2 70 Mr. S . V. Wood on the Events which produced and terminated 



these impressions could not have thus occurred at horizons so 

 (vertically) distant from each other during the progress of a 

 delta-deposit; while the freshwater character of the deposits 

 with which they are associated precludes the assumption of 

 their being formed on the shores of an estuary undergoing a 

 process of subsidence but silting up sufficiently fast to keep its 

 shores in a condition to be exposed by receding tides. Thus 

 it seems to me that the several phenomena which the charac- 

 ter and grouping of the Wealden formation of England pre- 

 sents, demand the concession of geographical conditions at once 

 peculiar and not to be paralleled by any known existing lake 

 or river, in order that such phenomena may not conflict with 

 each other. In the Wealden we have in places a thickness 

 of perhaps 1400 feet, divided into at least two, if not more, 

 horizons considerably separated, yielding evidences of having been 

 at some time left dry to receive air-breathing animals, and after- 

 wards silted over, while all these 1400 feet of deposit were accu- 

 mulated in the immediate contiguity of the sea of the period. I 

 cannot find that any of the great lakes with which we are familiar 

 would furnish any conditions parallel to this ; most of our exist- 

 ing lakes are depressions in rocky countries, high up in the 

 course of large rivers. Even the Caspian, to which Mr. Robert- 

 son refers for the parallel of the Wealden, furnishes no parallel ; 

 for although that basin receives a very considerable sediment 

 from the large rivers which flow into it, and seems to be laying 

 bare the vast Aralo-Caspian deposit that surrounds it by a pro- 

 cess of shrinking in its bed, yet that inland water, having a 

 closed basin preventing the outflow of the water it receives, 

 becomes in those parts which are distant from the embouchures 

 of the rivers a brackish-water lake, harbouring a fauna which is 

 not that of a purely freshwater basin. Moreover, the Caspian 

 and its allied basin of the Aral, as well as the lakes of Switzer- 

 land and the great freshwater lakes of North America and of 

 Central and Southern Africa, are situated remote from the ocean, 

 while the grouping of the synchronous marine strata of the 

 South of England show that the sea was in the very closest conti- 

 guity to the waters which furnished the Wealden deposit. Again, 

 the process under which a delta formation is accumulated is one 

 which seems to preclude the occurrence of purely freshwater 

 strata to a thickness such as is presented by those of the 

 Wealden. The thickness of the delta of the Mississippi is given 

 by Sir Charles Lyell * at about 600 feet ; the boring through 

 the delta of the Ganges at Calcutta f reached, at a depth of 420 

 feet, beds which appeared to indicate that the bottom of the delta 



* Principles of Geology, 1850, p. 219. 

 t Smith, in Proc. Geol. Soc. vol. iv. p. 4. 



