Vol. 65.] SUCCESSION AROUND PLYNLIMON AND PONT EKWYD. 465 



as being arranged in ascending order from west to east or from 

 north-west to south-east, but he admits that the actual order of 

 superposition is obscure. He included in the 'Plynlimmon Group' 

 most of the rocks to the east of the Aberystwyth Group, as far as 

 Llangurig. 



It is now known that, although Sedgwick's groups are not 

 mutually exclusive, and the order of superposition is not what he 

 believed it to be, yet the limits of age assigned to the groups 

 collectively are remarkably near the truth. 



About this time the area was being examined by the officers of 

 the Geological Survey, and the maps were published between 184S 

 and 1850. A horizontal section across the southern portion of the 

 district was issued about 1845. The mineral veins were examined 

 by "Warington W. Smyth, and the results were published in 1848 (2). 

 From certain letters which Sir Andrew Ramsay wrote to Aveline 

 in 1856, it appears that he regarded the grit-groups of Aberystwyth, 

 Cwm Ystwyth, and Plynlimon as one and the same group, which he 

 tentatively placed at about the level of the Lower Pentamerus- 

 Sandstones [= Lower Llandovery] or the sandstones of the Cefn-y- 

 Gamrhiw ridge near Rhayader [? = Upper Bala and Lower Llan- 

 dovery]. A few months later, Aveline seems to have seen reason 

 for raising the Aberystwyth Grits to a higher level. 



In 1866 the official memoir on North "Wales, compiled by 

 Ramsay (3), appeared. On the map accompanying it the letter 6 4 

 (indicating Lower Llandovery age) is sprinkled over a large part of 

 Central "Wales, especially on the grit-outcrops such as those of 

 Plynlimon, Aberystwyth, and Cwm Ystwyth. 



In 1869 Mr. Hopkinson (4) recorded the following species of 

 graptolites from Aberystwyth : — Monograptus priodon, M. Hisingeri, 

 M. SedgwicJci var., and gave a figure of the last-named. He 

 assigned the rocks to the Caradoc age, in accordance with the then 

 current views on the geological horizon of these species. The value 

 of the graptolite as an index of age had not at that time been 

 discovered ; and even when, several years later, its use had been 

 recognized, the significance of Mr. Hopkinson's find was not realized, 

 else from the presence of Monograptus priodon the Gala or Tarannon 

 age of these rocks would have been suspected. 



In 1872 W. S. Symonds in his ' Records of the Rocks ' refers to 

 the rocks of Plynlimon and of Devil's Bridge as of Lower Llan- 

 dovery age, and mentions Atrypa [now Meristina] crassa as having 

 been found at the latter place. 



The first systematic attempt to reduce the rocks of this district 

 into order was made by "Walter Keeping (5), who communicated 

 his results to the Geological Society in 1881. He established three 

 groups in ascending order : (1) the Aberystwyth Grits ; (2) the 

 Metalliferous Slate Group ; and (3) the Plynlimon Grits. Groups 



1 & 2 he united under the name of Cardiganshire Group ; groups 



2 & 3 constituted the Plynlimon Group of Sedgwick. 



He followed his predecessors in regarding the grits of Plynlimon 



2k2 



