306 A. GEIKIE ON THE SUPPOSED 



rocks below, from the waste of which they may have been mainly 

 derived. In such cases they must be regarded as littoral deposits ; 

 and in this respect they possess importance from the light they 

 throw on former geographical conditions. But it is equally certain 

 that pebble-beds and conglomerates have again and again been 

 intercalated, without discordance of any kind, in the midst of a 

 continuous and strictly conformable series of sandy and even of 

 muddy and calcareous deposits, often marked throughout by a com- 

 munity of fossil contents. In such positions they may possess local 

 value as stratigraphical horizons, but they evidently cannot be re- 

 garded as marking important geological breaks in the succession 

 either of formations or of organic remains. Under these conditions 

 they present certain common features that recur over and over again 

 throughout the stratified formations of the earth's crust. Unlike 

 the basal conglomerates just referred to, they are composed of well 

 waterworn pebbles, for the most part comparatively small in size, 

 derived from some distant and, in many cases, unknown source, and 

 consisting usually of quartz, quartzite, or other exceptionally durable 

 rocks. 



These features are characteristically displayed in the conglomerate 

 of St. David's, which is the earliest of the British examples yet 

 known. A long list of similar intercalated pebbly bands might be 

 drawn up from all the later geological systems down to the shingle 

 beds of the present sea-bottom ; but a few examples may be cited in 

 illustration*. 



The Lower Silurian rocks of Anglesey contain bands of conglom- 

 erate made up of pebbles of quartzite, sometimes from 6 to 8 inches 

 in diameter, and mostly well rounded f. Conglomerates of quartz 

 and black slate occur high up in the Skiddaw Slates J. Conglom- 

 erate bands of white quartzite and vein-quartz occur in the " Plyn- 

 limmon Group " of Central Wales §. 



In the Old lied Sandstone, bands of quartz conglomerate appear 

 on many different horizons. One of the most striking examples is 

 the coarse and thick mass that comes in conformably above the fine 

 Ludlow shales and mudstones of Lanark and Ayrshire. 



In the Carboniferous system lines of quartz conglomerate occur 

 on many platforms. The Carboniferous Limestone contains excellent 

 examples in the north of England, hundreds of feet above the base 

 of the formation. The Millstone Grit affords a familiar illustration ; 

 and occasional instances occur in the Coal-measures. 



The Bunter pebble-beds, composed of white and liver-coloured 

 quartz, are notable examples of the occurrence of conformable con- 

 glomerates in a continuous series of sediments, with occasional 



* I ani indebted to Mr. W. Topley for kindly furnishing me with most of the 

 references in the list given above, which have been supplied from the field-notes 

 of my colleagues in the Survey, Messrs. Bristow, Whitaker, H. B. Woodward, 

 Goodchild, De Ranee, Ussher, and Strahan. 



t Ramsay, Geology of North Wales (Geol. Surv. Mem. vol. iii.), 2nd edit. p. 247. 



j J. C. Ward, Geol. Mag. dec. 2, vol. vi. p. 51. 



§ W. Keeping, Quart. Journ. Geol. Soc. vol. xxxvii. p. 156. 



