« PreviousContinue »
NOTES ON THE CORRELATION OF THE LATER AND POSTPLIOCENE TERTIARIES ON EITHER SIDE OP THE IRISH SEA, WITH A REFERENCE TO THE FAUJA OF THE ST. ERTH VALLEY, CORNWALL. Br ALFRED BELL.
[communicated BT DR. Scharff.]
[Read Notembbb 14, 1892.]
With the exception of the country bordering the western end of the Caledonian canal crossing Scotland, there is very little possibilitr of the discovery of fresh deposits of later Tertiary age in the United Kingdom. So many specialists having taken part in the investigation of the faunas of the different members of this group, a fairly comprehensive view of the whole is now available for study; and a comparison and correlation of the various organic assemblages may suggest some lines of research as to the course of events that have led to the delimitation and configuration of this portion of the earth's surface.
The fauna of the Post-Pliocenes is larger than is generally sapposed, a brief summary of a catalogue compiled during some years past from all available sources presenting the following results:—
For present purposes the marine division of the fauna, and primarily the molluscan element will be mainly referred to, as affording the most ready means for collating the synchronism of the different horizons in the limits of the Post-Pliocene stage, and the least persistent in time if compared with the land and freshwater equivalents of the same epoch.
Practically speaking one ont of seven of the land and freshwater species are either exotic, or the habitats are unknown, or, excluding some half dozen chiefly from the Norfolk Forest bed as being doubtful species, are as few as one out of ten, the marine forms having more than one-fourth part under the same categories.
In compiling a catalogue of Tertiary fossils, I found that with very few exceptions, such as that of the Turbot bank, Co. Antrim, the various deposits of the Post-Pliocene age fall under the ensuing headings.
1. Pre-Glacial.—The Norfolk beds in the Bure Valley, and the Weybourne Sands—the marine or "shelly" Wexford sands and gravel, and the forest bed, and overlying laminated clays—in the Norfolk Cliffs, the marine deposits being characterised by the abundance of Tellina lalthica, a hitherto unknown form in the British area.
2. Arctic, or the sands and clays at Bridlington, East Scotland, and in a few western localities which yield almost a purely northern, or Arctic fauna, such as now lives within the Arctic circle.
3. Mid- or Inter-Glacial, including most of the valley gravels, and the marine deposits at Selsey in Sussex, and Killiney Bay, and those deposits which indicate by the southern elements in their contents the influence of a much increased temperature, probably exceeding that of the present day.
4. Late Glacial, embracing many of the cave deposits, the Lancashire and other N. "W. of England drifts, the clays and sands of Balbriggan Bay, Ballygeary, Ballyrudder, Howth, and the most elevated gravels on the Dublin Mountains, Moel Tryfaen in N. Wales, and elsewhere in Shropshire, Cheshire, and Lancashire, indicative of deep submergence, Palaeolithic man first appearing in
E.I.A. FBOC., VOL. in., SEE. LT. 3 A
the lowest beds of Kent Hole towards the close of the Arctic stage, and more fully developed in the Inter-Glacial period. The Later Glacial representing the more advanced or reindeer stage.
5. Putt-glacial or Neolithic.—Most of the Scottish beaches and other deposits yielding a few boreal forms still doubtfully existing on the northern coasts, and later the. Estuarine clays in Ireland, the Carse clays of Scotland, the Buttery clays of the Fen lands, and the marine accumulation of Portrush, Largo Bay in Fife, and elsewhere.
6. Recent.—Blown sands, Scrobicularia beds, submerged forests,
Acting upon these lines, I purpose endeavouring to trace the sequence of the different horizons, and to synchronize the deposits on either side of the channel by means of their geological surrounding;, and the similarity or differences in the faunas they have preserved as far as possible.
The terms Upper, Middle, and Lower have caused great confusion, the Irish drifts so qualified, having nothing in common with the N. W. English Boulder clays, and neither of them with the Upper, Middle, and Glacial beds of East Britain, except in a very limited way.
It would serve no useful purpose to enter into minute details, it will be sufficient to notice the more salient points, the more important deposits, and the general fades of the contained faunas.
