TUB CA.MHKIAN SYSTEM 79 



1. The Cambrian of Wale* 



Though the exposed tracts of Cambrian rocks in Wale* are 

 rather far apart it will be convenient to treat the whole as one 

 area, because the several tracts are doubtless portions of one 

 continuous subterranean mass, and because in process of time the 

 same stages and subdivisions will probably be recognised in all the 

 separate districts. 



Both in this and subsequent chapters I shall assume that tin- 

 student possesses a copy of the cheap general Geological Map of the 

 British Islands, published by the Geological Survey (see p. 27), so 

 that he may see at a glance the relative positions of the areas 

 described. The descriptions here given will in some cases be 

 illustrated by special maps, and for other parts of the country tin- 

 reader can obtain the Index maps of England and Southern Scotland, 

 which are published in sheets (see p. 28). 



The Cambrian rocks of North Wales were first described and 

 classified by Professor Sedgwick, and the lowest series of beds, in 

 which he found no fossils, were termed by him the Harlech grit*. 

 The beds in Pembrokeshire were first properly identified and 

 described by Dr. H. Hicks, 5 who obtained fossils from the equi- 

 valents of the Harlech yrits, and was thus led to divide the series 

 into two groups which he named respectively the Caerfai Beds and 

 Solva Beds. He was also the first to separate the Menevian from 

 the Lingula flags, arid to make a detailed map of the St David's 

 area which is reproduced in Fig. 16. 



We shall now indicate the representatives of the three divisions 

 of the Cambrian (Lower, Middle, and Upper) in each of the separate 

 areas, so far as it is possible to do so. 



Olenellian or Lower Cambrian. In Pembrokeshire this 

 series, named Caerfai Beds by Dr. Hicks, is estimated to have a 

 thickness of about 1600 feet, and to consist of the following seta of 



beds in descending order : 



FwU 



Purple sandstones with Annelid tracks 



Rl shales with Lingulella primccva, Discina, and fragment. < 



ofOlenellus 



Flaggy sandstones with Annelid markings . 



Conglomerates 60 to 150 



About 1600 



The basal bed of this series is the Cambrian conglomerate so 

 frequently mentioned in the preceding chapter. Near St. David's 

 it is about 60 feet thick, and consists of well-rounded pebbles set 

 in a matrix which is a mixture of sandy and ashy materials; 



