THE BAGSHOT BEDS OF THE LONDON BASIN. 



379 



and many of the smaller concretions are undistinguishable from the 

 bog-iron-ore obtained from the Canadian and Swedish lakes. More- 

 over they are often found on analysis to contain a basic crenate of 

 iron. Fragments of lignite are, in my experience, not uncommon 

 both in the Middle and Lower beds. The process by which iron is 

 carried down in solution by peaty acids and deposited mainly as 

 hydrated ferric oxide (bog-iron-ore) is well known *. This is, no 

 doubt, the true explanation of the agglutination by ferric oxide of 

 the gritty sands and conglomerates of the plateau-gravels known 

 locally as ' pan ' or ' rust.' It may still be witnessed in most of the 

 streams of the peaty waters of the present Bagshot country. Many 

 of the more sandy nodules were first agglutinated by iron-pyrites, 

 since such are common in their original state and may be observed 

 in all stages of oxidation. Sometimes they contain pure 'silver sand ' 

 in the interior. 



6. Results of Chemical Analysis. — These have been alreadj 7 in 

 part published t, so far as regards combined carbon. Those results 

 have been confirmed by several scores of analyses since made of 

 samples of these carbonaceous beds and of the waters they yield J. 

 Independent confirmatory evidence has also appeared §. Recently 

 I have made about 25 combustion-analyses ||, in order to get out the 

 percentages of elementary carbon. These percentages range from 

 0*312 up to 1*950, the highest being given by the very dark green 

 earthy sands of the Middle Bagshot. On the other hand, an average 

 sample from the basal beds of the Upper Bagshot in this neighbour- 

 hood gave only 0*17 per cent. The samples analyzed came from dif- 

 ferent parts of the district, the majority of them from well-sections. 

 Elutriation of some of the green earths removes nearly the whole of 

 the green colouring-matter, the fine amorphous earthy matter acting 

 apparently as a ' mordant,' and leaves behind white quartz-sand 

 mingled with some black and olive-green grains. This probably 

 partly explains the tenacity of colour of some of these green earths ; 

 yet chemistry teaches us that they must, and observation shows us 

 that they c?o, lose their colour by oxidation. Slow oxidation is only 

 a less intense phase of the process of combustion in oxygen ; and 

 the burnt residues of these green earths are so completely discoloured 

 as to be undistinguishable from those of the brown sands and clays ; 



* See Roth, loc. cit. pp. 596-598 (worth the attention of those who are 

 sceptical as to the existence of the Humussaure) ; Credner, El. der Geol. (3rd ed.) 

 p. 255 ; Julien, "On the Geological Action of the Humus Acids," Proc. Am. 

 Assoc. Sci. (1879) ; also the author's papers in the Geol. Mag. January 1885, 

 and elsewhere. 



t Geol. Mag. dec. ii. vol. x. (1883). 



| See remarks by the author in Proc. Inst. Civil Engineers, vol. lxxxv. 

 (1885-86). 



§ See (1) Quart. Journ. Geol. Soc. vol. xlii. p. 162 ; (2) Sanitary Record, 

 June 1886. Lecture by W. Eassie, Esq., O.E., F.G.S. A reply to Mr. Blyth's 

 criticisms of that lecture will be found in the following (July) number of the same 

 periodical. 



|| Made by combustion in a stream of purified and dried oxygen. The 

 accuracy of the method was tested by analysis of a specimen of graphite from 

 Borrowdale. 



