Vol. 56.] THE SILUKIAN SEQUENCE OF RKAYADEK. 117 



maximum thickness is exposed beneath the overlying group. In the 

 section the beds of the Gwastaden Series are taken as horizontal, 

 and all overlying rocks are plotted from off this datum. At Glan 

 Elan Earm the Gafallt Shales would begin to peep out beneath the 

 Rhayader Group, were it possible to remove the river-alluvium 

 from the underlying rock. A little farther west the Monograptus- 

 Sedgwickii Grits appear beneath the shales, and still farther west 

 the conglomerates beneath the grits. We have, in fact, in the 

 vertical section, the same set of conditions as those that exist along 

 the line separating the two groups in the field. The lower down 

 we go into the Gwastaden Group, the lower are the rocks that make 

 their appearance in the Caban Group. The beds are shown 

 thinning out and overlapping one another, as on the ground. 



A study of this section enables us to infer the various stages in 

 the history of the period in which these rocks were deposited, and 

 at the same time furnishes us with a clear conception of the 

 meaning of the various overlaps that have taken place. It is 

 probable that at the close of the period of deposition of the Gwa- 

 staden rocks the sea-floor was elevated into dry land. During this 

 elevatory period the newly-deposited rocks were denuded to a 

 surface represented by the outline shown in fig. 17 (p. 116). The 

 least amount of denudation occurred in the area now occupied 

 by the Wye Valley ; east and west of that area more material 

 was removed. To the west of the Elan a great hollow was worn 

 out, the land-surface sloping rapidly away from Glan Elan towards 

 Caban Coch. Subsidence of the land now commenced, and into the 

 eroded hollow the sea gradually encroached, laying down the beds 

 of the Lower Conglomerate (Ba^). It is possible, and even probable, 

 that these beds were deposited in the estuary of a river, which we 

 may assume was responsible during the ' dry-land period ' for the 

 scouring-out of the hollow. 



As subsidence went on the sea extended farther inland, so that 

 in Gafallt times the sea-waters were laying sand and mud not only 

 upon the Caban sediment, but upon the old surfaces which were 

 dry land while the conglomerates were being laid down. In 

 this way the disappearance of the conglomerate-group in certain 

 areas is easily accounted for. 



At the close of Gafallt times and at the beginning of the period 

 of the deposition of the Rhayader Group, we may conclude that the 

 old hollow was filled up by the previous deposits, so that the sea had 

 now an almost flat floor over which to extend as subsidence went 

 on. A sinking of a very small amount consequently would mean a 

 rapid inland progression of the sea-line. It would therefore seem 

 to follow that after a comparatively short epoch the shore lay 

 probably several miles to the eastward, bounding some broad sea 

 in whose quieter waters the pale shales and mudstones of the 

 Ehayader Group were laid down. 



Such, then, appears to be the history recorded by these rocks. How 

 far westward the old hollow extended is at present a mystery. 



