Museum, has kindly given 
descriptions of these in the Appendix (Nos. 1, 2, & 3). Speaking 
generally, the rock may be described as a “microcrystalline mass 
of quartz-grains with some interstitial light-grey substance, haying 
but little action on polarized light.” But the chief peculiarity consists 
in the manner in which the quartz is separated away into nests, so 
as to give that curious porphyritic appearance already mentioned. 
The mode of behaviour of the quartz also here is particularly in- 
teresting and instructive in regard to the changes which many 
crystalline rocks have undergone, especially the gneisses. In some 
cases the quartz is seen in distinct fragments, but yet coalescing, 
as if attracted together by some natural affinity from the surrounding 
material. In the next place the grains are so compressed together 
(and yet distinctly fragmentary) that all other material is removed, 
and nests of pure quartz grains only are seen, having a very crys- 
talline appearance. By this selective process also the darker material 
is brought together and made to fold round the nests, so that a 
banded or imperfect flow-structure is given to the rock, All this 
