180 
ELEMENTS OF GEOLOGY. 
melting. Glaciers or land-ice would, on the contrary, chief¬ 
ly discharge their cargoes at the bottom of valleys. Traces 
of an earlier and independent glaciation have also been ob¬ 
served in some regions where the striation, apparently pro¬ 
duced by ice proceeding from the north-west, is not explica¬ 
ble by the radiation of land-ice from a central mountainous 
region.* 
Glaciation of Wales and England. — The mountains of 
North Wales were recognized, in 1842, by Dr. Buckland, as 
having been an independent centre of the dispersion of errat¬ 
ics—great glaciers, long since extinct, having radiated from 
the Snowdonian heights in Carnarvonshire, through seven 
principal valleys towards as many points of the compass, 
carrying with them large stony fragments, and grooving the 
subjacent rocks in as many directions. 
Besides this evidence of land-glaciers, Mr. Trimmer had 
previously, in 1831, detected the signs of a great submerg¬ 
ence in Wales in the Post-pliocene period. He had observed 
stratified drift, from which he obtained about a dozen spe¬ 
cies of marine shells, near the summit of Moel Tryfaen, a hill 
1400 feet high, on the south side of the Menai Straits. I 
had an opportunity of examining in the summer of 1863, to¬ 
gether with the Rev. W. S. Symonds, a long and deep cut¬ 
ting made through this drift by the Alexandra Mining Com¬ 
pany in search of slates. At the top of the hill above-men¬ 
tioned we saw a stratified mass of incoherent sand and grav¬ 
el 35 feet thick, from which no less than 54 species of mol- 
lusca, besides three characteristic arctic varieties—in all 57 
forms—have been obtained by Mr. Darbishire. They be¬ 
long without exception to species still living in British or 
more northern seas ; eleven of them being exclusively arctic, 
four common to the arctic and British seas, and a large pro¬ 
portion of the remainder having a northward range, or, if 
found at all in the southern seas of Britain, being compara¬ 
tively less abundant. In the lowest beds of the drift were 
large heavy boulders of far-transported rocks, glacially pol¬ 
ished and scratched on more than one side. Underneath 
the whole we saw the edges of vertical slates exposed to 
view, which here, like the rocks in other parts of Wales, 
both at greater and less elevations, exhibit beneath the drift 
unequivocal marks of prolonged glaciation. The whole de¬ 
posit has much the appearance of an accumulation in shal¬ 
low water or on a beach, and it probably acquired its thick¬ 
ness during the gradual subsidence of the coast—an hypoth¬ 
esis which would require us to ascribe to it a high antiquity, 
* Milne Home, Trans. Royal Soc. Edinburgh, vol. xxv., 1868-9. 
