THE ARCILEAN ROCKS . r 5 



ward continuation of the Lower Cambrians, the apparent break 

 between them being only a surface of contemporaneous erosion.' 1 



Still more recently (1908) the district has been studied and 

 mapped by Mr. J. F. N. Green, 22 who appears to have finally 

 established the relative positions and ages of the rocks concerned. 

 He confirms the existence of a basal Cambrian conglomerate and < .f 

 an extensive pre-Cambrian Series, which is the Pebidian of Hicks. 



These Pebidian rocks occupy an area of about 7 miles in length, 

 with a maximum width of 2 miles ; and according to Mr. Green 

 they consist almost entirely of trachytic and felsitic tuffs, more 

 basic in the lower part, more acid and felspathic in the higher 

 part, the total thickness being over 3000 feet. A well-marked 

 band of conglomerate occurs in the middle of the series and 

 lenticles of shale are seen at several horizons. There do not seem 

 to be any interbedded contemporaneous lava-flows, but rounded 

 pebbles of rhyolite, trachyte, and andesite occur in the tuffs. 



The materials of this series are evidently the products of a large 

 volcano or group of volcanoes spread out under water, but whether 

 on the floor of lake or sea cannot be determined. At some later 

 epoch, but still in pre-Cambrian time, the series was invaded by a 

 large intrusive mass of granite and by a number of quartz- porphyry 

 dykes which probably proceed from the deeper parts of the granitic 

 mass. 



We arrive, therefore, at the conclusion that Hicks was correct in 

 his reading of the main stratigraphical sequence, but wrong about 

 the age of the granite ; while Sir A. Geikie rightly interpreted 

 the relations of the granite, but was mistaken about the base of 

 the Cambrian and in denying the existence of Archaean rocks in 

 Pembrokeshire. 



4. England 



Rocks of Archaean age rise to the surface in the following localities: 

 (1) Shropshire (Longmynd, Wrekin, etc.), (2) Warwickshire 

 (Nuneaton), (3) Leicestershire (Charnwood Forest), (4) The Malvern 

 Hills, (5) Devon (Start Point area), (6) Cornwall (Lizard area). 



Shropshire. The largest and most complete area in England is 

 that of the Longmynd, Caradoc, and Wrekin districts. These tracts 

 are shown in Fig. 21. That of the Longmynd has a length 

 of about 10 miles along its western border, with a further out. 

 on Haughmond Hill ; its greatest breadth is about 6 miles, and it 

 is bounded on each side by lines of fault, and inside the eastern 

 fault another narrow strip is exposed to the south of the main 

 outcrop. 



The exact age of the Longmynd rocks cannot be determined by 



