FIELD GEOLOGY. 217 



water and fluviatile Eeading-beds, which thin out 

 against the higher part of the Chalk, and rest uncon- 

 formably upon that formation. 



Very great and important generalisations in geology have 

 been made from scientific observation of the character 

 and mode of occurrence of beds at distant points, bringing 

 all apparently discordant features into one harmonious 

 whole, with sometimes grand and unexpected results. 

 As an example may be cited Professor Prestwich's 

 arguments in favour of obtaining a supply of water 

 for London from the Lower Greensand.* These affirm 

 the probable existence of that formation beneath the 

 metropolis, in a water-bearing condition, and are based 

 upon the physical conditions of its outcrop many miles 

 away from the point in question. The conclusions 

 have been amply justified so far as the reasoning is 

 concerned, and although the Lower Greensand has not 

 been found under London, it is owing to what may be 

 called an accidental circumstance, which has, however, 

 given rise to another important generalisation. That is 

 the underground extension of a ridge of Palaeozoic 

 rocks which rise up sufficiently high to reach the Gault, 

 and thus to cut out the Lower Greensand, that other- 

 wise must have been there.*f- 



The Secondary rocks were, of course, deposited in a 

 fairly level position, and in this area have not since 

 been subjected to much disturbance. They should 

 consequently be found, when not thinned out, in regular 

 sequence ; and if a tract of high ground existed in the 



" Water-bearing Strata of the London Basin." 



f " Account of the Kentish-town Well-section." Prestwich : 

 Quart. Jour. Geol Soc. vol. xii. p. 9. 



