part 2] PALEOZOIC rocks or the llajs^golleis^ district. 201 



were originally bands of laminated silt, alternating with bands of 

 mud. The thickness of individual layers varies from perhaps 

 1/10 inch to several inches, or occasionally a foot or more, but 

 the monotonous alternation persists throughout. Except in the 

 Pen-y-glog Slates, the mud-bands usually exceed the silt-bands in 

 thickness, and there appears to be a fairl}^ constant thickness -ratio 

 of about 1 to 3 or 1 to 5, as between silt-band and mud-band as 

 measured now in the indurated and cleaved condition. There is 

 little doubt that the alternations represent seasonal variations of 

 the transporting power of the rivers which supplied the sediment 

 to the sea. 



The mud-bands have assumed a more or less perfect cleavage ; 

 but the silt-bands are frequently devoid of it, although in many 

 cases the cleavage passes through both. This factor, together Avith 

 the original thickness of the bands, and the angle between the dip 

 of cleavage and the bedding, produces an endless variet}" of 

 appearance within somewhat monotonous limits. 



Apart from the Pen-y-glog Slates and Grit no subdivision of the 

 group possesses any striking lithological characteristic by which it 

 can be readily identified and traced. Nor do the fossils give much 

 help, on account of their rarity and poor preservation. Hence 

 there is considerable doubt about the accuracy of the lines 

 representing the boundaries of the subdivisions of the Salopian 

 shown in the map (PI. V), which must be regarded as illustrating 

 the structure, and as offering a solution (admittedly imperfect) to 

 the riddle of the folding (see pp. 212-17), in the complex Llan- 

 gollen Sjaiclinorium. 



It appears probable that most of the accepted graptolite-zones of 

 the Wenlock and Lower LudloAv (as developed in the Welsh 

 Borderland, Shropshire, and elsewhere) are represented. The 

 general sequence and the zones are indicated in Table V (p. 202). 



(2) Wenlock Series. 



The Wenlock is thin in comparison with the Lower Ludlow 

 Series, and is relatively well known. ^ Its lowest member, com- 

 prising mudstones and slates with the fauna of the Cyrtograptus- 

 murcliisoni Zone, succeeds the Monograptus-crenulatus Beds of 

 the Upper Valentian conformabl}^. 



The lithology of this and the succeeding 3I.-riccartonensis Zone 

 may be described as banded earthy-weathering slates,- the upper 

 zone providing the roofing-slates of Glyn-Ceiriog, Moel-Fferna, 

 Pen-y-glog (Corwen Slate-Mine), and Moel-y-faen. In the 

 southern part of the district the roofing-slates are succeeded by 



1 G. L. Elles, Q. J. G. S. vol. Ivl (1900) p. 397 ; P. Lake, ibid. vol. li (1895) 

 pp. 9-22. 



2 They are of interest, on account of the problematical fossil Bem-ynia 

 carrutliersi (a plant of unknown affinities, now preserved as an anthracitic 

 substance). 



