ae 
Dr. C. Callaway—Pre-Cambrian Geology of Anglesey. 117 
As rapidity of cooling appears to be essential to their production, 
it may perhaps be inferred that a similar condition has limited the 
natural perlites. In that case, it is suggested that a gradual passage 
from the complete structure on the surface to the existence of the. 
rectilinear cracks only, and finally to the unbroken glassy magma 
at the centre, might be traceable in the perlitic masses of the field ; 
and it would be of interest to learn from geologists who have 
opportunities for such study whether a gradation of this kind 
actually occurs. : 
VI.— Some New Pornts in tHe Pre-CamBrraAN GroLoGy oF 
ANGLESEY. 
By C. Catztaway, M.A., D.Sc. London, F.G.S. 
Witu Norrts on SOME OF THE Rooks. 
By Prof. T. G. Bonney, M.A,, ete. 
ECENT researches in Anglesey, made with a view to assist my 
investigations into the Pre-Cambrian rocks of Shropshire, 
have led me to certain results, some of which I now submit to the 
geological public. In venturing to differ, in some respects, from so 
high an authority as Prof. Ramsay, I wish to bear testimony to the 
great value of his descriptions of the Anglesey rocks in his magnificent 
work on the Geology of North Wales. I have had the satisfaction 
of finding that Prof. Bonney’s determination by the microscope of 
some of the more difficult rocks substantially agrees with my own 
opinion formed on hand specimens, and am under great obligations 
to him for permitting me to append his notes to this paper. 
The term “gneiss” is restricted in this article to a schistose 
compound of quartz, felspar, and mica or hornblende, and is not 
applied to a rock in which foliation is not quite distinct; the term 
“ oranitoidite ” being employed for certain granitoid rocks for which 
the word “ gneiss” bas sometimes been used. Chlorite is common 
in the Anglesey gneiss, but it is hardly rash to infer that it is 
a decomposition product of hornblende or a magnesian mica. 
A.—Evipence or Pre-Camprian AGE. 
It is well known that the Survey has mapped the schistose rocks of 
Anglesey as metamorphic Cambrian and Silurian, with a great 
granite band of intrusive origin; but recently the clastic origin of 
this “granite” has been maintained by Prof. Bonney after microscopic 
examination, and that author and Dr. Hicks are of opinion’ that the rock 
is contemporaneous with the Dimetian of Twt Hill and St. Davids. 
Of the accuracy of these views I have no doubt, and I am able to 
furnish confirmatory evidence on both points. First, as to the 
clastic origin. North of the Holyhead Road, north-west of Gwalch- 
mai, is a faulted mass of the granitoidite with a steep escarpment 
to the south-east (Fig. 4). The lower part of the scarp is a bedded 
breccia, containing fragments of a sort of hornstone. It dips at a 
high angle to the north-west, and passes up into granitoidite with 
distinct bedding, which, in its turn, passes up into the ordinary 
1 Quart. Journ. Geol. Soc. vol. xxxv. pp. 302 and 307. 
