588 ON ROCK-SPECIMENS FROM ANGLESEY. 
can only have been the result of earth-movements on a great scale, at 
a time long anterior to the formation of the conglomerate, and of 
denudation continued through a period so long as to allow the very 
foundations of a mountain-chain or upland district to be exposed. 
This conglomerate, in fact, must have been formed from and on a 
floor composed of outcropping beds of gneisses, schists of various 
kinds, argillites, and grits. 
(2) Volcanic materials, such as are so common in the conglome- 
rates in Caernarvonshire and near Beaumaris, belonging either to 
the very base of the Cambrian or to a somewhat older series (as I 
believe), cannot be certainly identified in these Anglesey conglo- 
merates, though a few fragments may belong to rhyolitic rocks. 
Hence we may conclude that voleanoes were few or absent in this 
district. 
(3) Since these conglomerates were formed they have been sub- 
jected to great earth-movements, sufficient to produce in their finer 
constituents a rude cleavage, together with those minor mineral 
changes which are facilitated by compression. 
(4) A comparison of 116 with 114, 115, the first occurring as 
a fragment in the beds mentioned above, seems to indicate the 
presence of a conglomerate composed of similar materials to that 
mentioned in (1), but decidedly more ancient than it; so that Dr. 
Callaway is justified in referring the two conglomerates to distinct 
geological formations. 
These investigations, then, lead us to the same conclusion as the 
researches of my friend Dr. Hicks in the district about Llanfaelog, 
namely, that the granitoid rocks, whether some of these be true 
granites or not, and the schists of Anglesey are of Archean age, 
and are the outcrops of a great floor of ancient rocks upon which 
the continuous series of the older Paleozoic rocks of North Wales 
was deposited. It is probable that these were to a very great 
extent, if not entirely, concealed from view comparatively early in 
the Cambrian period. Some of its coarser grits, as in the Harlech 
district, are evidently formed from the ruins of Archean rocks, and 
obviously all its sediments must have been derived from existing 
land-masses, composed, of course, of earlier rocks; but, so far as I 
have at present studied the ordinary Cambrian slates and grits, I 
should say that they indicated a derivation from a comparatively 
distant source. The Cambrian period of Sedgwick, the Cambrian and 
Lower Silurian or Ordovician of later authors, appears to have been 
one of continuous depression and sedimentation over at least all the 
North-Wales area, and the land, I conjecture, lay to the north-west, 
where now “rolls the sea.” At the end of this time began another 
epoch of “ mountain-making,’ when the border-zone of the newer 
sedimentary deposits was subjected to great lateral pressure, the 
cleavage of North Wales was produced, and a new age of denudation 
and of transference of sediment was begun. 
