Chap. IV.] CAEADOC FOEMATION AT BALA, NORTH WALES. 
71 
this work was published. This distinction is of great interest to myself; 
for it brings out the value of the original classification of the Lower Silu- 
rian rocks r which indicated a great inferior schistose group charged with 
large Trilobites (Llandeilo), as followed by more arenaceous strata — 
Shelly Sandstones, or the mass of the Caradoc formation. 
Looking at the general section under the Map, the reader will at once 
see how the beds of Llandeilo age are surmounted throughout North 
Wales by strata of the Caradoc age, and how the last are followed by 
other formations, of which hereafter. In truth, this identification of the 
Bala fossils with those of the Shelly Sandstones of Caer Caradoc has re- 
moved all difficulty in applying the Silurian succession to great masses in 
Wales, the rocks of which, being in a crystalline and slaty condition, are 
very unlike their unaltered equivalents in Shropshire. 
It has now transpired that the physical relations and palseontological 
distinctions between the Llandeilo flags on the east flank of the Berwyns 
(the most westerly point which I formerly examined) and the overlying 
fossiliferous strata of the tract watered by the Tanat and the Ffyrnwy, as 
given in the * Silurian System,' were perfectly correct. 
In the diagram before given (p. 60) we see how the Llandeilo flags 
with the Trilobites, Asaphus tyrannus and others, as exposed in lofty 
crags (Craig-y-Glyn) overhanging the gorge of the River Twrch above 
Llanrhaiadr, dip under those shelly sandstones of the lower hills of the 
valleys of the Tanat and Ffyrnwy. The latter strata, though affected, 
like those on which they rest, by a slaty cleavage, were described as again 
emerging from beneath Upper Silurian rocks in the environs of Welsh 
Pool, and were all, as before said, distinctly identified with the Shelly 
Sandstones of Caer Caradoc. This comparison was a subject of no diffi- 
culty to me ; for it was evident that the sandstones of Montgomeryshire, with 
their included courses of impure limestone, including Trinucleus Caractaci, 
Strophomena expansa, and numerous other fossils, were identical with the 
shelly sandstones of Caer Caradoc. 
In short, through other undulations (see Map and its Sections), these beds 
extend westwards until they constitute the slates of the summit of Snow- 
don, where the very same fossils occur as in the low hills of soft shelly 
sandstone in Shropshire, the district from which such forms were first 
described. 
It appears from the recent publication of the Geological Survey (Memoirs, 
vol. iii. p. 86) that the Bala beds of North Wales are by no means of such 
large vertical dimensions as the Caradoc Sandstones of Shropshire ; for, 
abstracting the porphyries, ashes, and other intercalated volcanic rocks, the 
limestones and sandstones of this age do not present a thickness of more 
than 1050 feet ; whilst those of Shropshire are about 4000 feet thick. 
In South Wales we have the same physical order and the same changes 
in organic life between the Llandeilo and Caradoc groups as are observed 
