118 ME. H. LAPWOETH ON THE [Feb. I9OO, 



The attenuation of the conglomerates in the western areas, and the 

 gradual lessening of the amount of denudation of the Gwastaden 

 rocks indicate an ancient land-surface sloping from west to east, 

 and facing the western side of the old hollow already described ; 

 it is not improbable, therefore, that in the absence of the unfortunate 

 Abernant Fault we should find no great extension of at least the 

 lower portions of the Caban Group, were we able to follow them 

 to the westward. Their appearance on a map would most likely 

 be that of a huge lenticle dying out eastward and westward 

 beneath the Rhayader Group, and somewhat resembling a hand 

 thrust from beneath a blanket. 



An interesting question that is likely to be raised is the deri- 

 vation of the materials of the Caban Conglomerates. The large 

 grit- and slate-pebbles seem to be of Gwastaden age, and their 

 size points to the fact that they cannot have travelled from any 

 great distance. It appears, therefore, most probable that, being of 

 local occurrence, they were derived from the bordering cliffs of the 

 old Caban coast, from which fragments dropped into the shallow 

 water of the sea, forming banks of shingle, the pebbles of which 

 successive tides rolled into their present rounded forms. 



The finer fragments of igneous rock — felsites and the like — are 

 more difficult to account for. The nearest masses of igneous rock 

 occur in the Carneddau and Llanwrtyd hills on the south. It is 

 therefore just possible that these rocks in the localities mentioned 

 furnished many of the felsites in the Caban Conglomerates, and are 

 at the same time responsible for the felspathic fragments of the 

 matrix. The confirmation of this suggestion, however, awaits 

 more extended investigation, and until such is made the matter 

 must remain uncertain. 



One word more before we leave this most interesting Caban 

 Group. Is it not possible that the synclinal form of the Caban 

 Series is partly the result of the deposition of the beds on steeply- 

 sloping shores ? To prove this would be difficult. The base 

 of the Rhayader Group runs nearly parallel to the strike of the 

 Gwastaden rocks, and it seems only natural to suppose that once 

 the Caban rocks were laid down the old Gwastaden conditions were 

 resumed. On the other hand, the syncline is carried well into the 

 Pale Shales between Coed-y-mynach and Llanfadog-Uchaf. Much 

 of the folding, therefore, appears to have taken place after the Pale 

 Shale period. 



Finally, we notice that the Cerig Gwynion Grits and the Caban 

 Conglomerates attain their greatest thickness in the approximate 

 axis of the Caban syncline, and it is therefore not improbable 

 that the syncline is more or less a tectonic structure throughout, 

 which commenced in Gwastaden times, was continued through Caban 

 times, and had not come to rest until long after the deposition of 

 the Rhayader Pale Shales. 



