514 H. HICKS ON THE PRE-CAMBRIAN 
that they are the result of crushings and of subsequent infiltrations 
of secondary minerals along joints. 
Evidences of similar conditions are present, as has been frequently 
mentioned by Prof. Bonney, in like rocks in North Wales; and I 
have also specially referred to these brecciated bands in my recent 
paper before the Society in describing the Dimetian rocks of Angle- 
sey *. I know of no place, however, where they are so marked as 
in the Allan Valley at St. David’s, particularly immediately behind 
the small farm called Halfesh, about midway between St. Dayid’s and 
Porth-clais, and in the face of the hill some few hundred yards to 
the south of that farm. To all who are interested in the study of 
the extraordinary changes which have taken place in these rocks as 
the result of crushing and subsequent infiltration, those places will 
well repay a visit. It is interesting to note that all these brecci- 
ated portions of the Dimetian which I had referred to were entirely 
missed by the Director-General and his assistants in their examina- 
tion, and that they were compeHed therefore, as stated at p. 274, to 
arrive at the conclusion that I had included in my Dimetian “ por- 
tions of the undoubted bedded rocks (quartz-schists, quartzites, 
shales, &c.) which flank the massive rock of the ridge.” Whether 
or not the evidences fail, which we have thought might tend to 
confirm our views that these rocks are of a metamorphic character, 
it seems perfectly clear that the arguments and deductions in the 
Director-General’s paper can have no weight in solving the question ; 
for they are all made subservient to one purpose, that is, to prove that 
the Dimetian is an eruptive granite of later date than the Cambrian 
rocks, to show, as stated in Dr. Geikie’s own words at p. 274,— 
“that this rock is not only a granite, but one which has been 
erupted through the Cambrian strata, and must therefore be younger 
than they.” 
e. Petrological Characters—With regard to the petrological cha- 
racters of the Dimetian I have not much to add to that which I have 
written in former papers, or which is contained in papers by Prof. 
Bonney, Prof. Hughes, Mr. Tawney, &c. Mr. Davies will, however, 
refer to some fresh slides which have been prepared, and will explain 
the reasons why, when these rocks were first examined by him, he 
thought the term quartz-schist seemed to be the most appropriate for 
the peculiar conditions exhibited by them under the microscope. 
The term granitoidite suggested by Prof. Bonney seems perhaps on 
the whole best applicable to these rocks as exhibited at St. David’s 
and in some parts of North Wales. On the other hand in parts 
of the same group in Anglesey the rocks are sufficiently schistose 
to be called true schists; and the rocks already referred to in 
South Pembrokeshire, if proved to be of the same age, as they 
appear to be, so far as the evidence at present goes, are also highly 
schistose. In the latter area the proportion of basic silicates is 
greater than at St. David’s, and this is necessary to give the charac- 
teristic banding of the true gneisses. The accidental absence, as I 
* Quart. Journ. Geol. Soc. vol. xJ. p. 187. 
