BALA SERIES. 73 



The Snowdonian slates, as exhibited in the pass of Nant Francon, 

 are full of fossils, occurring in five or six bands between the beds of 

 ash and felspathic trap. The species include StropJiomena expansa, 

 Orthis flabdhdum, and Bellavphon} 



North of Moel-Siabod (according to Sir Andrew Ramsay) the 

 Bala beds assume a markedly different character from that which 

 they possess between Dinas-Mowddwy and Dolwyddclan, for they 

 contain a much greater number of interbedded felstones and 

 volcanic ashes, which range northward to Conway, and thence south- 

 westwards along the higher Caernarvonshire mountains. Carnedd- 

 Llewelyn, Carnedd-Dafydd, Y-Glyder-fawr, Snowdon, and Moel- 

 Hebog are the chief mountains in this, the wildest and grandest 

 part of North Wales. And these, like the ranges of Cader Idris, 

 the Arans, and the Moelwyn, consist in a great degree of volcanic 

 products. These volcanic rocks belong to two sections of the 

 [Cambrian] period, for the felstone-porphyries and felspathic 

 ashes, and perhaps even the intrusive greenstones of Merioneth- 

 shire, were formed during the deposition of the Llandeilo strata, 

 while the same species of thick-bedded traps and ashes on Snowdon 

 and the surrounding mountains are high in the Bala or Caradoc series 

 (see Fig. i o). In both cases they form the highest mountain ranges in 

 Wales, not from upheavals caused by the intrusion of igneous masses 

 in special areas, but simply from the circumstance that long after 

 their formation, and after the volcanoes had become extinct (having 

 been already much denuded and lying deep below thousands of feet 

 of newer strata), the whole of the rocks of the area have been 

 disturbed ; and the hard igneous masses now rise so high because 

 they have better withstood degradation than the slaty rocks with 

 which they are interbedded. The ranges formed of the lower por- 

 phyries, etc., of Cader Idris, Aran Mowddwy, Arenig, and Moelwyn, 

 lie in the Arenig beds midway up in the strata of the great 

 Merionethshire anticlinal, while the peaks of the still newer series 

 of igneous rocks that form the range of Moel-Hebog, Snowdon, 

 and Carnedd-LIewelyn, actually lie in the middle of a basin. The 

 whole form but minor parts of an old mountain system, of which 

 Wales is only a fragment.^ 



The shales and andesitic ashes of the Berwyns are grouped with 

 the Lower Bala and Borrowdale Series by Mr. J. E. Marr.^ 



North-west of Bala, the Rhiwlas limestone, a grey limestone 30 

 to 40 feet thick has been considered to be the equivalent of the 

 Bala limestone. It has yielded many fossils, but these, according 

 to recent researches, suggest a higher horizon. In Montgomery- 

 shire, the Meifod slaty beds, which belong to the upper part of the 

 Bala beds, have also yielded many fossils. In Denbighshire many 

 specimens, including Orthis alternatUy have been obtained from 

 Cerrig-y-Druidion. 



1 J. W. Salter, Q.J.ix. 177. 



2 A. C. Ramsay, Geol. N. Wales, edit. 2, pp. 130, 131, 141. 

 ^ Q. J. xxxvi. 278, 



