Vol. 67.] ASSOCIATED ROCKS OF NORTH-EAST MONTGOMERYSHIRE. 445 



area itself is surprisingly like that succession. 1 It appears, how- 

 ever, that the succession in South Shropshire is not complete. 

 Eepresentatives of the Pwll-y-glo Group do not seem to occur, 

 while the Ashgillian has not previously been recognized. I would, 

 however, correlate the black shales of Gwern-y-brain with the 

 Trinucleus Shales of the Onny Eiver, placing the latter in the Ash- 

 gillian of Dr. Marr. This I have confirmed, and I may here record 

 for the first time the fact that I have found Diplograptus (Ortho- 

 gvaptus) truncatus var. socialis in considerable numbers in the 

 Trinucleus Shales. This would correlate the horizon with the 

 Barren Mudstones and the zone of Dicellograptus complanatus at 

 Dobbs Linn, and would place them in the lower part of the 

 Ashgillian. It would seem, therefore, that the Shelve area was in 

 all probability isolated from the Welshpool district in Caradoc 

 times, just as it was from the Caradoc area, and that the beds in 

 the former area were laid down in a northern*extension of the Caradoc 

 Sea which swept round the Longmynd. If this be so, there is 

 in all probability a prolongation of the Longmynd ridge under the 

 Long Mountain. 



(3) The Long Mountain. — The Silurian succession in the 

 Long Mountain [7] [9] [10] is similar in most details to that of 

 the Welshpool area. The graptolitic zones, absent in the Long- 

 Mountain succession, are absent here also. There are, however, 

 one or two points of difference, the principal being the occurrence 

 of limestones, locally, in the Wenlock Shales. ]Nor does the zone 

 of Monograptus vulgaris seem to be so well developed in this 

 western area ; otherwise the zones correspond fairly well. 



(4) South Wales. — The strata here are easily correlated, 

 although deep-water conditions seem to have lasted in South Wales 

 right through the Caradocian period. Unfossiliferous black shales, 

 overlying beds with Trinucleus seticornis and underlying mudstones 

 which contain fossils somewhat similar to those of the Powis- 

 Castle Beds, occur near Haverfordwest, and may represent, in part, 

 the similar shales of Gwern-} T -brain. 2 



(5) Central Wales (Rhayader). — There are some points of 

 resemblance and some points of difference between the succession at 

 Ehayader 3 and the sequence at Welshpool. The characters of the 

 ' cleaved black slates ' agree almost entirely with those of the black 

 shales of Gwern-y-brain. In Gwern-y-brain, however, they are 

 not cleaved and are, in places, very fossiliferous. Moreover, at 

 Ehayader, these black slates apparently pass up conformably into 



1 C. Lapworth & W. W. Watts, ' The Geology of South Shropshire ' Proc. 

 G-eol. Assoc, vol. xiii (1894-95) pp. 312, 319-20.* 



2 F. R. C. Reed, 'The Base of the Silurian near Haverfordwest' Geol. Mag. 

 dec. 5, vol. iv (1907) pp. 535 et seqq. 



3 H. Lapworth, ' The Silurian Sequence of Rhayader ' Q. J. G. S. vol. lvi 

 (1900) pp. 124 et seqq. 



