ANGLESEY AND CAERNARVONSHIRE. 199 
hypothesis of the metamorphic alteration of the stratified rocks in 
part, as at Twt Hill, beyond the point of fusion, and while the whole 
lay deeply buried under overlying strata during part of the Lower 
Silurian epoch. When compared with the porphyry of Llyn Padarn, 
it often exhibits more free silica, and in lithological character, as a 
whole, it may be said sometimes to come between the quartz-porphyry 
of Llyn Padarn and the imperfect granitic rocks of Anglesea.” 
Conclusions. 
As fragments of the Twt Hill and Anglesey (Dimetian) rocks, and 
of the typical quartz-porphyries and quartz-felsites mentioned in the 
foregoing quotation, have been discovered in abundance in the basal 
conglomerates of the Cambrian, in the immediate vicinity of those 
rocks, there cannot remain, at least to my mind, the shadow of adoubt 
that those rocks in the condition in which they are now found in 
those areas were there in Pre-Cambrian times; and therefore that 
the views of the Surveyors of their being metamorphosed Cambrian 
and Silurian strata, or intrusions posterior to the deposition of those 
strata, are most erroneous. 
Finally I must state that I am truly sorry that I have been 
compelled to draw further attention to so many points which I can- 
not characterize otherwise than as serious blots on the maps of the 
Geological Survey. It must, however, be borne in mind that much 
of the older work of the Survey was done before the great im- 
portance of the microscope as an aid in the study of rocks was 
recognized. | 
I have been driven to do this because in the Memoir referred to 
we are charged with claiming these rocks as Pre-Cambrian on “purely 
theoretical grounds” (p. 199); and also because in the attack made 
by the present Director General of the Survey on my St. David’s work 
he has obliquely attacked also my work in North Wales. I shall 
shortly have very much to say on the main questions, where they 
refer specially to St. David’s. But as I have not had an opportunity 
of revisiting that district since the publication of the paper in the 
Journal, I thought it advisable to lose no time in bringing these facts 
before the Society from the only areas I have since been able to 
reexamine. In self-defence I have had to show that work to 
which others as well as myself have devoted years of study in the 
field, and with aids which recent scientific knowledge has suggested, 
has a good and solid foundation, and that it has not been built 
up like a house of cards to be demolished by the first storm of 
opposition. Moreover, if the work has cost us so much labour, we 
can hardly be expected to allow these rocks and the names we have 
suggested for their definition, which have, to our minds, meanings of 
some importance, to be so easily ‘*‘ dropped out of geological literature ” 
—as it has been suggested they should be by the present Director 
General of the Geological Survey. 
For the Discussion on this paper, see p. 207. 
