XVII.] GEOLOGY OF THE THAMES BASIN. 295 



varied contours of the present Thames basin. Thus, para- 

 doxical as it may sound, the river is older than the hills 

 and dales amongst which it flows, and which appear to 

 determine its course. 



If the question be asked, how long a time has been 

 occupied in the formation of the floor of the Thames basin ; 

 the only reply which can be given is, that most certainly it 

 was of enormous duration, but that there are no means of 

 estimating it with accuracy. The whole mass has been 

 constructed, as has been seen, of the products of denu- 

 dation or of those of vital processes. There is not the 

 least reason for supposing that either of these products were, 

 on the average, formed more rapidly in those ancient times 

 than they are now ; and there is independent evidence, that 

 some of these rocks, such as the chalk, were deposited very 

 slowly. It may be taken to be certain that the thickness 

 of chalk which represents a year's accumulation in the 

 Cretaceous ocean, is but a small fraction of an inch. But 

 suppose it were an inch ; then, as the chalk beneath 

 London is 600 feet thick, it follows that this bed alone 

 represents 7,200 years. 



In point of fact, however, not only is it almost certain 

 that we should be much nearer the truth, in assuming that 

 the chalk beneath London took ten times as long as this to 

 accumulate ; but, it can be proved, that the strata which 

 overlie the chalk, in the London basin, represent but a mere 

 fraction of those which have been deposited elsewhere, 

 since the time at which the chalk was formed. The most 

 niggardly computation which lies within the bounds of 

 probability, presents us with a sum total of several hundreds 

 of thousands of years, for the time which has elapsed since 

 the sea, of which the chalk is the bottom-mud, flowed over 

 the site of London. 



