PRE-CAMBRIAN ROCKS IN PEMBROKESHIRE. 991 
aspect; and this is increased also by the frequent presence of crystals 
or fragments of crystals derived directly from voleanos, and some- 
times also from the denudation of igneous rocks. Then, again, 
rocks made up of large fragments and rough materials are, on the 
whole, less easily attacked than those of a more homogeneous nature. 
Hach of these formations, however, by the majority of the rocks 
composing them, show a state of alteration peculiarly characteristic’; 
and irom this fact, as well probably as from the original nature of 
the materials out of which they were formed, the rocks of each group 
are easily recognized after a very slight acquaintance with them. 
Indeed so curiously marked is this in the various areas examined, 
that by lithological characters alone one was frequently instinctively 
lead to recognize their presence where they were previously quite 
unsuspected. It is evident also that these are not local peculiarities 
only, but of very wide application, since the descriptions given by 
some Swedish geologists of similar rocks in their own country indi- 
cate that they are, approximately at least, of the age of these rocks. 
Again, rocks of a like nature have been described in America by 
Professors Hunt, Hitchcock, and others; and these, again, appear to 
hold a somewhat equivalent position in geological succession. 
Apprnntx on the Microscorican Srructurz of some Arvontan Rocks 
from PemproxesHiIreE. By THomas Davis, Esq., F.G.S. 
1. Loch Castle, Pembrokeshire (p. 286).—This rock, which presents 
the external aspect of a hornstone, when examined in a prepared 
section under the microscope is seen to consist mainly of a erypto- 
erystalline ground-mass which, in numerous nests and fissure-like 
groups, is developed into a microcrystalline structure. Examined 
with a high objective, this cryptocrystalline mass is resolved into 
grains which exhibit, both in ordinary and in polarized light, the cha- 
racteristic aspect of quartz, and is found to contain as an interstitial 
ingredient a lght-grey, somewhat indefinable constituent, having 
but little action on polarized light, and which, from its great resem- 
blance to the known felsitic substance of many of the quartz-felsites, 
constitutes the felsitic portion of the rock. The nests and fissures 
(resembling groups of coarser structure) present a rude parallelism 
suggestive either of an incipient foliation or of a stratification. 
The whole mass is traversed by numerous well-defined fissures 
(quite distinct from the fissure-like groups), which are filled with 
a clear crystalline quartz, probably of subsequent origin. Very 
numerous acicular crystals and spots of an undeterminable sub- 
stance (which, with a high power, are found to depolarize light and 
to assume a rich brown colour) are disseminated through the whole, 
accompanied by an opaque black mineral resembling magnetite. 
2. Roch Castle (p. 286).—The compactness and dull splintery 
fracture of this rock recall still more than the preceding the charac- 
teristics of ahornstone. Its structure likewise, as exhibited under the 
microscope, bears a marked resemblance, but is characterized by some 
well-defined differences, In places an exceedingly fine dust in the 
