Yol. 56.] FORMATION AND ITS GRAPTOLITE-FAUNA. 439 



such in the southern and south-western parts of the district. The 

 highest zone, that of Monograptus leintwardinensis, as developed at 

 Aberedw and at Bhiw Bhwstyn, agrees lithologically with the 

 beds which I have referred to the same zone in the valley of 

 the Duhonw, and would seem to be of considerable thickness. 



In attempting to offer an explanation for the variations in the 

 palseontological characters of the rocks in areas so nearly adjacent, 

 it is first of all necessary to consider briefly the conditions favourable 

 to the existence and preservation of graptolites in general. As 

 Prof. Lapworth has maintained, it is probable that the graptolites 

 were not entirely free-swimming animals, but were attached to 

 various floating bodies, such as seaweeds, which would tend to 

 accumulate in dense masses in still waters such as those of the 

 Sargasso Sea. A quiet though not necessarily deep sea, in which 

 slow sedimentation would take place, would appear therefore to be 

 the best condition for the existence of graptolites, while at the same 

 time the fine-grained sediments thus formed would be most favour- 

 able for their preservation. It is clear, then, that coarse-grained 

 and rapidly accumulated sediments, pointing as they do to the 

 proximity of land and to the existence of current-swept seas, would 

 be unfavourable both for the occurrence and for the preservation 

 of graptolites. Such conditions seem to have largely prevailed 

 during the deposition of the Ludlow Beds, comparatively fine- 

 bedded argillaceous shales alternating rapidly with coarse-grained 

 arenaceous and calcareous flags and mudstones. The upper beds, 

 especially, of the Lower Ludlow Group were probably formed under 

 more or less continuous shore-conditions, with occasional pauses 

 in the rapid sedimentation. Graptolites are everywhere abundant 

 in the shales, while they are almost entirely absent from the coarser 

 sediments. It is, therefore, possible that while the shales con- 

 taining Monograptus scanicus were being laid down in comparatively 

 quiet waters in the south-eastern area, the more rapidly moving 

 waters of the southern and south-western portions of the district 

 which lay too near the shore-line were unfavourable for the existence 

 of graptolites. The fact that the beds belonging to the M.-Nilssoni 

 zone south of Builth are of about the same thickness as the 

 M.-Nilssoni and M.- scanicus beds combined in the eastern area, seems 

 to lend support to this view. But against it, on the other hand, is 

 the fact that the uppermost horizons of the M.-Nilssoni zone near 

 Builth, though graptolitiferous, have yielded no forms of the 

 M -scanicus or M.-chimcera type. The thickness, too, of the beds 

 in adjacent areas cannot be relied upon, for the strata in the west, 

 lying as they do nearer the old coast-line, would naturally be thicker 

 than those farther east. 



Another explanation suggests itself, but its application in this 

 case could be ascertained only after a detailed mapping of the 

 country. According to Miss Elles, in the Wenlock Shales of this 

 district there has been an overlap of some of the higher zones on 

 to the lower zones, and a glance at the geological map of the area 



