262 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 16, 



the southern cretaceous fauna, as do also the fragments of Hippu- 

 rite which have been found in the Chalk of Sussex and Kent. 



Means of transport of extraneous materials in the Cretaceous 

 Sea. — The nature of the White Chalk formation, and the fact of the 

 presence there of extraneous blocks and pebbles, lead naturally to 

 the inquiry as to the agency by which they have been conveyed there. 

 Most geologists will doubtless at once call to mind the account which 

 Mr. Darwin * has given of a rounded block of greenstone which he 

 found on the outer coast of a small atoll belonging to the Keeling 

 group. The Keeling Islands are 600 miles from the nearest land 

 (Sumatra) ; they consist wholly of coral-formation, and Mr. Darwin 

 came to the conclusion that the block (which was rather larger than 

 a man's head) had become entangled in the roots of a tree which had 

 been washed into the sea, floated thus far, and finally liberated on the 

 beach where it was found. 



If only one single block could have been conveyed thus far by 

 such an agent, much more similar material must have been dropped 

 and lost than ever reached so remote a coast-line. The sedimentary 

 deposits of open oceans, such as the Indian or Pacific, where so many 

 of the lines exposed to coast- abrasion consist of coral-formation, 

 must be identical in composition with our White Chalk, so that the 

 rocks which are being conveyed away from the mainlands of those 

 seas may be presented at some future time under precisely the same 

 conditions as the extraneous materials now are in our Chalk : the 

 correspondence between the two cases is so close, that it is difficult 

 to avoid the consequence, that one case explains the other, and is 

 evidence of like agency at very distant times. 



Mr. Darwin's Keeling observations would fully account for the 

 presence of all the extraneous materials which have been met with in 

 the White Chalk and other deep-sea sediments up to the time of 

 the Purley discovery. 



The peculiarities of this case are these: — Apart from the large 

 boulder there was a smaller one, weighing upwards of twenty pounds, 

 some coarse shingle, and a quantity of loose sea-sand ; and these had 

 all sunk down together without separating : for this they must have 

 been firmly held together, both during the time they were being 

 floated away, as also whilst falling from the surface to the depths of 

 the cretaceous sea. 



If we suppose, in the case we are now considering, that all the 

 coarser materials were so firmly bound up in the knitted roots of 

 some tree which which had grown on a sea-margin, as afterwards to 

 be carried about, until the tree became water-logged and sank, it is 

 difficult to conceive how the finer sand should not have been washed 

 out ; and, as wood is well preserved in chalk, we might expect that 

 traces of the tree itself (and it must have been a considerable one) 

 would have been discovered in immediate juxtaposition with these 

 materials : such, however, was not the casef. 



* Journal, 1839, p. 549. 



t Fossil wood, and even lignite, has been found by Mr. Simmonds in the chalk 

 of the neighhourhood. 



