142 



SCIENCE-GOSSIP 



A HISTORY OF CHALK. 



By Edward A. Martin, F.G.S. 



(Coiiiiiiued /7-Qiii page 117.) 



rr DMOGKNEOUS as the Chalk is in itself, there 

 -*■ ■*■ have been found in various places, scattered 

 through the mass, what are known as erratic boulders. 

 Occasionally piecesof granite, greenstone, quartz, sand- 

 stone, schist, and even coal, have been foimd. When a 

 tunnel was being pierced through Chalk, near Lydden 

 Hill, on the London, Chatham and Dover Railwa)', 

 a large block of coal or lignite was found embedded 

 therein. It measured about 4ft. square, varying from 

 4in. to loin, thick. A large block of syenite 

 weighing 40 pounds, which has been thought to be of 

 .Scandinavian origin, was brought to Hght in 1857, 

 fiom a chalk-pit at Haling, south of Croydon, and 

 this has been described in some detail by Godwin- 

 Austen. Beside the larger granite boulder there were 

 pebbles of smaller dimensions, together with fine 

 sand ; and all of them were waterworn. "In common 

 with the other specimens of the same rock, the 

 largest boulder was much decomposed. The smaller 

 pebbles were wholly decomptjsed, and readily fell to 

 pieces, forming a sharp sand." In addition to this 

 there was a fine waterworn beach sand, "derived 

 from the waste of a coast-line of some crystalline 

 rocks." These interesting erratics were found in the 

 Upper Chalk, and as none of our English chalk-beds 

 show any signs of the approach of. shore conditions, 

 their presence is not thought to be explicable, except 

 by the agency of floating ice. 



Amongst the fragments of foreign rocks which are 

 occasionally found in the Chalk, rounded quartz 

 pebbles are, perhaps, the most frequent!)- met with. 

 They have been found in the Upper Chalk of Charlton 

 and Gravesend, in Kent. Prof. Seeley accounts 

 for their presence by the habit which man)' marine 

 animals possess of swallowing and re-depositing 

 gravel. He cites the walrus, the larger fishes, and 

 reptiles of the present day as possessing this habit- 

 It is quite possible that in cretaceous times these 

 pebbles may in part have been transported from 

 _ districts where veins in crystalline rocks were exposed 

 to denudation. The larger blocks of coal and 

 granite may have been transported by ice-action, or 

 may have been floated up by the action of liviiig sea- 

 weeds, which carried them away and dropped them 

 far out at sea. Where the blocks of coal come from 

 is an exceedingly interesting and debateable question, 

 but it seems very probable that some portions of the 

 coal-seams which are now known to exist beneath the 

 south of England, were at this age exposed to view, 

 and to the denuding action of a neighbouring coast. 

 Other boulders of smaller size have since been 

 excavated trom the Chalk, the neighbijurhood of 

 Norwich having yielded various specimens. Among 

 those more recently discovered are two which were 

 obtained from the Mitldle Chalk of Betchworth, in 



Surrey, by Mr. W. P. D. Stebbing, F.G.S. The 

 two boulders in ([uestion were of granite, although 

 different in character, and were both very much 

 weathered. They weighed 7 lbs. 7 oz. and 

 3 lbs. 12 oz. respectively, and measured, the one, 

 5-8 in. X 6'25 in. X 4'135 in., and the other, 

 3 '6 in. X 5 '8 in. X 4'S in. The transportation of 

 these boulders, which Professor Bonney judged 

 might be of Scandinavian origin, was attributable to 

 one of four causes : by adhesion to seaweed, drift- 

 wood, by marine animals, or ice. It was particularly 

 Interesting to notice that to the largest, valves of 

 Spondyliis latus and Serptda, were still attached. In 

 the presence of these and other larger boulders in the 

 Chalk, we have undoubted evidence that the sea in 

 which was laid down the Middle Chalk was occa- 

 sionally traversed by stray icebergs. The manner of 

 formation of boulder clay is not thoroughly under- 

 stood. If caused by land-ice, the thought at once 

 arises that if there had been no land where now the 

 boulder clay is found, possibly no deposit might have 

 been laid down at all, and no evidence furnished of 

 the glaciation for future geologists. In such a case, 

 these boulders in the Chalk may be sufficient evidence 

 of a veritable glacial epoch occurring even in Creta- 

 ceous times, although there is no Cretaceous boulder 

 clay. They may show its principal effects on lands 

 which have since suffered extreme denudation, or 

 which are now still beneath the sea. Here, too, 

 would be a satisfactory explanation^of those puzzling 

 bands of clayey-chalk which occur at all levels in 

 the true Chalk, and to explain which, it is almost 

 impossible to believe in repeated temporary shallowing 

 of the sea-bottom. Icebergs from the nearest coast 

 might be the bearers of fine clay frozen in its mass. 



Breaking the continuit)' of the Chalk, in addition to 

 the numerous bands of flint, with which we shall deal 

 elsewhere, Jukes noticed in a cliff of Upper Chalk, 

 east of Dieppe, a band of brow n clay or marl, perfectly 

 interstratifiedwith the Chalk. It was not connected with 

 any pit-holes, by which it could have been swept in. 

 This band measured about eight inches thick, and was 

 20ft. in length. The pit-holes, or "pipes," here re- 

 ferred to, often consist of Eocene sands fillingirregular- 

 shaped hollows, formed since the deposition of the 

 beds by the decomposition of the Chalk. These appear 

 to be more prevalent where there is but a small 

 capping of the Thanet Sands, since as a rule there 

 is a hard-and-fast even line ot division between the 

 Chalk and the sands. 



A peculiar feature of the Chalk at its junction with 

 the Eocene Reading Beds, near Bedwins, in Wiltshire, 

 is the occurrence of a bed of "Reconstructed Chalk." 

 This bed, some 20 feet in thickness, consists of blocks 

 of Chalk, with broken bands of flint, contained in a 



