360 



TF. WJiitaker — Chalk of the London Basin. 



A few practical applications of these results may be mentioned in. 

 conclusion. 



(1) The usual method of finding the average temperature gradient 

 over large areas may be incorrect, unless it is known that all the 

 stations have not recently experienced the effects of a considerable 

 cliange of mean annual temperature. For instance, the change of 

 gradient in Scotland and the north of England since the Glacial 

 period may be different from that which has taken place in France 

 or Cornwall, and the average of the best determined gradients in 

 these districts is probably not the correct average for the present 

 epoch due simply to the original temperature of solidification. 



TABLE II. 



(2) Estimates of the earth's age founded on Lord Kelvin's 

 solution and conditions, if derived from a temperature gradient in 

 a recently glaciated district, will err by being too great. For 

 instance, if the duration of post-Glacial time be 10,000 years and 

 if that of the Glacial period were four times as great, the normal 

 temperature gradient at the present time would be nearly one degree 

 in 44'1 feet, instead of one degree in 50 feet. The coi'responding 

 value of the earth's age would be about 75 million, instead of about 

 100 million, years. 



(3) Lastly, supposing it in our power to make very accurate 

 observations of underground temperature, it would be possible, by 

 comparing gradients in glaciated districts in the two hemispheres, 

 to determine roughly whether their respective Glacial periods ended 

 nearly simi;ltaneously or at widely different epochs; and thus to 

 obtain some suggestion of the direction, cosinical or terrestrial, in 

 vi^hich to look for the origin of the glacial cold. 



V. — On the Chalk of the London Basin in kegard to Water 



Supply. 

 By W. Whitaker, B.A., F.R.S., F.G.S., Assoc. Inst. C.E. 



[Reprinted from the Report of the Royal Commission on Metropolitan "Water 

 Supply, Appendices to Minutes of Evidence, pp. 433-5 (1893).] 



1. Thickness of the Chalk. 

 kVER the area in question the total thickness of the Chalk 

 (where it can be measured from top to bottom, or nearly so) 

 varies from 623 feet at Streatham to 1146 feet at Norwich, and in 

 the latter case the topmost beds are absent; so that the full thickness. 



