568 C. CALLAWAY ON THE ARCHEHAN AND 
slates. Even were no fault visible, a break might be inferred from 
the fact that the rocks to the north of the junction are crumpled 
into very sharp contortions, and have undergone partial metamor- 
phism, while the shales immediately to the south of the line are 
apparently unchanged and have suffered no contortion. 
I have traced the junction of the two groups from this point 
westward to Llanfflewin, three miles from the west coast, and 
nowhere have I found the slightest evidence of a passage, though 
the two kinds of rock are frequently seen within a few feet of each 
other. Some details on this point may be of interest. 
A little north of Byttia, nearly two miles west of Porth-y-corwg, 
the two formations are exposed within a few yards of each other. 
The Pebidian is a pale-green altered slate, of the ordinary type. 
The Paleozoic is a conglomerate, dipping with the Pebidian; yet 
there are no signs of beds of passage. Moreover, I traced the con- 
glomerate along to the east, and ascertained that it was the same 
bed which at Porth-y-corwg lies south of the fault, so that the dis- 
location must extend at least as far west as Byitia. 
West of Byttia the line of contact curves round to the south- 
west, and the conglomerate is no more seen, a conjunction of cir- 
cumstances more naturally explained by faulting than thinning-ont. 
Black shales, standing vertical, or even dipping away from the 
Pebidian, are now in contact with the older group for nearly a mile 
to the south-west. 
At Nantglyn, 27 miles W.S.W. of Byttia, we come to an excellent 
junction. The two types are seen almost in contact in the road, 
and the shales are well exposed in the quarry at the house. But 
there are no shale-beds in the contorted series or altered beds in 
the shales. 
Black shales may be seen at several points between the last 
locality and Llanffiewin. They sometimes approach to within a few 
yards of the Archean; but nowhere along this line have I been 
able to find the slightest evidence of a passage between the two 
formations. 
On the west coast, south of Carmel’s Point, I again observed 
Paleozoic shales and altered Pebidian rocks in cliff-sections, and the 
faults were perfectly clear. There is, then, no reason to doubt that 
the Porth-y-corwg fault extends right across the island, throwing 
down the Paleozoic strata against the Pebidian of the Amlwch area. 
We are not, however, left to the negative evidence of a fault. 
The conglomerate which fringes the Pebidian from the east coast to 
Byttia furnishes us with positive proof of the superior antiquity of 
the contorted series. The included fragments are mainly of types 
such as are found in the Pebidian of the Amlwch area. Some of 
the pebbles are of ordinary Pebidian slate (No. 113, p. 584), such 
as occurs in central Anglesey, as well as in the north, so that their 
teaching is not decisive. Others are of quartzite, and their testi- 
mony is equally inconclusive. But the predominant fragments are 
of grey compact limestone, and many of them are of great size, 
indicating a neighbouring source of derivation. This rock (No. 119, 
