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Vol. 56.] ~ S Lowr Downloaded from http://jgslegacy.lyellcollection.org/ at University of Liverpool on June 30, 2016 Vol. 56.] ~s LowR~ r.UDLOWFOaXA~ION. 415 24. The LOWER LUDLOW FOR~IATION and its GRAPTOLITE-FAIII~A. By Miss ETH~L M. R. WooD. (Communicated by Prof. C. LAPWOl~Tn, LL.D., F.R.S., F.G.S. Read March 21st, 1900.) [PLAvES XXV & XXVI.] ConvEnTS. I. Introduction ......................................................... ~4~~ II. Literature ..................................... : ...................... 417 III. General Stratigraphy of the Lower Ludlow Formation ... 420 IV. Description of Typical Districts ................................. 423 (A) The Ludlow District. (General Map on p. 424.) (]3) The Builth District. (General Map on p. 430.) (C) The Long Mountain District. (Map, Pl. XXVI.) (D) Supplementary Districts. V. General Summary ................................................... 447 VI. Palmontology ......................................................... 450 (A) General Characters of the Graptolite Fauna. (B) Description of the Graptolite Species. (C) Synonymy. I. I~TRODUCTro~. Lower Ludlow Shales, which form the lowest section of the Ludlow division of the Silurian System, are most typically developed in the neighbourhood of the picturesque old town of Ludlow. It was in this district that they were first defined and described by Murchison. Lithologically, they are here essentially an argillaceous group of strata, the lower and upper limits of which are well marked by two distinct calcareous beds--the Wenlock Limestone below and the Aymestry Limestone above. Pal~eontologically, the Lower Ludlow Shales are rich in fossils: brachiopods, cephalopods, crustaeea, and graptolitos. When the Lower Ludlow Beds are followed from the Ludlow district north-eastward along their line of strike as far as the valley of the Severn, south-westward as far as Aymestry, and eastward as far as the Malverns and Mayhfil, they are seen to retain their lithologieal characters unaltered, and the Wenlock and Aymestry Limestones are both well developed. But when these beds are traced from the typical area to the west and north-west, the limiting calcareous strata dwindle away and eventually disappear, until, finally, the Lower Ludlow Shales merge lithologically on the one hand into the Wenlock Shales below, and on the other into the Upper Ludlow :Flags above. It is clearly impossible, therefore, in these westerly districts to separates on purely lithological grounds, the Lower Ludlow Beds as a distinct group from the formations which overlie and underlie them. The proofs that the Lower Ludlow Beds of Britain contain a eharacteristie graptolite-fauna have been gradually accumulated by the researches of Hopkinson, Lapworth, Watts, Marr, and Lake (see pp. 418-19); and the evidences of the existence of a similar 2F2 Downloaded from http://jgslegacy.lyellcollection.org/ at University of Liverpool on June 30, 2016 416 Mzss R. M. R. WOOD ON THE LOWER LUDLOW [May I9oo, fauna at a corresponding horizon in Europe have long been known from the discoveries of Continental geologists. But little or nothing has hitherto been worked out with respect to the vertical range of the individual graptolite-species within the limits of the formation itself, or with regard to their geographical distribution in the equivalent strata of Wales and the West of England. During the last few years I have devoted much of my leisure-time to the study of the graptolitic fauna, the rock-sequence, and the vertical distribution of the various graptolite-species in the recognizable subdivisions of ~he Lower Ludlow Shales of the typical Ludlow district and of their equivalents along the Welsh Border, in order to ascertain (1) what are the truly characteristic graptolites of Lower Ludlow age as distinct from those of the Wenlock formation, and (2) to what extent the Lower Ludlow Beds are capable of subdivision into graptolite-zones. My results and conclusions are embodied in the present paper. It is now generally acknowledged that graptolites are among the most reliable fossils for purposes of correlation in the Lower Palaeozoic rocks. And the graptolite-fauna of the Lower Ludlow Shales is of more than ordinary interest, for it is the last distinct assemblage of these fossils presented to us before the extinction of the group. Notwithstanding the interest of this particular assemblage of graptolites, it has hitherto received but little attention in Britain. The reason for this neglect is evident. Most of the species of graptolites hitherto recorded from the British Lower Ludlow Beds belonged to forms origilmlly described by Barrande in his famous work on the 'Graptolites de Boh6me,' as early as the year 1850; but in some cases he included more than one species under a single