Rev. Dr. Irving — An Ancient Estuary. 361 



established by more extended observations is that a pure massive 

 clay deposit is found only in the Middle beds, and that the conditions 

 favourable to its deposit were feebly anticipated in the upper 

 horizons of the older fluviatile sands, and were feebly repeated in 

 places in the earlier stages of deposition of the younger Upper Sands. 



In the Quart. Journ. of the Geol. Soc. for August, 1887, I have 

 given many of the results of my studies in the Bagshots, not the 

 least interesting being the discovery of Freshwater Diatoms in some 

 of these beds. In a later number of the same Journal (May, 1888) 

 I have pointed out that the prevalent rounded form of the grains 

 of sand in the greenest beds points to their having suffered a vast 

 amount of ^olian attrition, in the shifting of sand-dunes by the 

 wind, before they were deposited in the lagoons, in the same manner 

 as is now going on on some parts of the shores of the Baltic. To 

 this kind of evidence we may now add the frequent occurrence of 

 thin beds and laminge of very pure clay in the very heart of these 

 green-earth beds, where they are most fully developed, telling us 

 of the still waters of the deeper portions of these lagoons, never 

 ruffled by the wildest storms, too deep as yet to allow of the growth 

 of the vegetation which lined their margins and covered the inter- 

 vening swamps. We can even in some cases, with the data now 

 to hand, measure the rate of the thinning out of the green-sand series 

 as well as of the Lower quartz-sands, and thus at the same time 

 approximately map the original outlines of some of the lagoons, as 

 well as measure roughly the amount of local and contemporaneous 

 subsidence which the underlying clays suflfei'ed.^ 



In the former paper referred to (1887) I ventured to suggest a 

 natural classification of the beds of the Bagsbot Series of the London 

 Basin into — 



a). An Upper Marine-estuarine Series ; 



b). A Lower FresJi-ivater Series of riverine, delta, and lagoon origin ; 

 in the place of the more empirical division into Upper, Middle and 

 Lower, which is of no importance except for cartography, and 

 even there is often as misleading as not. 



The fact that occasional fossiliferous bands or zones are met with, 

 and that some of the forms bespeak a marine habitat, no more proves 

 the marine origin of this complex of sands, green-earths, and clays 

 than the marine shells found driven some miles inland by a great 

 south-westerly gale in the Rhone Delta ^ prove the marine origin of 

 its beds ; it might as well be argued that the Coal-measures were 

 a " marine series " because of the occurrence at certain horizons of 

 the well-known "mussel-band," or even of occasional lines of real 

 marine shells. Colonies of creatures of a more or less marine 

 character doubtless established themselves here and there in the 

 more saline parts of the lagoons, and in the Bagshot beds these are 

 preserved usually as casts in the cemented sand ; but in the rare cases 

 in which actual shell-structures have been preserved, it is certainly 



^ Perhaps tlie Norfolk Broads present some analogy to the conditions which 

 prevailed in this old Tamisian estuary in later Eocene time. 

 2 Described by Lyell in the " Principles." 



