THE CAMBRIAN PERIOD. *J'J 



CHAPTER VIII. 



THE CAMBRIAN PERIOD. 



The traces of life in the Laurentian period, as we have seen, 

 are but scanty ; but the Ca^jibriafi Rocks — so called from their 

 occurrence in North Wales and its borders (" Cambria") — have 

 yielded numerous remains of animals and some dubious plants. 

 The Cambrian deposits have thus a special interest as being 

 the oldest rocks in which occur any number of well-preserved 

 and unquestionable organisms. We have here the remains of 

 the first fmma, or assemblage of animals, of which we have at 

 present knowledge. As regards their geographical distribu- 

 tion, the Cambrian Rocks have been recognised in many parts 

 of the world, but there is some question as to the precise limits 

 of the formation, and we may consider that their most typical 

 area is in South Wales, where they have been carefully worked 

 out, chiefly by Dr Henry Hicks. In this region, in the neigh- 

 bourhood of the promontory of St David's, the Cambrian Rocks 

 are largely developed, resting upon an ancient ridge of Pre- 

 Cambrian (Laurentian?) strata, and overlaid by the lowest 

 beds of the Lower Silurian. The subjoined sketch-section 

 (fig. 27) exhibits in a general manner the succession of strata 

 in this locality. 



From this section it will be seen that the Cambrian 

 Rocks in Wales are divided in the first place into a lower and 

 an upper group. The Lower Cambrimi is constituted at the 

 base by a great series of grits, sandstones, conglomerates, and 

 slates, which are known as the " Longmynd group," from their 

 vast development in the Longmynd Hills in Shropshire, and 

 which attain in North Wales a thickness of 8000 feet or more. 

 The Longmynd beds are succeeded by the so-called " Mene- 

 vian group," a series of sandstones, flags, and grits, about 600 

 feet in thickness, and containing a considerable number of 

 fossils. The Upper Cambrian series consists in its lower por- 

 tion of nearly 5000 feet of strata, principally shaly and slaty, 

 which are known as the ''Lingula Flags," from the great 

 abundance in them of a shell referable to the genus Lingula, 

 These are followed by looo feet of dark shales and flaggy 

 sandstones, which are known as the " Tremadoc slates," from 

 their occurrence near Tremadoc in North Wales ; and these 

 in turn are surmounted, apparently quite conformably, by the 

 basement beds of the Lower Silurian. 

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