274 SEE— TEMPERATURE, SECULAR COOLING [April 20, 



thereby afiforded beneath. Thus a mountain system such as the 

 Himalayas, Alps or Andes represents a vast number of such ele- 

 vations and depressions, and this is the way in which such move- 

 ments are to be explained. A theory which is capable of furnishing 

 such a simple explanation of such complex phenomena should 

 have a strong claim to acceptance. 



The principal escapes from the Indian Ocean seem to be chiefly 

 under the mouths of the Ganges and Indus, and hence we see why 

 these regions are so often disturbed by violent earthquakes. It is 

 noticeable that few important changes of level have been produced 

 there, except subsidence, because these troughs were arched down- 

 ward, and the surface movement therefore usually has been small. 

 Some geologists have inferred that earthquakes are common 

 in the deltas of all great rivers. But observation shows that this 

 is not universally, nor even commonly, true. The deltas of the 

 Mississippi and Amazon, so far as known, have never experienced 

 severe earthquakes, and if deposits of sediment were the cause 

 they ought to have felt disturbances like those which have been 

 frequent in the deltas of the Ganges and Indus. But there is an 

 important difference between the Ganges and the Indus, on the one 

 hand, and the Mississippi and Amazon on the other. Under the 

 former are avenues of escape for the steam-saturated lava forming 

 under the Indian Ocean, while little if any escape takes place at 

 the mouth of the Mississippi and Amazon. The Gulf of Mexico 

 is only moderately deep and the crust broken and faulted chiefly 

 on the side towards Mexico, hence the earthquakes occur there ; 

 the Atlantic in the region of the Amazon is shallow, and the crust 

 along the coast but little broken. 



It appears therefore that the _ claim that earthquakes occur in 

 the deltas of great rivers must be given up. Thus we get also a 

 clearer view of the relief of the Indian Ocean, by which the Hima- 

 layas were formed; and the earthquakes occuring there now be- 

 come more intelligible. 



§ 24. On Professor Siiess' Theory that the ^gean Sea has Col- 

 lapsed within Recent Geological Time, and on the Origin and 

 Secular Movements of Asia Minor and Syria. 

 Professor Suess holds that the basin of the ^gean Sea has sunk 



