62 A. J. Jukes- Broivne — Chalk-pebbles on the 



able yet in Hants or Sussex to trace in practice any easily 

 recognizable line marking the general disappearance of this fauna 

 or even that any two of its elements disappear simultaneously. At 

 the same time if a means could be devised for including fossils from 

 any beds in which this fauna lingers with those of the zone of 

 0. pilula without sacrificing the field value of the upper boundary 

 I have drawn, it would be a substantial improvement on any 

 suggestion yet put forward. 



III. — On somk Chalk-pebbles from the Floor of the English 



Channel. 

 By A. J. Jukes-Browne, F.E.S., F.G.S. 



IN 1908 Mr. II. H. "Worth described a number of stones which had 

 been dredged by Mr. T. R. Crawshay, of the Marine Biological 

 Association, from the floor of the English Channel to the east and 

 south-east of the Lizard.^ These stones included a great variety of 

 rocks such as granites, felsites, diorites, and other igneous rocks, 

 gneisses, schists, slates, and quartzites, Devonian grits, Permian 

 conglomerate, Triassic marl, sandstones of different kinds, Liassic 

 limestones, liard chalks, and flints. 



The majority of these stones and all the chalk-pebbles were 

 obtained from depths between 40 and 50 fathoms in an area to the 

 south of N. lat. 50° and about midway between the lines of 4° and 

 5° W. long, (see Map, p. 63). Mr. Worth has enumerated all the 

 different varieties of rocks in this interesting assemblage, and has 

 given particular descriptions of most of them fi'om microscopical 

 examination. Of the chalk-pebbles he observes, " a very hard yellow 

 or cream-coloured chalk is of frequent occurrence ; generally the 

 exterior of the pebble is brown, and this colour extends some slight 

 depth into the stone, getting less in intensitj' until it fades into the 

 vellow or cream colour. Unless the stone happens to be much bored 

 it usually requires a considerable blow to break it." 



Mr. Worth prepared slides from five of these pebbles, and briefly 

 mentions the composition of four of them, but describes the fifth in 

 more detail. This pebble was about 3 inches long, stained orange- 

 brown outside and yellow for a depth of 7 mm., all the inner part 

 being cream-coloured. It included a nodule of green-coated chalk, 

 the interior of which was reddish with cream-coloured markings, 

 these latter proving to be borings made by some animal before 

 embedment and filled with the material of the surrounding 

 chalk matrix. 



After describing a slice cut through this included nodule, 

 Mr. "Worth remarked that "the affinities of this yellow chalk 

 appear to be with the ' Melbourn Rock ' described by Mr. A. J. Jukes- 

 Bi-owne and later by the same author in collaboration with Mr. W. 

 Hill", but he adds, "whether lithological similarity in this case 

 implies identity of age may be doubtful." He does not seem to have 

 made comparisons with any other part of the Chalk, and does not even 

 mention the possibility of its being Chalk Rock. 



^ Journ. Marine Biol. Assoc, for 1908, p. 118. 



