420 



University of California. 



[Vol. t. 



what the mere weight of the neighboring sediments would demand, 

 as pointed out by Dana for the Atlantic seaboard, and suggested in 

 this piper for San Francisco Bay. But even if this be not so, it is 

 difficult to conceive of subsidence, due to a load of sediment, being 

 extended to any distance outside of the immediate area of deposi- 

 tion, unless we assign to the materials of the earth's crust a degree 

 of tensile strength in excess of that exhibited by the best of 

 building stones, in which it is notably small. A pile of shot, placed 

 upon a sheet of india-rubber, resting upon a viscous liquid, might 

 exhibit a subsidence greater in area than the portion immediately 

 covered by the shot; but it is hardly to be expected that the earth's 

 crust, composed of materials which, as far as we know, are of very 

 slight tensile strength, should behave as does india-rubber. 



McGee has stated that most of the great historical earthquakes 

 not connected with "vulcanism or orogeny" have ' affected tracts of 

 rapid deposition," and that commonly after the tremor the land 

 stood lower than before. Like so much connected with the theory 

 of isostasy. there is a seductiveness about this generalization which 

 is apt to lead one astray. By excluding the earthquakes possibly 

 connected with "vulcanism or orogeny," we are practically limited, 

 as far as the land is concerned, to the low-lying plains, which are 

 generally areas of subsidence, or are covered by recent sediments. 

 And when, further, Milne tells us that "earthquakes chiefly occur in 

 volcanic and mountainous regions,"* that they are particularly 

 abundant along coasts which slope down at a high angle to great 

 depths,f that "many earthquakes occur in mid-ocean," % and that 

 "earthquake and volcanic regions are situated on areas where there 

 is evidenqe of rapid elevation, "§ the foregoing generalization rather 

 loses its force. When we consider too, that the rich alluvial 

 plains and low-lying deltas have been in all historical times the 

 portions of the globe most densely populated, and that for this 

 reason earthquakes in such tracts have resulted in appalling destruc- 

 tion of life and human works, it is not surprising that so many of 



* Earthquakes, p. 227. 

 f Ibid. p. 22S. 

 % /but, p. 228. 

 \ Ibid, p. 278. 



