358 Hev. Dr. Irving — An Ancient Estuary. 



power proportionately lessened, by the counter-force of the tides, 

 rushing up the estuarj'^ of the Humber ; a process, to the reality of 

 which every ' dredge ' employed bears testimony. This process, 

 which the French geologists aptly term ' atterrissement,' is well 

 exemplified in the fact that Sunk Island (a genuine island under 

 Charles II., then just raising its head above the water-line) presented 

 140 years ago an area of 1500 acres, and was, even then, being 

 brought under cultivation. About the beginning of the present 

 century its acreage was doubled to 3000 actually under cultivation ; 

 and in 1854 the survey returns gave between 6 and 7 thousand 

 acres, showing that in the present century the rate of alluvial deposit 

 taking place has greatly increased. The saying of the first Napoleon 

 that Holland naturally belonged to France, because it was made up 

 of the mud of a French river (as he considered the Rhine), is, I 

 dare say, well known. 



Prof. Green in his valuable and deservedly popular text-book of 

 " Physical Geology " has discussed the geological principles involved 

 in the formation of a great series of estuarine deposits, with their 

 constant alternation of fresh- water, brackish, chemical and terrestrial 

 formations, their current-bedding, and the wedge-shaped interlacing 

 or interdigitation of beds of different mineral composition. This 

 interdigitation arises for the most part when a series of rivers 

 draining tracts of country of different lithological character, con- 

 verge to a common estuary ; but in cases where the deposits have 

 been laid down, and the process of atterrissement has gone on, in the 

 neighbourhood of the mouth of one great river (with perhaps minor 

 affluents) this characteristic is less marked, and we then find that 

 the vertical variation of the rock-character of the series constitutes 

 its principal feature, the successive deposits being traceable over 

 miles of country. 



My own studies of the Bagshot Beds of the London Basin have 

 led me to regard them as in the main a mixed series of fluviatile, 

 terrestrial and truly estuarine deposits, laid down under some such 

 conditions in later Eocene time. There is a general vertical 

 sequence traceable through the area, as Prestwich recognized long 

 ago, with variations laterally of percentage of lithological con- 

 stituents and homogeneity of structure, of the several beds. By 

 the term ' estuarine ' I mean to indicate those deposits which 

 are laid down in the open area into which the river flows, and 

 to which the ocean-tides have access ; a connotation supported 

 both by etymology and the usage of earlier writers on geology. 

 Such deposits are thus distinguished in a real sense from those 

 which are deltaic or fluviatile. A parallel case might be cited 

 in the still older Eocene series known as the Lignitiferous Series 

 of the Soisonnais ; a series of numerous alternations of sands, 

 clays, and beds of lignite, while the fossils met with indicate 

 a special regime ; shells of mollusca essentially marine (as Ostrea 

 and Pedunculus) occurring along with others (such as Melania, 

 Melanopsis, Neritina, Cerithium) which are known to prefer the 

 mouths of great rivers for their habitat, while such strictly fresh- 



