352 S. V. Wood, Jun. — On Glacial Deposits. 



of thin sections or otherwise, can the differentiation of the fossil 

 Spongiadse be satisfactorily made. Occasionally the structure, espe- 

 cially in the silicified sponges, is so admirably preserved as to render 

 this not difficult ; but until their true affinities to recent species have 

 been studied from a strictly zoological point of vievp, our knowledge 

 concerning them must be wanting in scientific precision. The result of 

 such inquiries will probably be to reduce many genera to the lower 

 grade of species, and many species to mere varieties or conditions of 

 growth. In common with other forms equally low in the scale of 

 organization, the sponges appear to have endured through a long 

 range of time, subject only to modifications, which scarcely amount 

 to specific distinctions. 



lY. — Further Eemarks on Mr. James Geikie's Correlation of 



Glacial Deposits. 



By S^. V. Wood, Jun. 



IN a republication of the papers by him which appeared in successive 

 numbers of this Magazine, Mr. James Geikie has replied to the 

 objections which I offered to his views, and also to the views of 

 sequence which I myself advance, by asserting that the seaward 

 ends of glaciers never float ; and that my view that " wherever 

 the ice-sheet rested there no deposit occurred, the material produced 

 by its action incessantly travelling outwards to the ice-edge," is a 

 misconception. 



The question whether this flotation does or does not occur is one 

 of the things yet to be solved, and it is difficult to imagine that the 

 continuous Antarctic ice-wall followed without soundings for hundreds 

 of miles by Sir James Eoss does not float. ^ Perhaps in thinking that 

 it did, I too readily adopted the view of Mr. Archibald Geikie, the 

 Director of the Scotch Survey ; but the question is one wholly beside 

 the main issue, which is — 



1st. Is unstratified clay or Till deposited under the sea ? 

 2nd. Whence does the material of such Till come unless it be a 

 product of land-ice shed out from the sea extremity of that ice ? 

 3rd. How, if so shed out, can it be denied that the material is 



constantly travelling outwards ? 

 4th. If so travelling outwards, how can the material shed out 

 under the sea at the commencement of a period be synchronous 

 with the material that was under the sheet at the close of the 

 period. 

 The Scotch geologists have mostly insisted on a negative to the 

 first of these propositions ; and Mr. Croll, in arguing that the un- 

 stratified clay of the Holderness cliffs — a clay identical so far as its 

 physical structure is concerned with the Scotch Till — was due to a 



^ In supposing that, so soon as it has a tendency to float, the glacier breaks off 

 into bergs from the rise and fall of the tide, Mr. Geikie seems to me to have over- 

 looked the fact that in such deep water and open sea as that in wliich tlie Antarctic 

 ice terminates, the vertical movement of the tide is altogether insignificant. It is to 

 the Antarctic, rather than the Arctic regions, that we must turn to fiud the ice 

 conditions of our Glacial period. 