specific name, and consequently correct identifications of the British forms were impossible until a thorough revision of Barrande's Bohemian type-specimens had been made. This revision has now been carried out by Dr. Jaroslav Perner (' ]~tudes sur les Graptolites de Boh6me' pt. iii), and through his kindness I have also been enabled to examine Bar- rande's type-specimens for myself on the occasion of a recent visit to Bohemia. Much of the present paper is necessarily pal~eontological, and consists of descriptions and figures of the more important species of Lower Ludlow graptolites. I have endeavoured to limit the number of species and varieties as much as possible, and have restricted myself mainly to the description of such forms as are of greatest value for stratigraphical purposes. In working out the stratigraphy of the Lower Ludlow I have studied three districts in some detail, namely :-- (1) The Ludlow District, where the defining limits of the formation are well marked by calcareous beds of considerable thickness ; (2) The Builth District, where the calcareous limiting-beds are still faintly indicated, but the lithological boundaries are more or less indefinite ; (3) The Long Mountain District, where the calcareous limits are absent., and no definite lithological boundaries exist. Downloaded from http://jgslegacy.lyellcollection.org/ at University of Liverpool on June 30, 2016 Vol. 56.J FoaMA~IONAND I~S ORAr~0LIVS-aAW*. 417 I have also collected or examined grapbolites from other areas of Ludlow rocks in Britain in addition to these three main districts, as for example from the Dee Valley, the Lake District, Dudley, Abberley, etc. In the present paper I propose first to deal with the succession, lithology, and graptolitie fauna of each of the three main districts; and afterwards to deal briefly with the supplementary districts, and show to what extent the results arrived at accord with those worked out in the more typical areas. The last part, of the paper is devoted to the description of the Lower Ludlow graptolites themselves. II. I~rSRATURE. In reviewing the history of stratigraphical research I confine myself as much as possible to that in Great Britain, but when considering the purely pal~eontological literature on the graptolites of the Lower Ludlow Beds I briefly summarize some of the more important results arrived at abroad. (1) Stratigraphical. In the year 1839 Murchison first described and defined these beds in the typical area of Ludlow. In his ' Silurian System' he showed that ' they constitute a great argillaceous mass .... of mud- stone .... more argillaceous, less sandy and calcareous' (pp. 204- 207) than the Upper Ludlow rocks. :He pointed out that they are rich in fossil organisms,-and are limited both at their base and summit by well-marked calcareous beds. He also noted the presence of these rocks in other areas along the Welsh Border, as in the undulating country between the Vale of Radnor and the Wye, and in the Long Mountain, etc. He gave a complete section through the Wenlock and Ludlow Beds in the neighbourhood of Builth, noting especially 'the thin band of impure limestone ' (o_p. cir. p. 315), made up almost exclusively of the small Terebratula navicula, which he believed to be the representative of the Aymestry Limestone. In 1846 appeared the first volume of the Memoirs of H.M. Geological Survey) In this the Survey officers, referring to the Lower Ludlow rocks of the Builth district, grouped the beds in descending order as follows :-- ' (a) Argillo-arenaceous rocks, with much oblique bedding and other evi- dence of irregular accumulation. Many of the beds are arranged in large irregular concretions. 210 feet. (b) Thin limestone-beds. 10 feet. (c) Same rocks as at (a). 300 feet.' These Lower Ludlow Beds are described as being limited at their base by nodules and interrupted beds of limestone (Wenlock Lime- stone), and at their summit by a thin and interrupted band of limestone composed of little else than the remains of Pentamerus Knightii (Aymestry Limestone). I ~Iem. Geol. Surv. vol. i : ' Formation of the Rocks of S. Wales & S.W. :England' p. 23. Downloaded from http://jgslegacy.lyellcollection.org/ at University of Liverpool on June 30, 2016 418 Hiss E. M. R. wood o~ ~ wwr~ L~WW [May z9oo~ In 1880 Prof. Lapwor~h, in his brilliant paper on the ' Geological Distribution of the Rhabdophora, '1 grouped the Lower Ludlow as his ' zone of Monograptus JVilssoni ' and described it as the ' highest and most important graptolitic zone of the Wenlock-Ludlow formation" He noted that its collective fauna is ' specifically very distinct from that of the Wenlock Shales,' and suggested that the formation ' will probably in the future be found divisible into several distinct zones.' In 1883 Tullberg,
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