PRE-CAMBKIAN ROCKS OF ST. DATID's. 307 



illustrations of contemporaneous erosion. The Budleigh-Salterton 

 pebble-bed and its continuation through the middle of the Triassic 

 system is another well-known case. 



In the Jurassic series of Yorkshire examples of intercalated con- 

 glomerates have been noted*. 



The conglomerates of the "Weald generally occur near the top of 

 the sandstone beds and are succeeded by shales f . The conglomerates 

 of Faringdon and Godalming &c. exhibit similar phenomena in the 

 Neocomian series. 



Of the Tertiary illustrations I need refer only to the pebble- 

 beds of the London basin t, and the thick shingle-accumulations of 

 the Marine Series of Bournemouth §. 



Every geologist who has studied the subject in the field must be 

 familiar with the way in which the same far-transported pebbles 

 have been used over and over again in successive formations. In 

 some cases, as, for example, in the later conglomerates of the Isle of 

 Arran, it is possible to distinguish the freshly derived fragments 

 from those that had already done duty in an earlier conglomerate. 



It is evident, then, that the intercalation of a conglomerate band 

 in the midst of a continuous series of sediments has been of frequent 

 occurrence in the geological past. Sometimes no change has taken 

 place in the general character of the sedimentation after the conglom- 

 erate was deposited ; in other cases the sediment laid down above 

 the conglomerate differs in composition or texture, or in both, from 

 that which lies below. We may infer from these facts that the 

 occurrence of a pebbly zone in a conformable series of strata, need 

 have no more geological significance than the shifting shingle banks 

 on the bed of the English Channel at the present day. Undoubtedly 

 such a band of conglomerate points to a change in the system of 

 currents by which sediment was transported, the change sometimes 

 giving way to the former conditions, sometimes introducing new 

 ones ; but it furnishes no sufficient ground for an important strati- 

 graphical boundary line. 



3. Zone of Green and Bed Shales, Sandstones and fine Tuff's* 



This zone is chiefly important because it offers well-marked litho- 

 logical characters, capable of being employed in working out the 

 stratigraphical succession and general structure of the district. In 

 mapping the ground, indeed, Mr. Peach and I found it convenient 

 to distinguish two bands in this zone, viz. a lower set of green, and 

 an upper set of red beds. But the line of demarcation is not always 

 sharply defined between them, though they can be readily observed 

 in their respective positions on both sides of the axis. They are 



* C. Fox Strangways and G. Bari'ow, Mem. Geol. Survey, 'Geology of 

 Whitby and Scarborough, ' p. 38. 



t W. Topley, Mem. Geol. Survey, \Geology of the Weald,' pp. 56, 59, 

 64, &c. 



t W. Wbitaker, Mem. Geol. Survey, vol. iv. ' Geology of the London Basin ' 

 pp. 207, 222-224, 233. 



§ J. S. Gardner, Quart. Journ. Geol. Soc. vol. xxxv. p. 215 



