126 STRATIGRAPHICAL GEOLOGY 



From the above account it will be seen that the succession in 

 the typical Bala district may be summarised as below : 



Foet. 

 Upper /Grey shales and shaly sandstones .... about 1000 



Bala \ Rhiwlas limestone . . . . . . 10 to 20 



Lower f Bala limestone . . . . . . . 10 to 50 



Bala ^ Grey sandy shales ..... perhaps 1600 



About 2600 



The succession in the Berwyn Mountains to the eastward has 

 been described by Mr. D. C. Davies, and more recently by Messrs. 

 Groom and Lake. 12 From the account given by the latter of the 

 series exposed near Glyn Ceiriog, it appears that a great thickness 

 of beds (the Paudy Series) below the Bala limestones must be 

 included in the Bala Series. These beds consist of volcanic ashes, 

 sandstones, and slates, and some of them contain many of the 

 characteristic Bala fossils ; their total thickness being about 1600 

 feet. In the Ceiriog district the true Bala limestone appears to 

 be faulted out, but elsewhere there is from 200 to 300 feet of such 

 limestone, containing the usual trilobites, brachiopoda, and corals. 

 Above this are calcareous shales and bands of limestone containing 

 the fauna of the Rhiwlas beds. These are succeeded by shales of a 

 considerable thickness. 



To the north and north-west of the Bala district the limestones 

 appear to thin out or to pass into a nodular calcareous tuff. In 

 the Lleyn peninsula the place of the Bala Series is occupied by a 

 thick set of sandy shales and mudstones of an olive-green colour, 

 and including thin beds of felspathic grit which are largely made 

 up of volcanic detritus. 



The Bala Series can be traced northward to Conway, and the 

 succession near that place has been described by Miss G. L. Elles, 13 

 who has divided the beds into the following group* : 



Feet. 

 Deganwy mudstones ......... 80 



Bodreida mudstones or Trinucleus Beds 350 



p , ["Upper, shales with Orthograptus truncatus \ 



, ^ a | '-! Lower, flags with Mesograptus multidcns, Climaco- j- . 310 

 sna es ^ graptus, and Dicranograptus 

 Conway Mountain Volcanic Series . . . . . over 2000 



Miss Elles regards the Lower Cadnant shales as Llandilian, 

 but almost all the species of graptolites recorded by her from these 

 beds occur also in the Mydrim shales of Carmarthen, and as the 

 total thickness of the sedimentary beds at Conway is only 740 feet, 

 it is probable that the base of the Bala Series is to be found in the 

 \volcanic rocks. 



