266 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 1 G, 



along a line drawn from Engelholm on the west, to Karlskrona on the 

 east, rest on granite, gneiss, claystone- porphyry, and old sedimentary 

 slates and sandstones. 



It perhaps may be thought by some that, by the aid of such a col- 

 lection of extraneous materials as has been now brought together, it 

 would have been easy to have determined almost with certainty the 

 precise portions of the old cretaceous coast-line from which they had 

 been derived ; such, however, at present is not the case. 



M. Brongniart has noticed the very great resemblance which exists 

 between the crystalline rocks of the Cotentin and Scandinavia ; and 

 either district could have furnished the large block of quartzite now 

 in Mr. Catt's collection. On the whole, however, and guided mainly 

 by the peculiar greenstone-porphyry, and the condition of the large 

 boulder from Purley, from which the mica has been removed, as it 

 has from great masses of a Scandinavian granite, I am disposed to 

 look to that quarter as the source of the extraneous materials of our 

 Chalk-formation. 



There is a difficulty, however, which I feel will suggest itself to 

 some, with respect to the agency of floating ice in transporting the 

 materials imbedded at Purley : it may be asked whether in such case 

 we should not long since have become familiarized with the opera- 

 tions of such an agent with reference to the Chalk-period, seeing the 

 extent to which that formation is quarried. Such an objection is in 

 part negative only : much evidence akin to that at Purley may have 

 presented itself, and yet not have been noticed. But, even should the 

 assemblage met with at Purley be the first of its kind that has ever 

 been met with till now, we may feel sure that thousands of like cases 

 are concealed in the mass of the Chalk, from the incalculable im- 

 probability which exists that the Purley phenomenon should be the 

 only one of its kind throughout the formation. It is only with refer- 

 ence to such mixed assemblages of boulders, shingle, and sand, that 

 the agency of ice is required. 



The physical arrangement of the circumjacent portions of the 

 earth's surface at the Cretaceous period preclude the idea that floating 

 coast-ice could have been either a constant or a very powerful agent 

 of transport with reference to the White-chalk area of Europe. The 

 line of coast, from which it has been suggested that the Croydon 

 boulder was derived, lay a little to S. of the 60° of north latitude ; 

 and, though coast-ice is a powerful agent there at present, the inten- 

 sity of cold must have been modified then by the expanse of sea that 

 lay immediately to the south. The requisite degree of cold need not 

 have been greater than what has occasionally been experienced on 

 our own eastern coast, when ice has lifted and floated away far greater 

 weights. 



Many general results will some day be arrived at from the study 

 of the extraneous mineral materials of our old oceanic deposits ; by 

 their means we shall certainly arrive at a definite outline for the 

 ocean of the Cretaceous period. 



