CHAPTEE II. 



CAMBRIAN PERIOD. 



1. Stratigraphical Evidence. 



rocks rise to the surface in several parts 

 of the British Islands, but they nowhere occupy any 

 very large tract of country, so that we can only compare 

 their isolated exposures, and cannot trace their strati- 

 graphical variations from one district to another. In 

 England and Wales there are five districts where rocks 

 referable to the Cambrian system occur South Wales, 

 North Wales, Shropshire, Warwickshire, and the Malvern 

 Hills. 



Wherever the base of the system is exposed, the base- 

 ment bed is found to be a conglomerate containing frag- 

 ments of Archaean rocks, and resting unconformably on a 

 very uneven surface of those rocks ; the conglomerate being 

 probably in process of formation at different levels through- 

 out the whole of the period, and belonging, therefore, to 

 different stages at different places. 



The Lower Cambrian rocks are not known to exist east 

 of the Longmynd, but Upper Cambrian shales have been 

 found in Warwickshire, and may occur at intervals beneath 

 many parts of our Midland and Eastern counties. 



The stratigraphy of the Cambrian rocks is not yet com- 

 pletely understood; there are many unsolved difficulties 

 connected with them, and any account of them must at 

 present be regarded as provisional. 



