416 PROCEEDIXGS OF THE GEOLOGICAL SOCIETY. [Juiie 19, 



appeared to me to subsist between the Loess of the Belgian Plateau 

 and the Upper Glacial clay of England, and between the Sables de 

 Campine (which the Loess overlies and overlaps southwards) and the 

 Middle Glacial sands of England. Mr. Prestwich regards the Loess 

 as the equivalent of the brick-earths of the Thames, of the Somme, 

 and of the Seine * ; and he observes that, although the valleys of 

 Belgium are cut down from and through the Loess, this deposit is 

 in places shut in by higher valleys. Now that feature, so far from 

 being one of distinction, is one of identity, between it and the Upper 

 Glacial clay; for although, strictly speaking, every valley in the 

 neighbourhood of the Glacial clay is cut down from it, or through it, 

 or into it, yet this clay has been extensively bedded against the sides 

 of anterior depressions and erosions, of which the area of the Lower 

 Bagshot sands of Essex, as illustrated in the diagrams figs. 1-3 

 affords an instance. My knowledge of the structure of the Loess is 

 not derived from personal examination, but from the description of 

 others, such as M. d'Archiac, and principally from a study of the 

 admirable geological mapt of Belgium of the late M. Dumont. To 

 one familiar with geological mapping the latter appears to convey 

 a more perfect idea of the structure than any description ; and from 

 it we see that a great denudation has descended from the brow which 

 the Loess occupies on the north of the Mouse, in the same way as we see 

 that the Postglacial denudation has descended from the brow occupied 

 by the Glacial clay on the north of the Thames ; but while this clay 

 does not cross the Thames, the Loess has a few outliers on the south 

 of the Meuse, just as occurs with the Glacial clay on the west of the 

 Lea. As the Post-glacial denudation of England must to some ex- 

 tent be involved with that of Prance, these points of coincidence 

 deserve, I think, to be noticed in this place. 



Note. 



In his paper, in the present volume of the Journal (p. 109), Mr. Dawkins states 

 that the true Boulder-clay lies in the basin of the Eoding, as -well as in those of 

 the Blackwater and Colne. As such a statement, going to the root of the whole 

 question of the Grlacial and Postglacial structm-e of the South-east of England, 

 should be made only upon clear evidence, I applied to Mr. Dawkins to know where 

 such evidence could be found. In reply, in the case of the Eoding, he refers in 

 proof to the Boulder-clay of Navestock. Now, if my Survey-map and the sections 

 traversing this deposit be referred to, it will be seen that this valley happens at one 

 part to traverse one of those depressions whose structure is illustrated in the 

 diagrams figs. 1-3, which to one not familiar with the whole area would convey 

 an illusory impression of the Boulder-clay resting within the brow of the valley. 



* Phil. Trans. 1864. According to my views the deposits of the Seine and 

 Somme, like the gravels of Salisbury, belong to a much later part of the Post- 

 glacial period than the beds of the Thames valley. 



t From this map it would appear that, if we substitute the Loess for the 

 Upper GHacial clay, and the Sables de Campine for the Middle Grlacial, the 

 diagrams figs. 1-3, would represent as far as these beds are coyicerned, their 

 position on the north of the Meuse, and the way in which the valley now occu- 

 pied by that river is cut dovni from them, — the subjacent deposits, as well as 

 the structure they have acquired from the Post-glacial denudation, being of 

 course quite different from anything in the Thames region. 



