216 Dr. Irving—Surface- Chanyes in London Basin. 
surface-water being ponded back to form pools and lakes, which, 
as they got silted up, would form morasses or peat-bogs, some of 
which actually remain to the present day,' and illustrate the operation 
of the same processes of nature as do many peaty floors of modern 
valleys in the Alps and elsewhere, where valley-lakes have been 
silted up. 
Among the mineralogical changes which would be brought about 
under such and similar conditions, would be the formation of limo- 
nitic iron-ore and limonitic concretions of sand,’ in which (as is 
easy to see) occasional shells of a land- or freshwater-habitat might 
have been included. One is justified, therefore, in hesitating to 
accept the occurrence of such limonitic masses in some of the 
secondary gravels (standing alone) as a part of the wreckage of 
beds of the Bagshot formation. Again, a little consideration will 
show that even glauconitic green sand may have been formed locally, 
under sub-aérial conditions, where the action of decomposing organic 
matter played a part (as it must have frequently done) in aiding 
mineral change. The occurrence, therefore, of glauconitic matter in 
superficial deposits is no more a proof that Middle Bagshot Beds 
once extended over the particular locality where it is now met with, 
than the occurrence of limonite is proof of the former existence in 
other places of the Upper Bagshot Sands. In each case we have no 
more than a presumptive probability. In some cases we know 
positively that such glauconitic material cannot have been derived 
from the Middle Bagshot, because those very beds are known to be 
a considerable distance beneath the locality where the glauconite is 
met with. Thus, in the cutting at North Court on Finchampstead 
Ridges, which is demonstrably in Upper Bagshot Sands, grains of 
glauconite occur sporadically but conspicuously in the massive sand- 
beds there exposed; and in places this material is collected in holes 
and pipes, which simulate root-tubes so remarkably, as to make it 
almost impossible to doubt that the glauconite was formed in situ, 
through the agency of the organic matter furnished by the decay of 
the roots, which once occupied the tubes now filled with glauconitic 
sand. Precisely the same phenomena are met with at Bill Hill, 
Bracknell, as I have pointed out in the unpublished portion of the 
MS. (now at the Geological Society’s Rooms) of the paper read before 
that Society on Nov. 12th, 1890. A duplicate series of specimens 
illustrating this was exhibited at the time. The details which I then 
gave, and which anyone can verify, show that the beds exposed at 
these two places have such a lithological similarity (in fact almost 
an identity), that, if we took lithological characters alone as our 
guide, we should without any hesitation refer the two series of beds 
to one and the same horizon. While deferring fuller treatment of 
1 Such as that at Mirk Bottom (see 6-in. Ord. Map), where we have made 
extensive excavations lately. 
2 See my paper ‘‘ On Organic Matter as a Geological Agent’’; Proc. Geol. Assoc. 
vol. xil. pp. 227 e¢ seg. 1892. 
3 See the cases mentioned in the paper (pp. 48, 44), to which reference has been 
frequently made in this paper. 
