296 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 22, 



lapse of time, during which the Marine Sands of Weinheim were 

 being deposited. But other changes accompanied this gradual eleva- 

 tion ; as we approach the upper portion of these sands, we find 

 Cerithia more abundant, indicating, if not already a brackish condi- 

 tion of the water, at least a diminution of depth and the presence of 

 freshwater streams flowing into the sea at no great distance. And 

 this is precisely what a priori we should expect. The closing of the 

 communication by the Alpine range converted what had previously 

 been an open passage, like the straits of Gibraltar, connecting one 

 ocean with another, into a deep land-locked bay or sestuary, where 

 the depth of water would necessarily diminish independently of any 

 elevation, and the influence of the neighbouring streams would be 

 constantly greater in diminishing the saline properties of the water. 



But after a time, another change took place. The deposits changed 

 from an arenaceous to an argillaceous character. The blue clay for- 

 mation (Cyrenen-Mergel of Sandberger, and lower Brown-coal forma- 

 tion of former German writers) bears indications of important changes 

 in the neighbourhood. Volcanic eruptions may have caused the 

 rivers to bring down a thick and muddy sediment, which was carried 

 to the northern point of this long narrow sestuary ; or, more pro- 

 bably still, the partial and ever-increasing closing up of the channel 

 leading to the North Sea may have caused those particles of clay 

 and mud, previously carried out through the northern straits, to 

 have been here deposited, containing in some places those accumula- 

 tions of drifted vegetable matter which have contributed to the forma- 

 tion of the Brown-coal beds. How the closing up of the northern 

 channel was brought about, we are not yet in a condition to explain ; 

 but it may have been occasioned by the elevation of the Fichtelgebirge, 

 or by some of the neighbouring basaltic outbursts which I have not 

 yet been able to examine. 



At the same time a gradual, but general, elevation of the land 

 appears to have been going on elsewhere ; inasmuch as the pre- 

 sumed parallel beds of the Upper Limburg series were also becoming 

 brackish, resting on the purely marine beds of the Middle Limburg 

 formation. The analogy, however, is not complete. In Belgium 

 the succeeding deposits maintain their marine character, whereas in 

 the Mayence basin the blue clays are immediately succeeded by a 

 thick deposit of calcareous matter, the fossil contents of which not 

 only indicate the continued brackish nature of the water, but show 

 that it was gradually losing its saline qualities, and becoming more 

 and more fitted for the existence of freshwater molluscs. During 

 this period, when all communication vntli the Northern Ocean 

 appears to have been cut off", the calcareous beds of the Cerithium 

 and Littorinella limestones were deposited in a great inland lake 

 having no apparent outlet. Large streams of fresh water, flowing 

 from the S., the W., and the S.W., the parents of the present Rhine, 

 the Neckar, the Maine, and the Kinzig, brought down large acces- 

 sions of material, to fill up the bottom of the basin, and to raise the 

 surface of the water, imbedding in it at the same time, in par- 

 ticular localities, the debris of the terrestrial molluscs which had 



