Rev. Br. Irving — An Ancient Estuary. 359: 



wnter shells as Paludina are also found. ^ Now the Bagshot Series 

 do not present us with quite such a complex of alternating conditions 

 as is recorded in the Lignitiferous Series of the Soisonnais : they 

 rather mark a progressive series of changes from strictly fluviatile 

 conditions to those which prevail in a marine estuary. The physical, 

 the stratigrapJiical, and the paloeontological lines of evidence all 

 agree in testifying to this fact; a fact that can only be satisfactorily 

 explained by the phenomenon of a slow subsidence with inter- 

 mittent pauses of long duration, during which the relative levels 

 of sea and land remained pretty stationary.^ 



The starting point in the investigation of the physical history of 

 the London Bagshots was the discovery of the organic origin of the 

 green colouring matter which is so commonly found in these sands at 

 certain horizons. Various salts of iron are formed in this way, 

 which ultimately break up by exposure to atmospheric oxygen ; 

 their non-metallic constituents are resolved into carbonic acid and 

 water, while the iron is precipitated as limonitic mud. So com- 

 pletely was this proved to be a true process of nature, that I was 

 able to make it the basis of a process for the purification of water 

 contaminated by dissolved vegetable matter, for which a patent was 

 granted me in the year 1885. Not only limonite but iron pyrites 

 is formed, the latter by the sulphur, furnished by the decay of 

 vegetable albuminous matter, attacking iron. Both these minerals 

 are of very common occurrence in the Lower and Middle Bagshot 

 strata; and both serve as cementing material for nodules which we 

 frequently meet with both in the sands and in the more sandy 

 varieties of clay. Even lignite in a fragmentary state is not un- 

 common, reminding us of the great brown-coal deposits of the 

 Continent of about the same age. 



I'he limonite is deposited in large and small concretionary masses, 

 and sometimes in continuous layers, resembling in every way that 

 which is dredged up from the Swedish and Canadian lakes, to be 

 utilized as iron-ore ; the excellent quality of Swedish iron being 

 largely due to the fact that a pure chemical precipitate of iron oxide 

 is thus made the basis of its manufacture. As there the conditions 

 of primeval forest prevail over a large proportion of that Arcbgean 

 region, and the decay of forest litter furnishes the acid solvents for 

 the leaching-out of iron from the rocks, the iron being precipitated 

 as the water undergoes oxygenation in the shallow lakes ; so here 

 in this ancient Eocene delta vegetation has by its decay furnished 

 the solvents ; the same cycle of change, and the same laws of nature 

 have been in operation. As solvents, too, of silica in the presence 

 of strong bases, these organic acids may have played their part in 



^ See Prof. Stanislas Meunier, "Les Causes Actuelles en Geologie," pp. 269, 270. 



^ I may be allowed to refer to a sketch in a popular form, of what I conceive 

 to have been the outline of the history of the Thames Basin, as I put it forward in 

 a lecture last winter, a summary of which appeared in ' ' Science Gossip "for M ay 

 and June, 1891. I should like to draw particular attention to the many points of 

 similarity between the conclusions I have arrived at as to the Tertiary history of this 

 part of England, and those arrived at by Prof. Sacco of Turin, as to the Po Basin. 

 (See "Bull, de la Soc. Beige de Geologie, etc., tome iv. 1890.) 



