CHAP. II.] CAMBRIAN PERIOD. 17 



In Wales the normal succession of the Cambrian rocks 

 is as follows : 



Tremadoc slates, 1,000 to 2,000 feet . . . . \ ^ 

 Lingula Flags, a group of sandy flagstones and slates, I 



2,000 to 5,000 feet /Cambrian. 



Meuevian slates, 200 to 750 feet \ 



Harlech Beds, red, green, and purple grits and slates, i- ^ e * 



3,000 to 9,000 feet /Cambrian. 



The Harlech Beds of Merioneth are more than 8,000 

 feet thick, and the base is not there seen, but when they 

 reappear on the west side of the Snowdon range, they are 

 much thinner, and are probably not more than 3,000 feet 

 thick, including the basal conglomerate of Llyn Padarn, 

 which lies on the eastern flank of the main Archaean ridge. 

 On the west side of this ridge the basal conglomerate is 

 overlain by grits and slates, and, near Bangor, by sand- 

 stones and mudstones, which have been referred to the 

 Harlech series, but it is quite possible that they are shore 

 beds of Lingula Flag age. As they are directly succeeded 

 by Arenig slates (Ordovician), it does not seem probable 

 that the Lingula Flags should be entirely absent, for the 

 lowest beds usually thin out before the higher. If, there- 

 fore, we regard the greater part of the Bangor Cambrians 

 as Lingula Flags, it follows that in the space of about two 

 miles the Harlech series has thinned from 3,000 feet to, 

 perhaps, 100 feet of grit and conglomerate. This suggests 

 that the surface of the Archaean rock formed a steep slope, 

 inclining to the south-east, when the Cambrians were 

 deposited against it. 



In Shropshire the same thing seems to take place on a 

 still larger and more surprising scale ; for the rocks of 

 the Longmynd, which are supposed to be of Lower Cambrian 

 age, and which lithologically resemble the Llanberis grits 

 and slates, have a great thickness (possibly 10,000 feet), 

 and are faulted on each side against tracts of undoubted 



c 



