60 ON THE SAN FRANCISCO AND VALPARAISO EARTHQUAKES. 



dislocations are geologically of very recent date ; and any displace* 

 nients of the strata since the Pliocene period are comparatively 

 insignificant, and would be little thought of were it not for the 

 spread of civilised inhabitants and their works. 



Third — The third point I would like to refer to is the wonderful 

 accuracy, due to astronomical methods, by which the rate of 

 transmission through the earth's crust of the shock (or " wave ") has 

 now been attained. Dr. Warren Upham's remarks on this point are. 

 of great interest, where he shows the rate as transmitted through 

 an arc of a Great Circle,* as compared with that of its chord. In 

 proportion to the distance from the focus of disturbance, the relative 

 lengths increase till the diameter of the earth is reached. 



The respective rates are shown on the board, as given by 

 Professor C. F. Marvin, and seem to bear out his conclusion that 

 the density of the interior mass increases with the depth from the 

 surface. This is one of the physical problems on Avhich opinions 

 differ, as the effect of the increase of temperature due to depth is to 

 diminish density. "Which of these agents nliiinafeli/ prevails is at 

 present unknown. These observations have a strong bearing on 

 the question of the state of the earth's matter below the " crust.'^ 

 The great uplifts of the Middle Tertiary period appear to hav& 

 given place to gentle and moderate flexures of the rock crust " in 

 more recent times, as stated by the author. I regret very much I 

 cannot give a satisfactory reply to Prof. Orchard's question, except 

 that loose material gives way more readily than solid when shaken. 



* i.e., a circle, or arc of one, of which the centre ])asses through tlid 

 centre of the earth. 



