(reological Society, 



339 



f'C. YSTWTTH 



Stage. 



/ Pale mu( 

 C lamina 



Valentiax 

 '(Silueian). 



A B. POXT 

 Eewtd< 



Si:AGE 



■Haktfell 



(OttDO- 



dstones with numerous 

 ated grit-bands. 



5. Zone of Monograptus 

 Jialli. 



Pale-blue And green- 

 ish mudstohes 



with bands of dark 

 i|ei wen | 3_ 2o„e „£ Monographs , 

 ■ /'""P." I regularis. |graptoliferous shales, 



4. Zone of Monograptus 

 sedgwicki. 



Cvrmere 



a. Group. < 



334 feet. 



f A. Pltn- f 



LiMON ■{ Tj-'n-.v-maen* 



vician). I^ Stage. i_ Group. 



regularis 



2. Zone of Monograptus 



leptotheca. 



1. Zone of Mesograptus 



magnits, 



3. Monograptus spp. 



Beds. 



2. Zone of Diplograptus 

 acuminatus. 



and some thin 

 gl-een flags. 



bark rusty- 

 weathering 

 shales and 

 limestones. 



1. Zone of Glyptograp 

 tus persculptus. 



Dark mudstones, grits, and som« 



Mottled Beds ' and 

 blue mudfetones. 



congloraeVutes. 



The distribution and characttTS of these beds are described. The 

 ' Mottled Beds ' form the base of the Sikirian and rest sharply on the 

 underlying beds, and there is evidence of complete discontinuity at 

 this level ; they have proved of great service in elucidating the 

 structure. The Monograptus spp. Beds c'ontain gmptolites which 

 elsewhere pertain to the zones of Monograptus triangulatMS, 

 M. cypJius, and M. acinaces ; but another zone, that of M-. atavtis, 

 has not been proved, although it probably occurs. The Denven 

 Group consists of a regular alternation of mudstones and shale-bands 

 with graptolites, "which have also proved of service in mappings, 

 Only a small thickness of the Ystwyth Stage occurs, and no sub- 

 divisions are attempted. 



The rocks are sharply folded, and sometimes overfolded, towards 

 the east. Their axes range approxuuately north-north-east and 

 south-south-west ; the folds in the central area pitch northwards) 

 but north of the Dovey a southerly pitch sets in. Each large 

 fold is composed of a number of slnaller folds having pai^llel axes, 

 and changing in pitch more frequently than the larger folds-. 

 Strike-faults of considerable magnitude range nearly parallel with 

 the folding axes, and are in all cases overthrusts towards the east. 



Of greater interest are the transverse faults ranging nearly east^ 

 north-east and west-south-west. Most of these are small, but theif' 

 course across the higher ground is indicated by well-defined notches 

 in the ridges that they cross. Two of these faults, the Pennal 

 and Llyfnant Faults, are shatter- belts. The Llyfnant Fault dis- 

 places several folding axes, and overthrusts to the east on the north 

 side. Its vertical displacement is on an average about 300 feet^ 



