The Benudations of Porth Gales. 
BY AUBREY STRAHAN, M.A., F.GS. 
A Lecture given on 24th November, 1881. 
HERE are few districts which illustrate more forcibly than 
our own the fact that, while existing lands, being them- 
selves chiefly formed of stratified marine material, occupy the 
sites of the seas of former geological periods, on the other 
hand these seas oyerspread the ruins of still more ancient 
continents. The cleaved and contorted, but purely marine 
Silurian rocks, the vast accumulations of shells, corals, and 
encrinites forming the Carboniferous Limestone, and the great 
half-marine, half-estuarine deposits of the Millstone Grit and 
Coal-Measures, form such records of the oceanic periods as can 
hardly escape attention; while the unconformity existing 
between these and the over- and under-lying systems proves, 
perhaps less obviously but not less conclusively, the intervention 
of continental periods, during each of which the deposits of the 
preceding age were elevated above the sea and exposed to 
denudation. My object is to examine the evidence on which 
these statements rest, and to shew how it may be obtained in 
the structure and mutual relations of the formations. 
The earliest rocks in our own district are characterised by great 
volcanic activity. During the deposition of the Bala Beds great 
sheets of felspathic lava overflowed the sea floor, and showers 
of scoriz and ashes were ejected, which, falling back into the 
sea, became stratified as they sank to the bottom and mingled 
with the remains of the sea-animals which were struggling for 
existence round the volcanie regions. As in the process of time 
these rocks were buried under later formations, the ashes were 
hardened, and cleavage, metamorphism, and contortion were 
produced. Ata far later period, when the ceaseless movements of 
the earth’s crust had brought the beds once more to the surface, 
there began to be carved out of them that endless variety of 
cliff and valley which is familiar to every visitor to Cader Idris 
and Snowdon. So great, however, is the amount of alteration 
and denudation which these rocks have undergone, that it is no 
longer possible to point even approximately to the position of 
any of the volcanic vents from which they were ejected, or to 
recognise in all North Wales a fragment of any volcanic crater. 
