Llandeilo and Caradoc Beds, 71 



the genera Thecci and Conularia are found, and 6 species 

 of Bellerophon, and of Cephalopoda there are 5 species 

 of Orthoceras. Of univalve shells we have only 3 

 species Pleurotomaria Llanvernensis, Ophileta, and 

 Raphistoma, and several other fossils needless to enu- 

 merate. 



In all, 184 species are known at present in the 

 Arenig beds, mostly characteristic of these strata, for 

 only about 8 per cent, pass upward into this horizon 

 from the Tremadoc beds, a proportion of which go down 

 into the Lingula flags, and about 7 per cent, pass up- 

 ward into the Llandeilo flags. 



Though in Wales the base of the Arenig beds is 

 clear, it seems as yet impossible to draw any definite 

 physical boundary between the Arenig beds and the 

 overlying Llandeilo slates, for there is nothing like un- 

 conformity, and no marked lithological difference in the 

 passage from one to the other. We have already seen 

 that there is a very limited passage of species from the 

 Arenig slates into those of the so-called Llandeilo series. 1 



Just about this time an important episode took 

 place in the history of the Llandeilo and Bala beds over 

 large tracts of Wales and Cumberland, for a series of 

 volcanic eruptions occurred on a great scale while the 

 strata were being deposited (fig. 62, p. 322). To this 

 subject I shall by-and-by return. 



In North Wales the Llandeilo and Bala or Caradoc 

 beds combined, attain a thickness of from 4,000 to 6,000 

 feet, consisting chiefly of slaty rocks, sometimes inter- 

 stratified with grits and occasional bands of limestone, 

 of which the Bala Limestone is the most conspicuous. 

 The whole series ranges right round the mountains of 



1 The Llandeilo flasks of North Wales are very unlike those of 

 Llandeilo, which are generally called Upper Llandeilo beds. 



