I. 



The Evolution of the Thames. 



PROBABLY nothing has gained for geology a greater number of 

 recruits than a love of the scenery and a desire to understand 

 the origin of the physical features of the district in which one dwells. 

 The arrangement of river systems has been an especially useful 

 irritant ; for no one provided with eyes and wits can travel about 

 England without being puzzled by the singular anomalies in the 

 courses of our rivers, which often appear to have been planned before 

 the law that water should manage to find its own level had come into 

 operation. The Exe, for example, rises on the north coast of Devon- 

 shire, almost within stone's throw of the Bristol Channel, from which 

 it is separated only by the narrow " Hobby Drive " ; nevertheless, it 

 flows right across the county and enters the EngHsh Channel, instead 

 of cutting its way to the shore near at hand. The fact that no one 

 is allowed to go through the Hobby Drive without paying six- 

 pence, seemed at first to supply a possible explanation, since as 

 the river either would not or could not pay the toll, it had to go 

 round another way. In other parts of the country the rivers do 

 exactly the reverse, having cut their way through hills when an easy 

 course lay open to them round the flanks. The desire to get at some 

 explanation of these eccentricities which did not assign an element 

 of "cussedness " to the rivers concerned, has given the writer, and no 

 doubt many others, a first interest in geology. 



The history of the Thames has been a comparatively simple one, 

 and it has not had to record the sensational revolutions of such rivers 

 as the Rhine or the Hudson, though great differences of opinion 

 still exist as to its age and formation. During the past four years, 

 several very important additions have been made to our knowledge 

 of the subject ; but before considering these, it may be advisable to 

 refer to the main geological sequence in the district so as to get a 

 time scale for comparison. The western part of the Thames Basin 

 is formed of a series of clays and limestones belonging to the Oolitic 

 series. These are succeeded to the east by newer beds forming the 

 Cretaceous system, of which the two most important members in this 

 connection are the Chalk and some beds of chert formed of sponge 

 remains occurring in the Lower Greensand to the south of the 

 Thames. 



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