Yol. 56.] SILTJEIAN SEQUENCE OF EHAYAE-EE. 125 



such a wealth of palseontological evidence has been collected that 

 the various components brought together in proof of the order 

 of succession are now finally welded into a rigid and inflexible 

 whole. 



The Khayader Yalley, as I have shown, is floored by rocks 

 belonging to three great groups — the Gwastaden, the Cab an, 

 and the Eh ay ad er. The lowest of these, the Gwastaden 

 Group, has a maximum thickness of over 1800feet. It is underlain, 

 with apparent conformity, by a mass of highly-cleaved Dark- 

 blue Shales; and is overlain unconformably by both the Caban 

 and Rhayader Groups. The base of the Gwastaden Series is formed 

 of a thick mass of grits or greywackes, which thin out eastward 

 and thicken out westward. They contain graptolites of the genus 

 Climacograptus only, and pass up gradually into a series of flags 

 and grits, which yield both Climacograptidae and Diplograptidse. 

 These are succeeded by a thick group of shales and mudstones — 

 the Ddol Shales and Gigrin Mudstones — where the first 

 Monograptidae make their appearance : the variations in the forms 

 of Monograptus rapidly increase towards the summit of the group, 

 where this genus becomes predominant. 



The Caban Group which overlies the Gwastaden rocks has a 

 maximum thickness of 1500 feet. It is found only in the western 

 areas, between the Gwastaden and Rhayader Groups. Its lower- 

 most division — the Caban Conglomerates — is made up of two 

 massive lenticular conglomerates, with an intermediate set of shales. 

 The conglomerate-group passes up into a set of fine-grained grits, 

 the Monograj)tus-Sedgwichii Grits, and finally into the shales 

 and flags of the Gafallt Shale division. Each of the various 

 divisions throughout the whole of the Caban Group, when followed 

 eastward, is found to be overlapped or covered up by the next higher 

 bed, so that eventually the whole of the Caban Group completely 

 disappears beneath the overlying Rhayader Pale Shales. 



The Rhayader Pale Shale Group is made up of pale green, 

 blue, and grey shales and mudstones. It overlaps the Caban 

 Group, but not necessarily unconformably. East of the point of 

 complete overlap, these Pale Shales rest irregularly upon the Gwas- 

 taden Group, lying on lower and lower rocks of the group, as we 

 pass eastward ; and it is not unlikely that, if traced farther east- 

 ward out of the Rhayader district, they would be found to have 

 completely covered over the whole of the Gwastaden rocks. 



From what we have gathered, the history of deposition seems to 

 be somewhat as follows. After the Gwastaden rot-ks were laid 

 down, the sea-floor was elevated into land and denuded. During 

 the elevatory period a great hollow was scoured out to the west. 

 Rapid sinking then followed, and into the eroded basin the sea 

 entered, filling the cavity with the Caban sediments. At the 

 commencement of the Pale Shale period the hollow was practically 

 levelled up ; and, as sinking proceeded, the Rhayader muds were 

 deposited over the sediments of both the Caban and the Gwastaden 

 epochs. 



