PRESIDENTIAL ADDRESS. 185 



studies of the relations of such subsidiary crystallographic characters to the 

 environment in the case of crystals which can be formed under normal con- 

 ditions of temperature and pressure, and therefore under the immediate observa- 

 tion of the experimenter. Some work has, in fact, already been done on the 

 effects on these characters of the presence of other substances in the same 

 solution. 



In the .study of the secondary alterations of metalliferous depo.sits, especially 

 those which consist of the enrichment of mineral veins by the action of circulat- 

 ing solutions, either of atmospheric or intratelluric origin, the study of peeudo- 

 morphs gives, of course, valuable a.ssistance in determining the nature of the 

 chemical and pliysical changes that have taken place. 



A successful solution of the problem of the exact conditions under which 

 deposits of economic importance are found would be of incalculable value in 

 facilitating their discovery and exploitation, and would be the means of saving 

 a vast amomit of unnecessary labour and expense. 



The problem of the structure and nature of the earth's interior, inaccessible 

 to us even by boring, would seem at fir.st sight to be well nigh insoluble, except 

 as far as we can deduce from the dips and relations of the rocks at the surface 

 their downward extension to considerable depths. We can, however, gain 

 important information about the physical condition of the deeper portions from 

 the reaction of the earth to the external forces to whicli it is subjected, and still 

 more from a study of the 'preliminary' earthquake tremors that traverse it, 

 the time occupied in their passage, and the difference in intensity of those that 

 follow different paths. These methods are, however, not applicable to the 

 earth's crust. Its physical characters appear to be distinct from those of the 

 interior, but very iittle is as j^et definitely known about them, except of course 

 in the neighbourhood of the svu'face, and for this reason they are usually 

 ignored in calculating the paths of tremors traversing the earth. It seems to 

 be separated from the deeper portions of the earth by a surface of discontinuity 

 at which earthquake vibrations travelling upwards towai'ds the surface may be 

 reflected. Calculations based on the total time taken by these reflected waves 

 to reach the surface after a second passage through the earth's interior appear 

 to indicate that this surface of discontinuity, whatever its nature may be. is at 

 a depth of about twenty miles, though there can be little doubt that this 

 depth varies considerably from point to point. 



The main earthquake vibrations appear to follow the curvature of the 

 earth, and to be confined to its crust, instead of traversing the interior, as is the 

 case with the preliminary tremors. In these vibrations a period of about seven- 

 teen or eighteen seconds is usually predominant, and is believed to be due to the 

 natural period of vibration of the earth's crust. Wiechert =- assumes that there 

 is a node halfway down and a free movement above and below, so that the full 

 wave length would be twice the thickness of the earth's crust. Assuming a 

 velocity of propagation of Sg km. per second, he calculates the depth of the 

 crust to be approximately 30 km. There seems, however, to be no warrant for 

 supposing that the lower surface of the crust is capable of free vibration. The 

 fact that, not only waves of compression, but waves of distortion can traverse it 

 shows that it must possess very high rigidity so far as forces of brief duration 

 are concerned. The lower surface should therefore be regarded as a node, and 

 only the upper as capable of free movement, so that the whole would correspond 

 to a quarter of a wave length. On the other hand, the velocity of 3;^- km. per 

 second, which is that of the propagation of waves round the earth's crust, in all 

 probability a complex process, is not the same as the true velocity of vibrations 

 passing upwards and downward through the earth's crust. Those with a period 

 of about 18 seconds appear to consist partly of horizontal vibrations and partly 

 of vertical; the foi-mer would seem to correspond to waves of distortion, and the 

 latter to waves of compression. The velocity of the former would probably be 

 about 4 km. and the latter 7 km. per second, corresponding to the thicknesses of 



" Gottingen NachHcht.en, 1907, pp. 468-9. 



