ROCKS OF PEMBROKESHIRE. 513 
c. As to the Origin of the Dimetian Rocks.—At present the evidence 
is not completely satisfactory, nor, indeed, can it be said that it is 
so with reference to the origin of the majority of the massive gneisses 
in Scotland and elsewhere. Whatever it may have been in the 
case of those gneisses, it was, in my opinion, of like nature in the 
case of the Dimetian. Whether we adopt a neptunian or a volcanic 
origin, or, what is more probable, a combination of both, for these 
rocks, one important fact constantly presents itself to our minds— 
that is that these rocks show peculiarities, wherever exposed, which 
lead us to suspect their age even where the stratigraphical evidence 
is not conclusive. The peculiar mode of fracture I mentioned as 
characteristic of the Dimetian at St. David’s, is equally evident 
in the Johnston rocks, and in the so-called granite (Dimetian) of 
Brawdy, Hayscastle, and Brimaston. It is also equally marked in 
the granitoid rocks of North Wales, which have been classed with the 
Dimetian. This peculiar condition was also developed at a very 
early period, undoubtedly before the Cambrian conglomerates were 
deposited, as it is present in the fragments contained in those sedi- 
ments. The diabase dykes in the Dimetian have been called in my 
papers by different names such as melaphyre, dolerite, &c. Dr. 
Geikie uses the general term diabase for these; and as it is on the 
whole perhaps the best term, I shall also use it throughout this 
paper, and it will prevent confusion. In early papers, before the 
microscope was so generally used to differentiate the various rocks, 
some of these diabase dykes, from their frequent parallelism with 
one another, and from a peculiar cleaved structure developed in 
them, were included in the Dimetian as interstratified bands. 
In my paper in 1878 I clearly pointed out, as the result of a 
microscopic examination of these made for me by Prof. Judd 
and Mr. T. Davies, that they were igneous rocks, and therefore of 
no value in regard to the question of the origin of the Dimetian. 
The dolomitic bands found by me in the cliff at Porthlisky may 
also, as suggested by the late Mr. Tawney, who examined them sub- 
sequently with me, be merely of secondary origin as the result of 
infiltration, “‘due to the decomposition caused by water filtering 
down joints, removing alkaline silicates, and depositing carbonates of 
lime and magnesia” *. Hence they also may be of no value in any 
question of the origin of these rocks. Mr. Tawney, however, even 
though he supposed that this evidence could not be relied upon, still 
strongly believed in the metamorphic origin of the Dimetian, and 
emphasized this in one of the latest papers} published by him, 
in which he gives the results of a prolonged examination of these 
rocks in the field both in North and South Wales, and by careful 
microscopical examination. 
d. The Brecciated Portions.—The brecciated bands found at several 
places in the Allan Valley have been carefully described by Prof. 
Hughes, and there seems to be tolerably clear evidence to show 
* Proceedings of the Bristol Naturalists’ Soc. vol. ii, part 2, p. 116. 
t Geol. Mag., dec. 2, vol. x. p. 67. 
