164 L. Dudley Stamp — Limit between 



marked in solid black. According to this interpretation the 

 topography of Snowdon at the beginning of the glaciation was 

 essentially the same as it is now. Professor Davis's sketch probably 

 shows the nature of the mountain slopes in the early Pliocene ; but at 

 the end of that period the increased power given to the streams by 

 the uplift had enabled them to deepen their beds until they flowed 

 through steep-walled gorges. The aspect of the mountain if seen 

 from the north-west would probably have been approximately as 

 represented in Fig. 8. The earlier down-like relief had been 

 mostly destroyed owing to the excavation of the valleys of 

 Snowdonia, like those of Arran, by streams during the Middle and 

 Upper Pliocene uplift. 



References. 



Davis (W. M.). 1909. " Glacial Erosion in North Wales " : Quart. Journ. 



Geol. Soc, Ixv, pp. 281-350, pi. xiv. 

 Gtjnn (W.). 1903. In The Geology of North Arran . . . Mem. Geol. 



Surv. Scotland, Sheet 21. pp. viii, 200. 

 Jehu (J. T.). 1903. " A Bathymetrical and Geological Study of the Lakes 



of Snowdonia and Eastern Carnarvonshire " : Trans. Roy. Soc. 



Edinburgh, xl, pp. 419-67, 8 plates. 

 Mark (J. E.). 1906. " The Influence of the Geological Structure of English 



Lakeland upon its present Features " : Quart. Journ. Geol. Soc, Ixii, 



pp. Ixvi-cxxviii. 



1916. The Geology of the Lake District, pp. xii, 220, map. 



MoRT (P.). 1914. North Arran : A Physiographic Study. 72 pp., map. 

 Ramsay (A. C). 1841. The Geology of the Islarid of Arran. pp.78. Glasgow. 

 1881. The Geology of North Wales. Mem. Geol. Surv. Great Britain, 



iii, 2nd ed. pp. xii, 611, 28 plates. 

 Rtjskin (J.). 1856. Modern Painters, iv. 



Smith (James). 1862. Newer Pliocene Geology, pp. xi, 192, 4 plates. 

 Willis (Bailey). 1893. " The Mechanics of Appalachian Structure " : 13th 



Ammal Report U.S. Geol. Surv., pt. ii, pp. 211-82, pis. xlvi-xcvi. 



I 



Note on the Determination of the Limit between the 



Silurian and Devonian Systems. 



By L. Dudley Stamp, M.Sc. (Lond.), A.K.C., F.G.S. 



N a recently published paper on the " Highest Silurian Rocks of 

 the Clun Forest District, Shropshire ",^ the present writer gave 

 an account of an area where the series of " passage-beds ", marking 

 a gradual transition from Silurian to Old Eed Sandstone conditions, 

 is well seen. The succession there is practically identical with that 

 established by Miss EUes and Miss Slater in the Ludlow District.^ 

 From November, 1918, to October, 1919, whilst engaged in a study 

 of Belgian Tertiary rocks, I have had an opportunity of visiting 

 some of the more important sections of the Gedinnian in the Ardennes 

 and also of examining the paleeontological collections at Brussels, 

 Lille, and elsewhere. This work has thrown very considerable 



1 Quart. Journ. Geol. Soc, vol. Ixxiv, 1918, p. 221, 1919. 



2 Ibid., vol. Ixii, 1906, p. 195. 



