2G4 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 16, 



like manner, convey marginal shingle outwards ; but the transport 

 in this direction is far less in amount than it is in the other. 



Such a process as this has been in operation at all geological 

 periods ; and some years since I indicated it as the only one by which 

 large detached valves of such shells as Oysters (Exogyra gryphcea) 

 could have found their way into the fine sandy beds of the Lower 

 Greensand formation. 



Sea-plants have their limits of growth ; and the bulk of each mass 

 will be limited by the support or surface of attachment : conversely, 

 therefore, there is a limit to the size and weight of the materials 

 which can be lifted by such an agency. As far as my own observa- 

 tions go, the floating power of large masses even of Bladder-fucus 

 does not disturb shingle-stones weighing much above 1 lb. A good 

 deal of the extraneous shingle from the White Chalk is undoubtedly 

 under this weight, and so far therefore admits of being thus accounted 

 for : but the agent becomes wholly inadequate when considered with 

 reference to blocks of 12 and 14 lbs. weight, such as have occurred 

 in Sussex ; still less could any conceivable mass of sea- weed have ever 

 carried the Purley boulder. 



But, even if it be assumed that sea-plants of that time could have 

 lifted the largest boulder, and that a tangled mass of them could 

 have floated away at the same time all the smaller stones associated 

 with it, the difficulty as to the mass of loose sand still remains. 



Sea- plants, when detached from their proper zone, are constant 

 agents in transferring the animal forms belonging to it, as well out- 

 wards as upwards ; and it was in this way most probably that the White 

 Chalk acquired the very peculiar assemblage which it contains. This 

 agency, like that of floating trees, is not to be excluded with refer- 

 ence to past times, and may possibly have been engaged in trans- 

 porting some of the smaller pebbles, which are met with in beds 

 into which they could not have been drifted. Naturalists, who are 

 well aware how many forms are to be found amidst the roots of sea- 

 plants when these are washed up to the marginal zone, will see that 

 these must act as the distributors of the fauna of the same sea-zone 

 when the plants are drifted away outwards. 



Masses of ice floating at sea have two distinct sources of origin ; 

 and, as a consequence of this, they are also distinct in the external 

 characters of the records they leave. The larger masses known as 

 icebergs, which are the terminal portions of the glaciers of Polar 

 regions, which have reached down to the sea, have been reported 

 by a long list of observers to be laden with detritus : in this case the 

 detritus is sub-aerial, and its form angular. Yet I have not seen a 

 single angular fragment from the White Chalk, nor from any part of 

 the Cretaceous series. 



There is another form of floating ice, in which, though it is far 

 less imposing in respect of its mass, it is of infinitely greater power 

 as a means of transport. This ice has its origin along the coast-lines 

 of Polar lands ; it incorporates itself with all the materials of the 

 beaches to which it is attached ; it acquires increased buoyancy 

 from the snow-ice which collects on its surface ; and, when the annual 



