440 Br. R. D. Roherls — The Camhrian in Anglesea, etc. 



lip of quartz pebbles. At Penygraig, near Penlon, about four miles 

 south of Amlwch, a still more interesting example of variation in the 

 character of the conglomerate is shown. The basement bed resting 

 on the granitoid axis at Penlon presents all the characteristic featui-es 

 of the Twt Hill conglomerate. Higher up at Penygraig a band 

 occurs composed almost entirely of felsite pebbles ; different parts 

 therefore of the same conglomeratic series present lithological 

 characters as diverse as the characters of the felsite conglomerate of 

 Bangor and the quartz conglomerate of Twt Hill, Towards the 

 S.W. of Anglesea the conglomerate ceases to be a quai-tz conglomerate 

 and West of Llanfaelog is made up largely of fragments of green 

 felspathic rocks. 



In thickness the variations are also considerable. At Prys Owen 

 Fach, 1^ miles S.W. of Llanerchymedd, the conglomerate is repre- 

 sented by a thin band, only a few inches thick at the base, of the 

 fossiliferous brown sandstones, and resting upon the gneiss ; while 

 farther to the S.W., near Llanfaelog, the thickness appears to be 

 very great. The Cambrian succession in Angelsea may be represented 

 diagrammatically as follows : 



Generalized Section aceoss Lowest Beds of Cambrian. 



a. Pre- Cambrian axis. 



b. Conglomerate of variable thickness, sometimes a pure quartz conglomerate, 



sometimes made up of f elsitic material, and sometimes shomug alternations 

 of bands of sandstone or slate. 



c. Brown sandstones, sometimes containing Orthicles. 



d. Black or grey shales, sometimes banded. 



e. Black brecciated ashy-looking shales with fossils. 

 /. Black shales, containing Graptolites. 



Turning to Carnarvon the succession is seen to be precisely similar. 



At Pontseiont black shales containing Graptolites occur dijoping 

 S.S.E. Walking across the strike towards the Pre-Cambrian axis 

 of Twt Hill, brown sandstones are met with at the entrance of the 

 Twt Hill quarry, dipping S.S.E., which pass down at once perfectly 

 conformably into the well-known conglomerate of quartz pebbles 

 which rest against the granitoidite. 



The sequence is in every particular similar to that in Anglesea, 

 both in respect to the lithological character of the several beds, and 

 in their relation to the pre-Cambrian axis. No one who has studied 

 the beds in Anglesea can feel a doubt as to the position of the Twt 

 Hill bed. 



The resemblance between the two areas may be still farther 

 followed up. 



The Twt Hill conglomerate traced from Carnarvon towards Bangor 

 appears to thin out, and is not seen beyond the quarry between 



