28 SECTIONAL ADDRESSES 



secure the proper interpretation. Much to the surprise of the observers, 

 the gravity gradients found in the neighbourhood of the salt dome pointed 

 towards it, whereas the relatively low density of salt, in comparison with 

 that of the earth in which it was embedded, led to the expectation of 

 a minimum of gravity directly above the dome. The explanation of the 

 paradox lay in the recognition of the possibility of the existence of a cap 

 rock, or shell of relatively high density anhydrite, covering the salt in the 

 dome. Close to the dome this nearer material was the predominant 

 factor, and produced a gravitational attraction ; it was only at greater 

 distances, where the great bulk of the deeper salt more than compensated 

 for its depth, that apparent gravitational repulsion was actually observed. 

 I have, rather regretfully, to leave at this stage this part of my subject. 

 My recent practical experience with torsion balances has aroused in me 

 the greatest admiration for the work of the original inventor and his 

 successors, and for the skill and precision with which most of these 

 remarkable instruments have been constructed by the makers. It comes 

 as something of a shock, even though we do not doubt the universal law 

 of gravitation, to see for the first time a small mass of gold being attracted 

 by a neighbouring lead sphere a few inches in diameter. With a torsion 

 balance at our disposal the same becomes commonplace, and is indi- 

 cative of the great power of these instruments for geophysical purposes. 

 Accumulated evidence from the field confirms this view. There is 

 convincing proof that extensive underground features, such as the salt 

 domes mentioned, limestone anticlines and synclines, rock faults, and 

 deposits of hematite or of brown coal, produce, if not too deeply buried 

 or masked by complicating irregularities, gravitational disturbances 

 large enough to lead to their delineation by means of the torsion balance. 



The Seismic Method. 

 The seismic method of prospecting began to be used about 19 19, 

 chiefly owing to the initiative of Mintrop. To some degree it has replaced 

 the gravitational method, on account of the greater speed with which it 

 enables a given area to be surveyed — a most important economic criterion, 

 of course. But there are other important reasons why, under certain 

 conditions, it must be preferred. If, for example, the topography of the 

 country is too irregular for the corresponding corrections to be applied 

 reliably to torsion balance observations, gravity surveying is excluded ; 

 and seismic work, which is not so sensitive to surface conditions, may still 

 prove of value. Again, the structure to be determined may itself settle 

 the choice of method. For instance, if the problem were to determine 

 the depth of a horizontal interface of discontinuity between two strata of 

 very great extent, the torsion balance would not find anything to measure ; 

 the seismic method, on the contrary, would be confronted, as we shall 

 see, with its most direct and simplest task. But while admitting these 

 undoubted advantages, and recognising the many notable successes of 

 seismic surveying under suitable conditions, it is necessary to state that 

 this method does not yet rest on so sure a theoretical foundation as the 

 law of gravitation ; nor do the portable seismographs employed give 

 records so unambiguous as the readings of the torsion balance. 



