284 Mr. S. V. Wood on the Events which produced and terminated 



pointed to a truly delta origin in the absence of the peculiar 

 geographical conditions affecting it in common with theWealden ; 

 but the stage at which the elevatory movements of the oolitic 

 age terminated and converted the marine gulf into an inland 

 basin, concurs with the formation of this deposit. As Mr. 

 Robertson in the paper before quoted showed, the earlier 

 deposits of a basin recently severed from the ocean would be 

 those of brackish water; but this brackish condition could 

 scarcely have endured, under the influence of the passage of 

 fresh water through it, long enough for the accumulation of 300 

 feet of sediment. The brackish condition of the Caspian and 

 its allied basins, and of some other lakes, is due to their being 

 closed basins, where the evaporation balances or exceeds the 

 supply of fresh water, so that the salts washed from the soil and 

 held in solution in imperceptible quantities in the river- water 

 gradually accumulate. Where this is not the case, the basin has 

 an outlet and the waters are fresh. We can scarcely conceive 

 the conditions of the basin as regards river- supply and evapora- 

 tion to have been so different during the Purbeck epoch, to what 

 was the case during the Wealden, as to allow us to seek an 

 explanation of the brackish-water condition of the basin during 

 the former epoch by assuming that it was a closed one where 

 evaporation equalled the precipitation. We must rather infer 

 that during this epoch channels through the north-east open- 

 ing permitted the access of the tide, and thus made brackish 

 the waters, and that the final movements which terminated the 

 oolitic age rendered the fall of these channels too great for the 

 tide during the succeeding age to reach the basin. 



Of the many formations of oolito-neocomian age containing 

 freshwater mollusca, either solely or intermixed with estuary 

 forms, a few only would, if the foregoing conclusions are well 

 founded, be common to the same basin with the Wealden of 

 England. Of these, the ferruginous sands of Shotover near 

 Oxford, shown by Prof. J. Phillips* to repose at Coombe Wood 

 and Garsington in Oxfordshire, and in parts of Buckinghamshire 

 and Bedfordshire, on beds referable to the Purbeck, seem alone 

 to be unequivocally referable to the deposits formed in the basin 

 during the Wealden epoch ; but the underlying brackish-water 

 beds in these counties, and the brackish-water beds in the Bas 

 Boulonnais, appear clearly to be parts of the earlier deposits of 

 the same basin, and synchronous with the Purbeck deposits of 

 Dorset and Wilts. The unfossiliferous sands and marls of small 

 thickness containing slight vegetable impressions, described by 

 M. Cornuelas occurring in the Haute Marne, between the lower 

 greensand beds and the Portland limestone, and regarded by 

 * Quart. Journ. Geol. Soc. vol. xiv. p. 240. 



