360 Rev. Dr. Irving — An Ancient Estuary. 



the formation of the glauconite (essentially a mixture of hydrous 

 silicates of potash and the oxides of iron'), which gives such a 

 marked character to the green earthy sands of the Middle Group, 

 and helps to testify to their lagoon-origin. [The minute flakes of 

 glassy silica described in previous papers, as well as the secondary 

 crystals on the quartz grains, are, I believe, authigenous products ; 

 while the abraded scales of mica, so common in the sands associated, 

 with the clays of the formation, are in all probability allothigenous 

 material, testifying to the erosion and denudation in later Eocene 

 time of the crystalline rocks of the mountain-system to the west, 

 which furnished the head-waters of the great Eocene river.] 



" False-bedding," both on the larger scale and on that smaller scale 

 known as "current-bedding" or "oblique lamination" is a pheno- 

 menon of extremely common occurrence in the sandy beds of both 

 the Lower and Middle Gi'oup, testifying to the variations in strength 

 and direction of the currents which laid the deposits down in their 

 present position. Between the sand-beds (often current-bedded) 

 there frequently occur, both in the Middle and Lower Group, thin 

 seams, sometimes mere films, of pure pipe-day, telling us of alterna- 

 tion of quiet conditions and the settling-down in still waters of the 

 fine argillaceous materials, which, as every geologist knows, are 

 capable of suspension in water for a considerable length of time. 

 This alternation of conditions is so frequently and so strongly 

 marked in the interlaminated arrangement of clay and sand in certain 

 beds of the Middle Group, that, as a physical fact, it was one of the 

 first among those, with which my earlier observations of these beds 

 made me familiar, to suggest to my mind a deltaic origin for them. 

 Some of the older geologists were startled three or four years ago, 

 at my suggestion that the pipe-clay might be, in part at least, 

 derived from the Chalk strata, which must have been exposed to 

 subaerial erosion over considerable areas within the catchment- 

 basin from which the waters were concentrated to the area in 

 question. I was able to establish the probability of this by a very 

 simple piece of laboratory work. A small quantity of the marly 

 material of the Lower Chalk was pulverized and treated with very 

 dilute acetic acid. The material was washed and re-washed with 

 this, until no further escape of carbonic acid could be detected. The 

 insoluble argillaceous residue settled down after suspension in 

 water, to form a thin film of pure clay, similar in every way to the 

 filmy deposits of pipe-clay which we meet with in the Bagshot 

 Sands. "What a feeble organic acid could do in that case might 

 easily be done in the ordinary processes of nature, with sufficient 

 time and the conditions prevailing in a deltaic area, by the action 

 of the humus acids, which decaying vegetation furnishes. The 

 facts here stated seem, I think, to justify my refusal to accept the 

 presence of either current-bedding, or pipe-clay films, or of both 

 together, as infallible tests of horizons, as has been persistently 

 urged by one or two workers in the Bagshot Series. The fact 



^ See Naumann-Zirkel : "Elemente der Mineralogie ; " Leipzig (Engelmann) , 

 1885, Also, Justus Eoth's " Chem. und AUgem. Geol." p. 559. 



