24 SECTIONAL ADDRESSES 



to say that there are well-established cases where geophysical methods 

 properly applied have, at relatively low cost, either led to the discovery 

 of important mineral deposits previously unknown, or have facilitated 

 the precise location of such deposits, thus reducing abortive digging or 

 drilling to a minimum. 



It is mainly the physical basis of the work that I wish to review. And 

 here I should point out that this limitation will exclude ' divining,' 

 whether for water or any other underground feature. Innumerable 

 claims of successful use have been made for the divining-rod and similar 

 indicators, but the f nodus operandi has never been explained, and none 

 have been established on an acceptable physical basis. This is not to say 

 that all the claims are necessarily false, and I do not wish to use this 

 occasion to express scepticism. For I have discovered, to my surprise, 

 that the use of the hazel twig for water-divining finds credence among 

 scientific friends, the honesty of whose beliefs is above suspicion ; con- 

 sequently I am less ready to be dogmatic on the subject. Nevertheless, 

 I am glad to escape from this highly controversial ground by defining in 

 a sufficiently exclusive manner what is a geophysical method, and what 

 a geophysical instrument, in relation to the search for minerals. The 

 basis of every geophysical method is the differentiation, usually abrupt, 

 of some physical property as between rocks. The four principal methods 

 — gravitational, magnetic, seismic and electrical — depend, in fact, upon 

 differences, in the various media underlying the earth's surface, of density, 

 of magnetic susceptibility, of velocity of elastic wave propagation, and of 

 electrical conductivity respectively. Associated with these variations of 

 physical properties, either naturally or through stimulation by artificial 

 means, there are produced, at or near the earth's surface, calculable 

 physical effects which may be capable of measurement by suitable 

 apparatus. Such apparatus is a geophysical instrument, in the sense in 

 which the term will be used. Divining-rods do not belong to this 

 category ; nor, on present evidence, do those apparently more elaborate 

 instruments which are sometimes found advertised and illustrated in such 

 unexpected organs of publicity as the popular story magazines. What- 

 ever the outward form depicted may suggest, and whatever the validity 

 of the remarkable claims to infallibility in the text, these instruments 

 cannot be regarded as geophysical instruments until the mode of action 

 is revealed, and proved to be dependent on known physical laws. In 

 short, there must be something physical to measure, and the instrument 

 must be able to measure it. 



Considering the simplicity and obvious nature of the physical concepts 

 involved, it is not surprising that frequent suggestions have been made 

 over a long period of years to put the matter to practical test. As early 

 as the seventeenth century, indeed, the magnetic method was success- 

 fully employed to locate deposits of magnetite and other highly ferruginous 

 ore-bodies, by observing how the earth's magnetic field was distorted 

 locally by their magnetisation. And the beginnings of the electrical 

 methods appear to date back to 1830. But substantial progress was not 

 made until comparatively recent years, and practical geophysical pros- 

 pecting — that is, systematic surveys with instrumental equipment specially 



