CAMBRIAN AND ORDOVICIAN ROCKS. S5 



. where about j 2,000 feet of volcanic ash 

 and Honister id Sea- 



thwaite, mark the beginning of volcanic action 

 which continued through the accumulation of the 



cks. In North Wales 



Rhyplitic "... - ntinued to be I in the 



1 which followed. They arc seen 



and the Conway falls. 



Rhyolitic lavas arc seen in tl on the 



Llanb Bedd- 



irt, and about Snowdon. 



iture in the life of 



imbrian extraordinary 



■ 



ire found not only in South Wales and the 



.<■ Distri '. b il f forms 



and structure in North 



Trilobiti in number of - and 



rhe g eurotom pod 



y known at the present day from living 



in tl ' inction, ap- 



■ me in l r \ renig r< tcks 



• Llanvirn, near St. Davids. In this period an- 



Othe xphalus y which continues to 



mt during the primary period, is found 



the first tin. 



eral corals make their appearance in the 

 ndeilo rock- the most ancient 



representati ip in Britain. Among 



them is the chain coral Hal ;d a species of 



which are important genera 

 in the primary rocks. The Crinoids increase in 

 number. 



Several genera of Brachiopod shells appear, 

 two of which, Rhynchonella and Crania, afterwards 



