32 SECTIONAL ADDRESSES 



magnetisation under the influence of the earth's general magnetic field. 

 For the field distortion thereby produced at the earth's surface to be 

 marked it is necessary for the responsible rock structure to have a large 

 susceptibility ; this implies that only highly ferruginous rocks will be easy 

 to find. No difficulty, for example, presented itself in the case of the great 

 magnetic anomaly at Koursk, where two elongated deposits of magnetite, 

 totalling 20 billion tons, and several hundred feet deep, produce, over 

 a region of many square miles, prodigiously large variations'^ of all the 

 terrestrial magnetic elements. For this survey the magnetic method 

 was eminently suitable, and comparatively insensitive instruments served. 

 Ore-bodies of similar magnetic material, but of much smaller dimensions, 

 give rise to anomalies less marked but still unmistakable. Thus deposits 

 of ilmenite and pyrrhotite, as well as magnetite, if not too deeply buried in 

 relation to their size, can be both detected and located with considerable 

 precision by simple magnetic measurements. 



I do not mean to imply that the magnetic method of surveying is limited 

 to the detection of ore-bodies of this kind. Igneous rocks generally, and 

 particularly basalt, may contain considerable quantities of iron, and 

 consequently possess an eifective magnetic susceptibility much larger 

 than non-ferrous materials. There is abundant evidence that structures 

 of such rocks have been determined, under favourable conditions, by 

 the use of magnetic variometers. Moreover, in recent years these instru- 

 ments have been much improved in sensitivity, so that, at any rate 

 nominally, they are capable of measuring variations of about 5y only in 

 the vertical or horizontal force. This development is bringing within the 

 field of applicability of the magnetic method even sedimentary formations 

 only slightly ferruginous, if due care is taken to make corrections which 

 are unnecessary where the anomalies are great. But I should, neverthe- 

 less, reject the claims, which have sometimes been made, to have used 

 present-day instruments to locate salt domes by reason of the diamag- 

 netism of rock salt. In fact, the conclusion seems inevitable that the 

 susceptibilities, whether positive or negative, of materials not within the 

 ferro-magnetic class are too small to be responsible for field distortions 

 at present measurable with portable instruments. Variation in the iron 

 content of rocks has been the origin of the anomalies so far observed. 



If we are to hope to bring within the scope of the magnetic method 

 non-ferruginous underground formations, we must improve greatly the 

 sensitivity of the instruments, and at the same time exclude the operation 

 of certain disturbing factors. It is little use rendering apparatus more 

 sensitive if this involves enhancing also corrections of an uncertain 

 character. The chief difficulty with the variometers at present available 

 is the application of the corrections for diurnal variation of the earth's 

 field and for temperature changes. A certain degree of sensitivity having 

 been achieved by balancing the control of the normal earth's field against 



' The vertical anomaly has a maximum of about 2 gauss ; the horizontal 

 field has both positive and negative values ranging over about i • 3 gauss, and 

 the declination, accordingly, varies from 0° to ± 180°. These effects are larger 

 than could be attributed to the magnetisation of the magnetite under the in- 

 fluence of the earth's present field. The deposit has strong permanent magnetism 

 derived in a way not known. 



