ANGLESEY AND CAERNARVONSHIRE. 189 
tions made by Dr. Hicks himself in regard to the area of St. David’s, 
and have proved them to be untenable. If this is the result of the 
critical examination of his typical Pre-Cambrian district, over which 
he has spent most time, I can hardly anticipate that his more rapid 
traverses elsewhere will, when properly tested, be found to have been 
more successful.” 
After listening to the above statements which have been made 
by the present Director General, I hardly need ask the Members 
of this Society, in whose Journal they have appeared, to consider 
this paper only as a first instalment of that thorough criticism 
of the work of the Geological Survey which is demanded in the above 
paragraph, and as necessary in reply to the indictment, from the 
only district which I have been able to reexamine since Prof. 
Geikie’s paper appeared in the Journal. 
Though the title of my paper shows that I purpose mainly to deal 
with evidence derived from the Cambrian Conglomerates, it is clear 
that it will be advantageous to refer incidentally also to any new 
facts obtained relating to the Pre-Cambrian rocks themselves. The 
rocks which we claim to be of Pre-Cambrian age have, however, 
been so fully described in papers by Prof. Hughes, Prof. Bonney, Dr. 
Callaway, and myself that it will not be necessary to do more than 
eall attention very briefly to their characteristics. Prof. Geikie has 
also, in his descriptions of these rocks, included all peculiarities 
which have been pointed out by us as specially applicable to any of 
these North-Wales areas. 
In the Geological Map issued with the Survey Memoir referred 
.to, published in 1881, there is a great elongated mass coloured as 
“intrusive felspathic porphyry,’ reaching from Llanllyfni in the 
south of Caernarvonshire, to Bethesda in the north, a distance of about 
fifteen miles. It has an average width for the most part of from a mile 
and a half to two miles, but becomes narrower towards the north. At 
its south-western extremity and for a short distance along its south- 
eastern edge, altered Cambrian rocks are shown to be in contact with 
it, but at all other points ordinary Cambrian rocks. This so-called 
intrusive mass is stated in the index to be ‘“ chiefly of Lower Silurian 
age.” Another mass of the so-called ‘intrusive felspathic porphyry” 
is shown to extend from Caernarvon to Bangor, a distance of about 
ten miles, with a width atits broadest part of rather over a mile, and 
narrowing towards each end. At Bangor altered Cambrian rocks 
are shown along its north-eastern edge; further south along the 
same side the ordinary Cambrian colour is given; and beyond this, 
to Caernarvon, Lower Silurian rocks are indicated as being in contact 
with the mass. Along the western edge Carboniferous rocks are 
shown. This mass, like the one above referred to, is marked as 
“chiefly of Lower Silurian age.” 
In Anglesey, extending from the coast south of Llanfaelog to the 
north-east of Llanerchymedd, there is, on the same map, a great 
patch coloured as granite. At its broadest part it is three miles 
across, and its length is about eleven miles. It is stated in the 
index to be “ probably of Lower Silurian age.” What I have main- 
