CHAP. II.] CAMBRIAN PERIOD. 19 



In South Wales there are indications of a similarly rapid 

 disappearance of Lower Cambrian sediments. Near St. 

 Davids these beds are only 4,000 feet thick, which is 

 about half their thickness in Merioneth. Whether they 

 thin eastward in the trough between the two principal 

 Archaean tracts is not yet known, but at Trefgarn on the 

 south-east side of the eastern massif no Harlech Beds or 

 Menevian are found, and the Upper Lingula Flags with 

 a basal conglomerate appear to rest on the Archaean. It is 

 true there are some signs of faulting, and Dr. Hicks believes 

 the Lower beds are faulted out, but more evidence of this 

 is required. 



Dr. Callaway favours me with the following remarks on 

 the lithological composition of the Lower Cambrian rocks :, 

 "These sediments show that the adjoining lands were 

 partly built up of granitic, volcanic, and metamorphic 

 rocks. The purple conglomerates and sandstones of the 

 Longmyndian, reaching a thickness of several thousand 

 feet, as well as the broad band of pale-green slates which 

 skirts the eastern side of the Longmynd, are largely derived 

 from the rhyolites of the Uriconian and indicate the 

 proximity of an extensive land area. Bounded fragments 

 of granitic and schistose rocks are not uncommon in the 

 conglomerates, while the grains of quartz and felspar in 

 the grits have probably the same origin. These grits grow 

 less felsitic towards the south, and fragments of metamor- 

 phic rocks increase in number indications which suggest 

 the conclusion that the Malvern Hills are a worn fragment 

 of a mass of land which made a conspicuous feature in the 

 Longmyndian ocean. 



" The rocks of North and South Wales afford similar 

 evidence. The conglomerates of Moel Tryfaen and the 

 sandstones further west contain a very large proportion of 

 fragments of f el site similar to the Archaean rhyolite near 

 Carnarvon, while the massive grits of the Merionethshire 



