CHAP, xin 
ANGLESEY 
223 
given (Fig. 58). The lowest member (1) of the series is a white quartz- 
ite much jumhled in its bedding, hut yet distinctly interstratihed with the 
other sediments, and containing intercalated courses of green tuff and highly 
carbonaceous shale. Markings like worm-pipes are here and there to be 
seen. The next group of strata (2) consists of black sluile followed Ity yellow 
conglomeratic sandstone and pebbly tuffs. The shales enclose rounded and 
angular fragments of quartzite. The sandstone passes upward into pinkish 
and yellowish conglomerate (3), with an abundant lustrous phyllilic matrix, 
which wdien free fi’om pebbles closely resembles some of the tuffs of Llyn 
radarn. The next band (4) is one of yellow, sandy, felspatliic grit, 
quartz-conglomerate and fine tuffs, with leaves of dark shale towards the 
base. It was in the lower part of this baud that the Orthis above men- 
tioned was found. The black shales contain markings which are probably 
graptolites. Eeddish quartzite and quartz-conglomerate (5) next succeed. 
These strata have the same phyllitic base just noticed. The highest group 
here shown is one of black, yellow and green shales mixed with patches 
and bands of volcanic breccia and tuff, the whole being greatly disturbed, 
cleavage and bedding seeming as it were to be struggling for the mastery. 
Fio. 58. — Section of the strata on the .shore at Forth Wen, west of Amlwch. 
These last strata look as if they were about to pass up vertically into the 
ordinary dark Lower Silurian shales or slates. 
There can be no doubt regarding the serious amount of crushing which 
the rocks of this coast-line have undergone. Some of the bands might even 
be described as “ crush-conglomerates.” Yet the intercalation of seams of 
black shale and limestone, and the occurrence of the exactly similar but 
thicker group of black shales at Forth Frydd, which arc admitted to be 
Lower Silurian, unite the whole series of strata as parts of one formation. 
It thus appears that the area coloured “ altered Cambrian ” on the 
Survey map, and regarded as pre-Cambrian by some later observers, is 
proved by tlie evidence of fossils at its base, towards its centre and at its 
top, to belong to the Lower Silurian series, probably to the Bala division. 
That this was the geological horizon of part at least of the area was 
recognized by Sir A. Famsay, though he confessed himsell vxuable “ precisely 
to determine on the north coast of Anglesey how much of the strata are of 
Silurian and how much of Cambrian age.” ^ Frofessor Hughes was the first 
to suggest that the whole of these rocks should be referred to the Bala 
group.^ 
I have dwelt on the determination of the true geological age of the rocks 
^ Mem. Geol. Surv. vol. iii. 2nd edit. p. 242. 
- Proc. Oavih. Phil. Soe. vol. iii. (1880), pp. 341-348. 
