Notices of Memoirs—On sone Pre-Cambrian Rocks. 519 
except that, being impervious clays, they cut off impure surface 
waters, and are easier to bore through than the loose sands, etc., 
overlying them. 
The plane of the Chalk surface, whether under or beyond the 
Lower Tertiaries, is sufficiently uniform to render calculation of its 
depth in any part of the district an easy process. In the Bramfield 
boring, the latest of the series, the Chalk was reached at 48 feet 
below the ordnance datum, calculation from the three nearest points 
—Beccles, Framlingham, and Saxmundham—indicating 52% feet. 
VII.—On some Pre-Camprran Rocks In THE Hartecn Movunratns, © 
M&rRIONETHSHIRE, 
By Henry Hicks, M.D., F.G.S. 
URING an excursion into the Harlech Mountains in the summer 
of last year, I recognized, near the centre of the well-known 
anticlinal of Cambrian rocks, another group of rocks, which ap- 
peared to me to underlie the former, and to be part of a pre- 
existing formation. On further examination I noticed also that 
many of the fragments in the conglomerates at the base of the 
Harlech Grits seemed to be identical with the rocks below, and to 
have been derived from some such pre-existing group. Subsequent 
microscopical examination of some of the fragments, and of the 
underlying rocks, tended strongly to confirm this view. In order, 
however, to satisfy myself more fully on this point, I revisited the 
area this summer, accompanied by my friends, Prof. Hughes, Mr. 
Tawney, and Dr. R. D. Roberts, and the result has been to entirely 
confirm my previous conclusions. This discovery is of considerable 
importance, as it enables us to compare the thickness of the 
Cambrian rocks of North Wales more satisfactorily than has been 
hitherto possible with those of South Wales, and to realize more 
clearly the early physical conditions of the areas. Hitherto it seemed 
doubtful what the actual thickness of the Harlech Group could be, 
and very different estimates have been given. It now becomes 
possible to give a perfectly correct estimate, and it is satisfactory to 
find that it approximates far more nearly with that made out in 
other Welsh areas, than was previously supposed. 
The points where these older rocks come to the surface mainly 
occur along a line running nearly due N. and 8. from Llyn-Cwmmy- 
nach to about two miles to the §8.W. of Trawsfynnyd. Along this 
line the anticlinal is much broken, and denudation has taken 
place to a very considerable extent. It is mainly in consequence 
of this that the Pre-Cambrian rocks are exposed. The so-called 
intrusive felstones marked here on the Survey Maps are part of the 
Pre-Cambrian group, and are not intrusive in the Harlech rocks. 
They are highly felsitic rocks, for the most part a metamorphic 
series of schists, alternating with harder felsitic bands, probably 
originally felsitic ashes. They alternate with bands of purplish 
slates, which I once supposed might have been dropped amongst 
them by faults, but which I now think also belong to the Pre- 
