170 



ON THE GEOLOGY OP CENTRAL WALES. 



and the Plynlimmon synclinal. Secondary axes of upheaval bring 

 up the lower beds of the series at the Devil's Bridge and near Cwm 

 Symlog. Innumerable minor foldings preserve the same north-and- 

 south strike. 



The included fossil remains, especially the Graptolites, prove the 

 Aberystwyth Grits and Metalliferous Slates to belong to the same 

 general geological horizon — namely, on the parallel of the upper 

 Birkhill series of S. Scotland and the Coniston " Mudstones " of the 

 English Lake district. The Plynlimmon Grits are probably an 

 arenaceous development of the Tarannon Shales ; and the Cwm-Elan 

 conglomerates and Rhyader Pale Slates belong to the same series. 



Following up our Cardiganshire series to the lines of junction 

 with older and newer groups in parts of Montgomeryshire, South 

 Cardiganshire, Caermarthenshire, and Merioneth, we find no evidence 

 of a break in any part of the rock-groups ; but there is concordant 

 evidence of lithological passage from the Bala to the Llandovery 

 groups, and from these upwards into the Tarannon Shales and the 

 Denbighshire Grit series. On the contrary, in Denbighshire and 

 ]S".E. Merioneth, Professor Hughes has shown that, just as in the 

 English-Welsh border districts, there is an important stratigraphical 

 break at about this Llandovery period. Now the beds below that 

 break in N". Wales are the Bala group, i. e. inferior to our Cardigan- 

 shire series ; those above it are basement-grits and conglomerates 

 (the Cor wen Grit), covered by the pale slates, with some black 

 Graptolitic bands. These latter contain some species in common 

 with the Cardiganshire group, but also others characteristic of a 

 higher horizon than our fossiliferous beds. 



Thus all the facts are harmonious in pointing to a continuity of 

 the Silurian and Cambrian deposits in West and Central Wales, 

 while an important break exists in the east and north-east. Our 

 Cardiganshire group is only partially developed (its upper part) in 

 the latter district, being, in fact, there represented by a great strati- 

 graphical break — Sedgwick's original May- Hill unconformity. 



I conclude that, while in the latest Cambrian times (Sedgw.) and the 

 dawn of the Silurian era the elevatory forces, acting in a north- and- 

 south direction, lifted up the sea-bed to form a land-surface over the 

 west of England and the Welsh borders, these forces influenced the 

 greater part of Wales only in a less degree — producing, it may be, 

 the shallower water in which the Aberystwyth grits were laid down, 

 but not interfering with the continuous deposition of sediment and 

 the unbroken sequence of the geological record from the Cambrian 

 to the Silurian eras. 



