lxXXli PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March I913, 



extent the plumb-line is deflected by the attraction of a prominent 

 rock-mass. It appears that such experiments would be of little 

 value, unless they were accompanied by observations on the varia- 

 tions in gravity in the surrounding region where such variations 

 may exist ; although the configuration of the ground gives no 

 reason to suspect them. 



I have quoted these observations made so many years ago, in 

 order to show that the early observers realized the nature of the 

 problems on which they were touching. How far their conclusions 

 would be confirmed or modified by more precise modern methods I 

 do not know; but, taken in connexion with what is being discovered 

 in other parts of the world, the observations serve to prove that, so 

 far as regards the British Isles, the investigations ceased at a critical 

 stage. That there were variations in gravity was proved, and it 

 was recognized that some other cause than inequalities of the 

 surface must be looked for ; but the extent and distribution of the 

 disturbances have not been ascertained. A Gravity Survey seems 

 clearly to be called for. 



If such a survey were made, a comparison of the results with 

 those of the various magnetic surveys of Great Britain and Ireland 

 would be likely to yield much interest. The last of the magnetic 

 surveys was carried out by Sir Arthur "W. Riicker and Sir T. 

 Edward Thorpe. In the reports published in the ' Philosophical 

 Transactions' for 1890 and 1896 a map was presented, showing 

 the relation between the magnetic and the geological features of the 

 United Kingdom. In the words of the authors ' the magnetic 

 indications appear to be quite independent of the disposition of 

 the newer strata,' but it must also be admitted that their con- 

 nexion with the form and geological structure of the Palaeozoic 

 platform, so far as these are known, is not obvious. On the other 

 hand, instances are reported in Germany of coincidence between 

 magnetic ridge-lines and lines of equal gravity that can hardly be 

 accidental. 



I may here remind you that, at the last meeting of the British 

 Association a recommendation was made in Section A to the effect 



' tbat it is desirable that a detailed Magnetic Survey of the British Isles, on 

 the lines of Professors Eiicker and Thorpe for the epoch of 1891, should now 

 be repeated, in order to answer the question as to the local variations of the 

 terrestrial magnetic elements within 25 years.' 



I should regard a Gravity Survey as at least equally desirable. 



