Reports and Proceedings — Geological Socielij of London. 45 



with the Califoruian Earthquake of April 18, 1906." By Eicharcl 

 Dixon Oldham, F.G.S. 



At the time of the San Francisco earthquake movement took place 

 along a fault, known as the San Andreas Fault, which can he traced 

 for a distance of ahout 200 miles. A reraeasurement of the primary 

 triangulation in the region shaken by the earthquake revealed con- 

 siderable displacement, increasing in amount as the fault is neared, 

 and of such nature that places to the east of the fault were shifted 

 southwards A^-hile those to tlie Avest of it were shifted northwards. 

 The author points out that the extent and peculiar distribution of 

 these displacements negative the supposition that the fault was the 

 cause — it must rather be regarded as a consequence of, or an incident 

 in, the earthciuakc, this word being used to denote the disturbance in 

 its entirety. 



He also considers that the displacements cannot be explained in 

 a satisfactory manner on the supposition that they are the result of 

 strains affecting the crust of the earth as a whole, but may be 

 explained by the difference in character and behaviour of the materials 

 composing the greater part of it, where pressures are great enough to 

 produce the phenomena of solid flow, and of those in the outer skin, 

 where the pressures are not great enough to produce any material 

 difference in the behaviour of rocks from that which we associate 

 with solidity, as experienced at the surface of the earth. The 

 surface-displacements constituting the earthquake, as ordinarily 

 understood, arise from disturbances in the outer skin ; but in great 

 earthquakes, like the one dealt with in the paper, these may be the 

 result of more deep-seated disturbances affecting the whole crust of 

 the earth. A distinction is drawn between these two forms of 

 disturbance, and the term bathyseism is proposed for the deep-seated 

 disturbance : the wave-motion which impresses itself on distant 

 seismographs and consti'tutcs the teleseism or world-shaking earth- 

 quake being the product of the bathyseism. 



The deep-seated cause, or bathyseism, of the San Francisco earth- 

 quake is regarded as the result of a widespread strain, of the nature 

 of a shear, such as might have been produced by displacements 

 approximately parallel to the general direction of the coastline, and 

 b)' forces which must have been very different from those concerned 

 in the formation of the San Andreas Fault. This fault cannot, 

 consequently, be regarded as the cause of tlie earthquake, nor the 

 earthquake as an incident in the growth of the fault. 



Mr. W. "Whitaker called attention to specimens of impressions of 

 salt-crystals from a local sandstone in the Keuper Marl at !N^orth Curry 

 (Somerset). Pseudomorphs of salt-crystals were well known, but, 

 so far as he knew, the occurrence of impressions, not filled in (and 

 which might be taken as arrested pseudomorphs), had not been hitherto 

 recorded in this country. The only notice of such that he knew of 

 was from America, in 1842. Now that this occurrence Avas recorded, 

 probably other examples would be noticed. 



