'94 ME. H. LAPWOETH ON THE [Feb. I9OO, 



has been built for supplying the Elan Tillage with water, that the 

 red -weathered nags at the top of our second division (Ab^ make 

 their appearance. I have not discovered fossils in these beds above 

 the dam ; but immediately below the dam Diplograptus longissimus, 

 D. modestus (?), and Climacograptus normalis were detected. These 

 graptolites occur also in the loose blocks which have been removed 

 from the site of the dam-trench, and fragments of rottenstone 

 may be seen in the loose scree on the banks of the brook ; and 

 on the east side of the path above Gro-isaf beds of this material 

 can be seen in situ dipping into the hill. The unmistakable 

 appearance of this characteristic rock and the occurrence of the 

 typical fossils point conclusively to the horizon of the Rotten- 

 stone Beds (Ab ). 



jSTo further exposures are visible after reaching the river-alluvium 

 below Gro-isaf. A north-and-south fault passes through Gro-isaf, 

 crosses the slope above the crest of Craig Fawr in a well-marked 

 groove, and joins the Abernant Fault, somewhere in the river-flat. 

 It will be more expedient, however, to deal later with the ground 

 lying to the west of these dislocations. 



Having now examined those portions of the Gwastaden 

 Series that lie within the western area, we are in a position to 

 compare them with their equivalents in the central and eastern 

 districts. 



One fact is evident, that in the western area a fresh set of 

 deposits, differing entirely from any that we have seen before, has 

 made its appearance. The lowest bed in this new group, which 

 we have, as yet, merely touched upon, is a conglomerate, which 

 crosses irregularly over the lower zones of the Gwastaden Group. 

 In our first section through the western area this conglomerate was 

 found on the horizon of the Rottenstone Beds (Ab 2 ). In the second 

 it approached within a short distance of the Cerig Gwynion Grits 

 (Aa). Above Ty'n-y-graig it overlies the Diplograptus-modestus 

 Flags (Ab 3 ) ; and in the Nant-y-Gro the bed has disappeared. 

 A glance at the map (PL VII) shows that this conglomerate- 

 mass (J5«. ) creeps round from a north-easterly and south-westerly 

 strike into a north-and-south line; and on the west side of 

 Cnwch Hill it reposes in beds high up in the Diplograptus-modestus 

 Flags (Ab 3 ), and approaches nowhere near the Nant-y-Gro. "Now, 

 two explanations only can be given for the irregular position 

 of this new group : — (1) There may be a thrust-fault at the base 

 of the conglomerate, the plane of which, in order to bring about 

 the peculiar outcrops of the beds, must be nearly horizontal ; or 

 (2) there may be an unconformity between the Gwastaden Group 

 and this new conglomeratic series. For reasons which I propose 

 to bring forward later, the first cannot exist, and it will be shown 

 that this new bed — the Cab an Conglomerate — rests uncon- 

 formably upon the Gwastaden Group. 



