R. D. Oldham — Origin of the Himalayas. 533 



of cooling matei'ial must cause tension, as a diminution of the 

 circumference is resisted by the uncooled central core ; at the outer 

 limit, where no further cooling takes place, the consolidated crust 

 would be thrown into compression by the reduction in bulk of the 

 material below, and somewhere between these two would come a zone 

 where the radial and tangential contractions exactly balance each 

 other, so that the material would neither be compressed nor extended. 



The depth at this level of no strain beneath the surface would 

 depend on the initial temperature of consolidation and the 

 temperature gradient. The latter is appi'oximately known, the former 

 is doubtful; Mr. Mellard Reade, assuming a temperature of about 

 3,000° F., calculated the depth at about 1 mile; later, Mr. Fisher 

 calculated a depth of about 0*7 mile for an initial temperature of 

 4,000° F., and of 2 miles for an initial temperature of 7,000° F. 

 Of these three values for the initial temperature the lowest is the 

 most probable, and the calculations show that any rift, which might 

 be formed by tension in the ' sub-crust ', could not reach the surface ; 

 it would originate at the level of the greatest tensional strain, which 

 would lie at between 30 and 50 miles below the surface, and thence 

 extend upwards and downwards, but could not reach upwards to 

 within about a mile from the surface. These calculations never had 

 any geological interest, beyond showing that the hypothesis of an 

 originally highly heated, and gradually cooling, solid globe afforded 

 no sufficient explanation of the structure of the earth's outer crust, 

 as revealed by geological observation in the field ; they have been 

 rendered of little more than academic and historic interest by modern 

 researches in radio-activity, which have, incidentally, provided a means 

 by which fissures opening at the surface of the earth and penetrating 

 downwards could be produced; for if the earth is an originally cold 

 gL)be, getting gradually warmer, and expanding in bulk, by the 

 action of radio-active material, then such fissures would naturally be 

 formed ; but in that case none of the consequences which Colonel 

 Burrard has drawn from his rift would follow. 



In all this I have not been arguing for or against a fluid or a solid 

 earth, but against the introduction of an irrelevant issue. Whether 

 the earth is a fluid or a solid globe seems a matter which concerns the 

 astronomer, the physicist, or the mathematician, much more than 

 the geologist ; for all the processes which his observations demand 

 appear to be equally compatible with either hypothesis. Nor am 

 I arguing against Colonel Burrard's explanation as a whole ; it 

 may stand with the others as a possible hypothesis, to be tested 

 and examined on its merits, but the tests must mostly be of 

 a nature quite unconnected with geological observation. When, 

 however, Colonel Burrard postulates the existence of a rift, or narrow 

 band of subsidence, reaching 20 miles deep from the surface, we are 

 brought face to face with a phenomenon for which we have no 

 precedent, and before accepting it we must be satisfied, not only that 

 it explains the geodetic facts, but that no other explanation, in closer 

 accord with what is known of the geology of the regions, can be found. 



No one can gainsay Colonel Burrard's contention that geologists 

 must take count of the facts of geodesy, but it is equally true that 



