126 MR. H. LAP WORTH ON THE [Feb. I9OO, 



(b) Comparison with the Deposits of other Areas. 



The accompanying Tables (I & II) show the geological range and 

 distribution of the Rhayader fossils in Britain and abroad. The 

 first set of columns in each table indicates the arrangement of the 

 fossils in the various groups of the Ehayader district ; the others 

 their distribution in several British and foreign localities, for the 

 purpose of correlation. (See also Table III, p. 128.) 



Taking, first, those fossils of the Rhayader rocks which occur also 

 in the Silurian rocks of Southern Scotland. It is an easy matter 

 to prove that the Rhayader graptolite-forms answer as a whole 

 to the fauna of the Yalentian rocks of that region, and that the 

 ascending succession of the species is the same in both areas. The 

 Gwastaden Group represents the Lower Birkhill or Lower 

 Llandovery, the Caban Series the Upper Birkhill, while 

 the Rhayader Pale Shales present us with the fossils of the 

 succeeding Gala or Tarannon Group. 



A careful examination of Tables I & II (facing this page) shows 

 that, out of a total number of 29 species of graptolites contained 

 in the Gwastaden rocks (new and uncertain forms included), 

 21 have been found in the Lower Birkhill Group, while only 13 

 of these occur in the Upper Birkhill. Of this total quantity, the 

 number of species rigidly confined to the Gwastaden Group is 24, 

 10 of which are peculiar to the Lower Birkhill, and 1 only to 

 the Upper Birkhill (Diployraptus sinuatus, which may be found 

 in the lowest band of the latter). Among these characteristic 

 forms may be specially mentioned Monograptus flmbriatus, M. tri- 

 angulatus, Diplograptus acuminatus, I), modestus, and Climaco- 

 graptus rectangularis, — forms which have never been known to 

 occur outside the Lower Birkhill Shales, or their equivalents, 

 either in the South of Scotland or even throughout the whole of 

 the Silurian rocks of Europe. We can, in fact, parallel every zone 

 in the Gwastaden Group with its representative in the Moffat 

 area. The Cerig Gwynion Grit no doubt takes the place of the 

 ' gingerbread band ' at the base of the D. acuminatus-zone. At 

 Rhayader, as at Moffat, this zone contains only Climacograptus 

 nor malls and its varieties. The remainder of this zone is probably 

 represented by the lower half of the Dyffryn Flags, with its restricted 

 D. acuminatus. A portion of this latter group, with the addition 

 of the M. tenuis-zone, would appear to lie on the same horizon 

 as the zone of D. vesiculosus, in which the first Monograptidae 

 (M. tenuis, etb.) with JD. modestus present themselves. The suc- 

 cession of the life-forms throughout the remaining beds is identical 

 with that of the M. gregarius-zone. First in order comes Mono- 

 graptus cyphus ; this is followed by the ephemeral M. triangulatus, 

 in company with M. flmbriatus and gregarius, and the first species 

 of Rastrites ; while towards the summit of the group, the first 

 members of the prolific M. lobiferus-i'&m\\y come into view. This 

 is in perfect accord with the order of these forms, as they are 



