536 Prof. W. Keeping — Geology of Aberystwyth. 



than we find it at Aberystwyth. Our specimens probably contain 

 carbonate of iron, for the brown oxide is frequently found colouring 

 exposed weathered specimens. 



There is a remarkable contorted structure very common in our grit 

 rocks,^ which may be well seen in the blocks brought from Ystrad 

 Meurig Quarry, and now forming the stone pier at Aberystwyth. 

 Conci'etionary action may perhaps explain this in part, but I am 

 disposed to look to water as the more probable agent ^ — the water 

 percolating through the more sandy layers after the deposition of 

 the rock, and so altering the arrangement of the particles. 



Arrangement of the great BocJc-masses, and their Age. — The regular 

 alternation of old sandy and muddy deposits (grit and shale) is very 

 striking around Aberystwith, and for about five miles to the east. 

 Professor Sedgwick called these beds the Aberystwyth Group. Next, 

 further to the east, a great series of slaty rock succeeds — the Metal- 

 liferous Slate Group ; then at Plynlimmon we reach another grit 

 group (Ply nlimmon Group), in thicker beds than at Aberystwyth, 

 and with some quartz-conglomerates (part of the Plynlimmon Group 

 of Sedgwick). 



Altogether these several rock groups make up an enormous thick- 

 ness of strata. An actual measurement of only that part of the 

 series between Aberystwyth and the Devil's Bridge gave us a thick- 

 ness of over three and a half miles.^ 



Of all these deposits the Aberystwyth rocks are the lowest, for all the 

 beds to the east slope over them, just as the lines here drawn towards 



B incline over those nearer A. 



As we walk to the north or to the south, we shall find again that 

 the rock deposits slope away from, and are therefore newer than, 

 those at Aberystwyth. Even to the west, too, the beds slope away 

 seawards, as may be seen when we walk along the shore, or notice 

 the cliff sections. So that Aberystwyth is the centre of what is 

 known to geologists as an Anticlinal axis — the axis of a huge dome 

 of rock-masses. The slope to the north and to the south is much 

 less steep than to the east, so that an almost north and south line of 

 outcrop has its axis running through AUt Wen, Aberystwyth, and 

 over Constitution Hill on to Clarach. 



The exact geological age of the Aberystwyth rocks cannot yet be 

 fixed. That they are extremely old is clear enough, for just as we 

 see in the little diagram above, that the beds at A, slope under those 

 at B, so we might have drawn a long series of sloping lines on to C, 

 where G should represent the coast of the south-east of England ; 



1 This contorted structure forcibly brings to mind the irregular contortions which 

 are so generally found in foliated rocks, such as mica-schist. The same cause may 

 •well have produced the structure in both rocks. 



2 The great apparent thickness of a gi-oup of rocks in the Moffat District of 

 South Scotland has been explained by Mr. Lapworth as produced by frequent sharp 

 foldings, by which the beds are repeated over and over again. But we have no 

 evidence that such an explanation can be applied to the rocks between Aberystwyth 

 and Plynlimmon. 