The oldest deposit in the channel area is unquestionably the shelly sands of Wexford, which, prior to the forcing of the barrier of Ordovician rocks by the Slaney River, near Fitzstephen's Castle (Ferry Carig), extended, as I have tried to point out in some British Association Reports, 1887-1890, from the landward side of the Wexford range, near Rathaspick and Little Clonard, without break to Arklow, where it thins out by the Wicklow Mountains intersected by the Ovoca River. As I there noticed, the sands are exceedingly clean, with few pebbles, and exhibited in places decided lamination. This was very well seen in a pit near St. Peter's College, Wexford, where an angular mass of rock had pressed into and deflected the lines on either side. The sand passes upwards into a comminuted shelly sand with occasional perfect specimens, but only at considerable heights, the pits at Clonard and Pulregan being many feet above the sea level, and the whole is capped by gravel and calcareous clay, except where these have been obviously denuded.
The whole rests directly upon Palaeozoic rocks, and is remarkably free from signs of glaciation.
About 100 species of molluscs have been obtained from these sands (see Reports), embracing a number of forms of Pliocene age, and others of a northern type, the relative proportions suggesting that the northern fauna was gradually superseding the decaying southern and Pliocene one. Such a mixture of Pliocene survivals and Boreal species is only equalled in some of the very late East Anglian Pliocenes, the Pre-Glacial Weybourne and Bure Valley sands, and a deposit now being worked by Mr. Kendall, F.G.S., near Ramsey, in the Isle of Man.
"wexford Species—Exotic Ok Habitat Unknown.
Cyprsea lurida? *Nassa rectieosta.
* ,, semistriata. Purpura incrassata. Fusus contrarius.
,, rostratus. Leda pusio? Nucula cobboldiae.
* ,, proxima
Boreal. Scalaria grsenlandica. Meyeria pusilla. Pleurotoma pyramidalis. „ la3vis, n. sp. harpularia.
Of these Pliocene species, Melampus pyramidalis, Nana rectieosta, N. temistriata, Turritella incrassata, Nucula cobboldia, and Fusus contrarius are thoroughly characteristic crag-shells, and the remainder southern or Mediterranean. An interesting question arises here— How came these forms usually known as only Crag or Pliocene into the Irish area? Till lately, no very satisfactory answer could be given to the query, but the discovery of a fossiliferous deposit in the St. Erth Valley, near the extremity of Cornwall, to the south or S. S. E. of the Wexford coast, throws some light upon the subject. Upon the death of the late Mr. S. "Wood, and my brother, R. G.
Bell, it fell to my lot to take up the thread of their palaeontological work, and having had the majority of the specimens extant in the few museums through my hands, and others obtained in a personal examination of the section, I venture to offer a list of all the species recognised, and those described by Mr. Etheridge, F.R.S., and myself in preparing a monograph upon these fossils, the present list being as yet unpublished.
St. Erth Fossils.
Ostrea edulis, L., var. sinuosa, E. and B.
Flat, almost circular, with concentric lines of growth.
,, edulis, var. semileevjs, E. and B.
A triangular shell, with pointed apex; ribbed on the bottom, and partly so on the upper valves.
,, plicatula, Gmel. ,, ungulata, Gold/. Pecten brocehii, E. and B.
,, plebeius, Broc. non Lam.
Perhaps a variety of P. opercularis, finely costated, bat not imbricated. „ opercularis, L. ,, ,, var. audouinii, Payr.
,, pusio, L.
,, curvistriata, E. and B.
The ribs are straight; moderately close; the whole crossed by thin curved erect squamae. ,, divigata, E. and B.
ltibs made up of 3-5 close-set, smaller riblets; interspaces rather wide, with shallow curving imbrications, projecting forward; the whole covered with fine perpendicular lines, diverging on the cost®. ,, striatus, Midi.
Like the two preceding species, this is only known by a fragment, but the sculpture corresponds with Miiller's species in the finely divergent lines between the ribs. If not this species, it is new, and may be called "intertextilis." Pectuncnlus glycimeris, L. Limopsis minutus, Phil. Nucula nucleus, L.