OF THE BAGSHOT BEDS OF THE LON^DON BASIN. 183 



estuarine line of drainage, admitting of the simultaneous silting up 

 of the more central parts and local erosion along the margin, were 

 not a simple movement tending to produce a mere synclinal flexure, 

 but rather a series of movements connected with the later Eocene 

 physical history of central Mercian England, is, I think, an inference 

 which is justified by the data before us. 



Additional Note on the Geeen-earth Seeies. 



Taking into account such facts as the following — (i.) the atten- 

 uation of the green-earth series towards the north; (ii.) the solitary 

 instance (Wellington- College Well) in which the series of beds 

 between the two principal clays are made up (or said to be made 

 up) wholly of green earthy sands and pebbles, while even there, in 

 the well at the West Lodge, 1 ft. of light grey quicksand was proved 

 at the base of the green sands, representing several feet of a similar 

 sand in the Wick-Hill Section, other beds being often in the deep- 

 well sections intercalated with the green earths (e. g. Vd^ ft. of 

 ' light sandy clay ' at the Bagshot Orphan Asylum, 20| ft. of other 

 strata in the well at the South Camp, Aldershot, while the green 

 earths scarcely occur at all in the Brookwood Well) ; (iii.) the 

 possibility of their decoloration by slow oxidation in long-exposed 

 sections — we cannot admit the mere jDresence or absence of green 

 earths in small sections as a test of horizons of the Middle Group 

 (as has been persistently urged in some quarters), although the evi- 

 dence is undoubtedly strengthened by the presence of the green 

 earths. 



Again, it follows from their northerly attenuation (which is a 

 stratigraphical fact) that either the beds at higher horizons must 

 be unconformable to them (owing to contemporaneous denudation), 

 or else a gradual subsidence of the more central parts of the estuary 

 (which is far more probable) allowed of the gradual accumulation 

 of these green earthy deposits in extensive lagoons. In either case 

 the southerly dip of beds at lower horizons must, of necessity, 

 be greater than that of beds at higher horizons : and obviously any 

 application of stratigraphical methods of calculation which ignores 

 this consideration must be so far misleading. 



Then as to the common presence in them of well-rounded wind- 

 worn quartz-grains, to which attention was drawn in my last 

 paper *, a little reflection enables us to see in this fact additional 

 evidence of the lagoon-origin of these green beds, the shifting of 

 sand-dunes by the wind subjecting the sand to a vast amount of 

 aeolian attrition f. 



The Middle Group, then, with its lagoon-deposits coloured fre- 

 quently with organic salts of iron, its freshwater Diatoms, its 

 pebble-beds at more than one horizon, and its two persistent clay- 



* Quart. Journ. Geol. Soc. vol. xliii. p. 380. 



t For a remarkable example of this, within the present century, on the shores 

 of the Baltic (Kurische Haff), see Credner, 'Elem. der Geol. ' 6th ed. (p. 271, 

 fig. 83). 



