BENEATH THE LONDON CLAY. l49 



For it should always be remembered that, when the main broad 

 outlines, and even what at any particular time appear to be minute 

 details, in any subject of science have been worked out, there 

 always remain some residual small quantities or minor facts (as 

 they seem) to be accounted for, behind which possibly lurk some 

 of the grandest secrets of nature. The history of Science is full 

 of instances of this truth. 



I would urge therefore that the collection, and registration in 

 convenient forms for comparison, of any facts or observations, 

 however apparently trivial in themselves, is a worthy — perhaps I 

 might say, the most important — service which a society such as 

 our own can render to the progress of Science and the knowledge 

 of Nature. 



But leaving these general observations, let me now proceed to 

 my special subject. You will probably pardon me if, in order to 

 make clearly intelligible the question which I have to propound, I 

 summarise some facts as to the geological structure of our county 

 which are doubtless quite well known to many, if not most, of those 

 present at this meeting, only casting them in a mould which is suit- 

 able for my purpose. Let us suppose that we are proceeding to 

 sink a well from the spot where we are now assembled (Kilburn). 

 I am not aware whether there are any special peculiarities in the 

 surface soil and upper layers at Kilburn, but if not, I apprehend 

 we shall find beneath a foot or two of surface soil, a stiff brown 

 clay, in which, without much variation, except for an occasional 

 stony layer of a concretionary character from 6in. to i8in. thick 

 (known as Septaria), we should dig for some 220 to 250 feet, and 

 then for a foot or two through a more or less sandy bed, possibly 

 some more clay, and then a bed of well-rounded pebbles. This 

 group of strata is known as the London clay with its basement bed. 

 Throughout no water would enter our shaft, unless possibly a small 

 quantity in the sand of the basement bed. Then for the next 60, 

 or perhaps 80, feet we should pass through a much more variable 

 series of beds, consisting of light-coloured clays, streaked and 

 mottled with various brilliant colours like mottled soap, an 

 occasional layer of lignite coated with iron pyrites, and one or 

 more beds 2 to 4 feet thick of sand, with probably a thicker bed of 

 sand and pebbles lying on the surface of the chalk, which we might 



