Dr. J. W. Evans — Presidential Address. 561 



vibrations a period of about 17 or 18 seconds is usually predominant, 

 and is believed to be due to the natural period of vibration of the 

 earth's crust. "VViechert ' assumes that there is a node half-way 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 3i km. per second, he calculates the 

 depth of the crust to be approximately 30 km. There seems, how- 

 ever, 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 3J 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 former 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, coi'responding to the thicknesses 

 of 18 km. and 31-g- km. (11 and 20 miles). There is some evidence 

 in the case of a distant earthquake of a period approximating to 

 30 km. per second, which would correspond, with waves of 

 distortion, to a thickness of 30 km. (19 miles). However, in the 

 present state of our knowledge of these vibrations such calculations 

 are only of speculative interest. 



There must be numerous surfaces of discontinuity in the earth's 

 crust in addition to that forming its lower limit. Such would be 

 the boundaries between great tracts of granite or granitoid gneiss 

 and the basic rocks that in all probability everywhere underlie 

 them; the surface dividing gneisses and crystalline schists from 

 unmetamorphosed sediments overlying them unconformably ; that 

 between hai'd Palaeozoic rocks and softer strata of later age; and 

 the surfaces of massive limestones or sills. "VViechert observed at 

 Gottingen, at the time of the Indian earthquake of April 4, 1905, 

 small horizontal vibrations, superimposed on the others, with 

 a period of only 1£ seconds. He believed that these were due to 

 horizontal distortional vibrations of the local sandstone formation 

 with a node at its basal surface. He found the velocity of similar 

 vibrations at the surface to be 250 m. per second, and thence 

 calculated the depth of the sandstone stratum to be 90 m. 2 No 

 doubt similar correlations of terrestrial vibrations and the structure 

 of the earth's crust may be made in other cases. 



It deserves consideration, however, as to how far it may be 

 possible to add to our knowledge of the earth's crust by experimental 

 work with a view of the determination of surfaces of discontinuity 



1 Gottinger Nachrichten, 1907, pp. 468-9. 



2 Ibid., pp. 467-8. 



DECADE VI. — VOL. VI. — NO. XII. 36 



