MODERN SEISMOLOGY 247 



to assume the movement of a mass of water of proportions truly astound- 

 ing. The observations of Mr Forster.have given us experimental data 

 on a truly grand scale, and in connection with quakes at best of but 

 secondary strength. 



Modern Seismology and fundamental Conceptions of Geology • 



The earthquake cataclysms were in the days of the founder of uni- 

 formitarianism regarded as rare phenomena of the globe. The seats 

 of the western civilization and culture were not then so connected by 

 telegraph and cable with the earthquake zones of the globe as they are 

 today, and conditions within those provinces could not be adequately 

 judged. Of the known earthquakes at that time, moreover, much that 

 was generally ascribed to the natural exaggeration inspired by terror we 

 have now by investigation found to be literal truth. By natural infer- 

 ence the fundamental conception of modern geology, namely, that the 

 present is the key to the past, came to mean much more than its mere 

 words implied. It was interpreted to mean that geological phenomena 

 were to be mainly explained by those forces only which in countries of 

 low seismicity act so slowly that it is only by measurements extending 

 over many years, or even centuries, that they can be detected. Almost 

 infinitely slow warping of the crust, the forces of erosion, and vast 

 periods of time — these elements loomed large in the concepts which were 

 formed. 



Modern seismology has now afforded us a measure of the amount of 

 actual crustal movement which accompanies earthquakes of the first order 

 of magnitude on the land. At the surface this measure generally lies 

 between the limits for maximum vertical movement on a single plane of 

 10 and 60 feet — a movement which is begun and completed within a time 

 more frequently measured in seconds than in minutes. The observations 

 of Forster on the movements of the Mediterranean floor at the time of 

 earthquakes, almost the only data of the kind yet available, as we have 

 seen, indicate that the values above given for the amplitude of earth 

 movements on the land are probably but a tithe of those which, on the 

 floor of the ocean, accompany macroseisms. Yet of seisms capable of 

 producing movements of this order about 70 occur each year. 



The most impressive are, however, not necessarily the most important 

 phenomena, and we must take full account of the fact that, according to 

 Milne's estimates, 30,000 lighter shocks occur each year on the land areas 

 each of extent and intensity sufficient at least to be appreciated by a con- 

 siderable number of people. Of the number of such shocks on the sea- 



