10 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 11, 



channel through the sand-hills to the sea. This is now the mouth of 

 the Yistula, that passing Dantzic having been turned into a canal*. 



I do not intend here to discuss the question of subsidences and 

 elevations which have affected the surface of the earth so largely, 

 and have no doubt occurred in some localities during the period 

 under consideration. I would, however, remark that in Wealden, 

 Eocene, or Miocene deltas there is no instance of any large fluvia- 

 tile deposit having been elevated or depressed evenly over a large 

 area ; while aU over the world a perfectly even movement of subsi- 

 dence is supposed to have taken place just at the mouths of large 

 rivers in the Quaternary, or most recent, period, in order to account 

 for modern freshwater delta-deposits containing shells living in the 

 adjoining seas being now found hundreds of feet below the sea-level. 



It would be the safer plan, in considering the remarkable gravel- 

 and crag- deposits which characterize so distinctly the Quaternary 

 period, to infer the size of rivers, amount of rainfall, and elevation 

 of tides from the deposits themselves. Further acquaintance with 

 meteorological phenomena may find a fitting explanation of the 

 difiiculties we meet with in explaining the position of the gravel at 

 such heights above our present streams and freshwater clay, and 

 sands at such depths below the sea-level. 



If the hypothesis we have been considering is a true one, that 

 the sea-level fluctuated 600 feet in the Glacial period, falling gradu- 

 ally and then rising again to its former level, we ought to find the 

 best evidence in the Pacific Ocean among the vast littoral accumu- 

 lations of the coral-zoophytes. 



The same theory of gradual subsidence, as was proposed for ex- 

 plaining the delta-deposits, has been applied to explain the formation 

 of the remarkable coral-islands over a tract of ocean 5000 miles long. 

 The sea-bottom by this theory is supposed to subside so regularly 

 and slowly that the coral-zoophytes build up their reefs and coral- 

 banks at an equal rate with the fall of the sea-bottom. 



Mr. Croll's hypothesis of an immense mass of ice at the poles 

 sufiicient to make the polar diameter equal to the equatorial (Phil. 

 Mag. p. 302) is well known. He has (p. 305) assumed that the 

 quantity of liquid water would be unchanged, as the ice in the 

 southern hemisphere would be transferred to the northern hemi- 

 sphere. 



The author's hypothesis is different. 



He thinks the geological evidence of a Glacial period indicates an 

 immense collection of ice at the north pole, accompanied by a low 

 temperature in the tropics, and probably a very low temperature at 

 the same time at the south pole. 



This would involve the supposition of a fall in the ocean-level in 

 proportion to the quantity of ice collected at the poles. 



It is of course doubtful what quantity of water was abstracted 

 from the ocean in this Glacial period (which Mr. Croll considers 



^ Pfeiffer, ' On the Vistula,' Dantzic, 1849. The gorging of ice at the mouth 

 of the Thames, Seine, and Somme may have assisted in the production of some 

 of the remarkable gravel-beds in these rivers. 



