CAMBRIAN AND ORDOV1CIAN ROCKS. 8l 



are in the same I with volcanic ash, 



in terst ratified m schists, the whole being affected 



by II to which they have since been 



• British (i< gical gins with 



S which indicate volcanic outbursts, 



and the imulation of grits and 



. similar 

 the present day ; but 

 the rocks have been modified from their original 

 . chemical chai ij 



CHAP IKK XI. 

 .M];klAN AND 01 KS, 



Tm ciformity between the pre- 



mbrian r< m ks, which implies a 

 me, unrepresented by deposits 



in the I - which have examined. The 



ibrian r s thickness ; and 



in Britain are probably not less than 30,000 feet 

 thick in Wales and th< f Eng- 



land. 



There is di e of opinion as to the use 



of the term Cambrian. Some writers make it 

 include four groups of rocks, named Longmynd 



cs, Menevian beds, Lingula flags, and Tre- 

 madoc slates. I M irry the name higher 



and make it include the succeeding rocks named 



mig, Llanvirn, Lower Bala and Middle Bala 

 and Upper Bala, which have also been grouped 

 as Ordovician. 



The overlying strata, termed May Hill rocks, 



