in America and Europe. 279 
with the other series. At Clegyr bridge was seen the base of 
the Pebidian, already mentioned as consisting of a conglom- 
erate of Arvonian fragments. Another belt of the same crys- 
talline rocks was also visited, a few miles to the eastward of 
the last, and not far from Haverfordwest, forming, according to 
Hicks, a ridge.several miles in length and about a mile wide. 
Where seen, at Roch Castle, it was found to consist of Arvo- 
nian petrosilex, with some granitoid rock near by. The ridge 
is flanked on the northwest side by Pebidian and Cambrian, 
and on the southeast by Silurian strata, let down by a fault. 
n the shore of Llyn Padarn, near the foot of Snowdon in 
North Wales, the porphyritic petrosilex of the Arvonian is 
‘ while in contact with it, and at the base 
agrees with Messrs. Hicks, Hughes and Bonney that there is no 
ground for such an opinion, but that the conglomerate marks 
biieh, which here reposes on Arvonian 
rocks, and is chiefly made up of their ruins. In like manner, 
according to Prof. Hughes, the Cambrian in other parts of this 
padag includes beds made of the débris of adjacent granitoid 
rocks. 
These petrosilex conglomerates of Llyn Padarn are indistin- 
guishable from those found at Marblehead and other localities 
near Boston, Massachusetts, which have been in like manner 
Interpreted as evidences of the secondary origin of the adjacent 
petrosilex beds, into which they have been supposed to grad- 
uate The writer has, however, always held, in opposition to 
this view, that these conglomerates are really newer rocks made 
up of the ruins of the ancient petrosilex. He has found sim- 
