400 



PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [May 24, 



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22 



Thus on the high ridge of London Clay 

 at Highgate we have an outher of the 

 Bagshot Sand, and the same formation 

 is also to be seen at Hampstead, and 

 particularly on the sides of the road 

 descending towards Hendon. 



Thus also, when upon the London 

 Clay at Edgeware, if we ascend to Harrow 

 Weald, we first find brick-kilns upon the 

 higher level, showing the commencement 

 of a superjacent arenaceou.s deposit, and 

 then afterwards, fully developed upon 

 the high summit of Harrow-on-the-Hill, 

 we find another acknowledged outlier of 

 the Bagshot Sand*. 



The discovery of this ferruginous bed 

 of sand leads to an interesting inference 

 when considered in connexion with the 

 distinction of colour, which has been pre- 

 viously noticed as existing between the 

 upper and under portions of the subjacent 

 argillaceous formation. Two causes, ap- 

 parently distinct and unconnected, seem 

 to have been simultaneously in opera- 

 tion — the one producing an earthy, and 

 the other a metalliferous deposit. Thus, 

 while aluminous matter was being pre- 

 cipitated, there was an outpouring of 

 iron, dissolved most probably in water 

 containing carbonic acid gas. But this 

 ferruginous solution was not simply co- 

 extensive with the formation of the alu- 

 minous stratum ; it continued in opera- 

 tion even when the alumina had given 

 place to silex, although a substance che- 

 mically and essentially so diverse. The 

 origin of the ferruginous impregnation 

 was therefore independent of those causes 

 which produced the earthy deposits. 



The same circumstances occur in other 

 places. Thus near Bath they are to be 

 seen in the transition of the clay of the 

 Upper Lias into the sand of the over- 

 lying Inferior Oolite ; while near Wey- 

 mouth they are equally to be noticed in 

 the passage (e contrcwio) from the Oxford 

 Oolite into the superjacent Kimmerid^e 

 Clay. 



* The geognostic position of these kilns and 

 that at Scrap Gate in the Isle of Sheppey appears 

 to be identical. 



