Ixviii PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



species of the London Clay, which had disappeared during the period 

 of the intervening Bracklesham. Sands, reappeared with the reoccur- 

 rence of argillaceous strata. One of the most remarkable cases, how- 

 ever, is that of the Argile de Boom, which forms the very top of the 

 Eocene series of Belgium, — the Oligocene of German geologists. 

 This deposit is so like the London Clay in lithological character 

 that it would be almost impossible to distinguish them, while the 

 shells (especially the several species of Fusus, Pleurotoma, and Naticd) 

 so closely resemble those of the London Clay, from which it is sepa- 

 rated by the four or five divisions of the Upper Eocene, that they 

 might easily be mistaken for London-Clay fossUs. The exceptional 

 appearances of Colonies, whether in the older or newer rocks, are, 

 no doubt, mainly due to the recurrence at certain intervals of 

 similar lithological, thermal, and bathymetrical conditions. 



During the Middle Tertiary or Miocene period, it would seem 

 that a diflPerent distribution of land and water prevailed. The 

 Miocene beds of Skye and of Greenland, with their remarkable 

 floras, indicate land and fresh-water conditions, while at the same 

 time the Miocene marine beds of Prance and Germany are rich in 

 subtropical forms of Mollusca, Assuming part of the area which 

 now constitutes the Northern Atlantic area to have been then dry 

 land, the migration southwards of arctic species of MoUusca would 

 have been for a time interrupted. 



Approaching nearer to our own times, we have Pliocene beds in 

 Iceland, Quaternary deposits in Spitzbergen and on the western 

 flanks of the Scandinavian peninsula, while in this country Glacial 

 or Preglacial beds range to the height of from 1000 to 1400 feet 

 above the sea-level. There is reason, therefore, to believe that 

 the bed of the North Atlantic may have been from 1500 to 1600 

 feet or more deeper during the Pliocene and Glacial period than 

 it now is. If northern submarine currents are now checked, as 

 Prof. WyviUe Thomson supposes, by the shallower seas between 

 Scotland and Greenland, such an addition to its depth as these 

 emerged portions indicate would materially have afiected those con- 

 ditions, and have allowed of a freer passage of the north-polar 

 waters, and consequently of a freer dispersion of its fauna to the 

 abysses of the mid-Atlantic, where, in fact, so large a number of 

 them are now found to exist. This more open communication gave 

 rise, I conceive, to that great migration of northern Mollusca which 

 are now found fossil in Italy and Sicily, and some of which still sur- 

 vive in the Mediterranean and mid- Atlantic. 



