12 NOTES ON THE GEOLOGY OF 
The following is a list of those which I have happened to 
come upon in the localities named :— 
List oF Fossits FROM SILURIAN MupstroNeE AT PONTYRDDOL, 
Sr. ASAPH. 
Favosites fibrosa Rhynchonella navicula 
LHeliolites interstinctus ( ?) R. nucula 
Petraia sp. Spirifera elevata 
Encrinites Leptena small form (7?) minima 
Pterinea retroflexa Strophomena depressa 
Atrypa reticularis S. euglypha 
Chonetes lata and young (7) Phacops 
Orthis elegantula and var, lunata Orthoceras 
List OF FOSSILS FROM SILURIAN MupSTONES ON MokEt Fopta, 
DENBIGH. 
feliolites interstinctus Chonetes lata 
Nebulipora lens Orthis elegantula 
Cliona antiqua | Rhynchonella navicula 
Atrypa reticularis R. nucula 
Pterinea | Strophomena depressa 
Ctenodonta ! S. Sp. 
Atrypa reticularis Phacops (2) 
List OF FOSSILS FROM SILURIAN MUDSTONE NEAR PONTUCHAF, 
RHEWL. 
Nebulipora (?) | Rhynchonella navicula 
Encrinites R. nucula 
: Atrypa reticularis R. Wilsoni (?) 
Chonetes lata Strophomena depressa 
Orthis elegantula Orthoceras primevum 
A fuller list by SALTER, from what I consider to be equivalent 
beds to those of Pontyralltgoch, will be found in the Mem. 
Geological Survey, Vol. iii., p. 278. See also Aveline Explana- 
tion, Hor. Sect., Sh. 43. 
We have not the highest beds of the series in North Wales. 
They had all been swept away in the great period of upheaval 
and denudation that elapsed before the land went down again 
to receive the vast marine deposits of the Devonian and 
Carboniferous. How much is gone we have not evidence to 
show, because there is no means of identifying the exact place 
in the series of the highest beds which have been left. Probably, 
from the analogy of adjoining districts, there was a vast thick- 
ness of sediment deposited above the highest Silurian rocks of 
our area, and the sediment, under the great pressure to which it 
was exposed, was hardened into rock. Then in the crumpling 
and folding of earth crust it all began to rise again from below 
the sea level, and as it came up it was eaten away by rain, rivers, 
and sea. If we take just the bit that comes in the valley of the 
Clwyd, the minimum thickness of rock removed in that interval, 
