Correspondence — G. N. Bromehead. 381 



different data, less satisfactoiy on other grounds, might lead also to 

 a less satisfactory amount of contraction or even to an expansion. 

 This is an argument in favour of the data and of the theory, and not 

 against them. 



His assertion that 1 would have readers " believe that the thickness 

 of the radio-active layer has been fairly accurately measured", and 

 his charge of "dogmatism", are definitely untrue. It was because 

 it is not accurately known that 1 determined the available compression 

 on two hypothetical distributions of radio-active matter, both per- 

 missible on other grounds, but widely different ; the results they 

 gave were not very different and were stated in the article. 



I introduced no new theories regarding the properties of matter. 

 What I did was to classify in a convenient way the known behaviour 

 of different types of matter under shearing stress. The statement 

 quoted from Maxwell that liquids and perhaps most solids are 

 perfecth^ elastic as regards stress uniform in all directions is irrelevant 

 to my discussion, which was explicitly limited to the differences 

 between the stresses in different directions. In the light of present 

 knowledge the account of shearing stress in Maxwell's book needs 

 revision ; for it makes no reference to elastic after-working or to the 

 elasticity of such a substance as pitch, which in my classification 

 would be a plastic solid with a very low limiting stress-difference. 

 The common practice of regarding as a liquid a substance so elastic 

 that tuning forks can be made of it is exceedingly inconvenient. 



Had the conclusion, that my views on the solid and liquid states are 

 quite inadmissible, been accompanied by the slightest argument, it 

 might have been more impressive ; or it might not. 



Hauold Jeffreys. 



THE PEE-THANETIAN EROSION OF THE CHALK. 



SiK, — I have read with much interest the suggestive paper by 

 Mr. H. A . Baker on the " Pre-Thanetian Erosion of the Chalk in the 

 London Basin". I have for some time past been accumulating 

 ■evidence for a similar study, but in 1915 wrote that "the evidence 

 ... is as yet too slight to allow of a definite map being made" 

 {^Geology of Windsor and Chertsey, Mem. Geol. Surv., p. 14). 



Mr. ]3aker's map (Pig. 1) includes the area to which I referred, and 

 appears to be based upon less evidence than that which my work for 

 the Geological Survey had afforded. In the construction of such 

 a map it seems natural to ascertain as far as possible the zone of the 

 Chalk immediately underlj'ing the Tertiary at the boundary of the 

 latter, and to check the zones whose presence beneath the Tertiary is 

 deduced from borings by these facts. This has not been done by 

 Mr. Baker. The zone of Chalk on which the Tertiary rests has been 

 iiscertained by the Survey in the south-western part of the area 

 shown on Mr. Baker's map, and a portion of the results has already 

 been published (op. cit.). From the neighbourhood of Beaconsfield to 

 the western margin of the map forming Fig. 1 he shows the base of 

 the Tertiary as resting on the zone of A. quadratus. The fact is, 

 that the zone is that of M. cor-anyuinum at Beaconsfield, Marstqntes 



