124 STRATIGRAPHICAL GEOLOGY 



Bala Series. In Pembrokeshire this series was described for 

 the first time in any detail by Messrs. Marr and Roberts in 1885. 10 

 They examined the succession of beds near Haverfordwest and Nar- 

 berth, and established divisions which have been adopted by the 

 Geological Survey officers, who have since mapped the beds through 

 Pembroke and Carmarthen, and have at the same time fixed the 

 horizon of passage between the Llandilo and Bala Series in this area. 

 The limestones are seldom all developed in one locality, being 

 lenticular beds which thin out entirely in some directions, but 

 when each occurs it is on the same horizon and the complete 

 succession is given as follows : 



Feet. 



. , . 1V f Slade and Redhill Beds .... 2000 



'" \ Sholeshook limestone .... up to 200 



( Robeston Wathen limestone . . ,, 100 



Oaradocian -j Mydrim shales with Dicranograptus . . 500 



I Mydrim limestone . . . . to 150 



About 2900 



The Mydrim limestone is quite local, and in its absence the 

 horizon can only be distinguished by the change in the species of 

 graptolites, the lower part of the Mydrim shales being characterised 

 by Dicranograptus Nicholsoni, the middle part by Mesograptvs 

 multiden-s, Dicranograptus Clingani, and Diplograptus calcaratus ; 

 while the uppermost 100 feet yield Diplograptus truncatus and 

 Climacograptus minimus, 



The overlying limestones are most fully developed near 

 Narberth, the lower beds consisting of fairly pure limestone, black 

 in tint and crowded with corals of the genera Heliolites, Halysites, 

 and Favosites, with many Brachiopods, but few trilobites. The 

 Sholeshook limestone has quite a different assemblage, being grey 

 or greenish in colour, impure and sandy in texture, and char- 

 acterised by an abundant trilobite fauna including Staurocephalus 

 globiceps, Trinucleus seticornis, Phacops Brongniarti, and Cheirurus 

 juvenis, with many Cystideans, Gastropoda, and Cephalopoda. 



The overlying beds form a monotonous series of bluish-grey 

 mudstones which are only fossiliferous on certain horizons ; they 

 sometimes include bands of limestone containing Favosites fibrosus 

 and sometimes more sandy beds which contain many species of 

 Orthis, while the shales contain Leptcena sericea. 



Northward this Bala Series extends over a considerable area, and 

 probably occupies large parts of Central Wales, along the anticlinal 

 axes of the numerous flexures into which the rocks of that region 

 have been folded, but little information about them is at present 

 available. 



