22 R. D. Oldham — The Interior of the Earth. 



a spherical body with a definite outer surface, from which reflection, 

 takes place, and it Avas not unnatural to regard this as the outer 

 surface of the earth, but there are some very real difficulties in the 

 way of this interpretation. Just thirty years ago Dr. C. G. Knott 

 showed ' that, in the heterogeneous material of which the outer 

 layers of the earth are composed, simple condensational and dis- 

 tortional waves could not be transmitted, as each would undergo 

 a breaking up into two forms at every passage from rock of one kind 

 to that of another; ten years later Professor M. P. Pudzki further 

 showed 2 that only a very small proportion of known rocks possessed 

 those characters of elasticity which would enable them to transmit 

 unaltered the two simple forms of elastic waves, and the records of 

 seismographs show that the movement of the wave particle at the 

 surface is of a very complicated nature, having no relation to the 

 simple movements required by the theory of reflection. For these 

 reasons it seems probable that the reflection, of which we find 

 evidence in long distance records, does not take place at, but a short 

 distance below, the surface, and it is natural to place it at the limit 

 where the more heterogeneous rocks of the outer layer pass into 

 the more homogeneous material of the central core — in other words, 

 at the lower limits of the crust or at about 80 km. below the outer 

 surface. Professor Barrell's figures suggest that the limit is probably 

 sufficiently defined to give rise to reflection, and the fact that the 

 reflected waves are not always equally conspicuous is in consonance 

 with the natural assumption that the lower limit of the crust may be 

 more sharply defined in some places than in others, an assumption 

 which is strengthened by fact that these reflected waves are 

 especially conspicuous where the point of reflection lies under the 

 great nexus of mountains forming the Pamir Plateau and the "Roof 

 of the World". It is not unnatural to suppose that this region, 

 unique as regards surface features, should be equally singular in the 

 character of the under surface of the crust, and so give rise to the 

 more than usual prominence of the reflected waves, where incidence 

 takes place under this region. 



For the rest of the interior of the earth we are principally 

 dependent on the results obtained by the modern development of 

 seismology. When it was recognized that the long distance records 

 of great earthquakes represented the arrival of mass waves which 

 had travelled through the earth it must have occurred to more than 

 one worker that they would give information regarding the 

 constitution of the material traversed by the wave paths, but 

 I know of no published work previous to a paper by myself, read 

 before the Geological Society in February, 1906, on the "Constitution 

 of the Interior of the Earth as revealed by Earthquakes", 3 to 

 which, doubtless, I owe the honour of having been invited to address 

 you to-day, and in treating the subject the most convenient course 



1 Trans. Seismol. Soc. Japan, xii, pp. 115 ft\, 1888. 



2 Beitrage z. Geophysik., iii, pp. 519-40, 1898. 



3 Quart. Journ. Geol. Soc, lxii, p. 456, 1906. 



