MODERN STUDIES OF EARTHQUAKES. 367 



constant contraction dependent upon the cooling of the globe. It is 

 broken up into separate masses which in their turn are dislocated 

 horizontally or vertically; is lifted up and folded into immense moun- 

 tain ranges, the arches of which, breaking, may again suffer disloca- 

 tion. Thus continuous action in movement of masses and foldings is 

 constantly going on in the earth. Edouard Suess, the distinguished 

 Austrian geologist, has indeed constituted a special earthquake type 

 to correspond with this type of mountain formation. Since, in con- 

 sequence of this condition, tension is present everywhere in the crust 

 of the earth, it may come to pass that it shall be relieved by a distant 

 earthquake, and another earthquake, which may be called a relay or 

 transmission earthquake, be produced thereby. Hence we have, be- 

 sides the volcanic, the landfall, the tectonic (in the strict sense), and 

 the transmission earthquakes. The sources of earthquake force lie, 

 then, according to this theory, in the incompleteness of the earth's 

 crust, the effects of gravity, and the earth's loss of heat. 



And is the supposition not very probable ? Do we not see similar 

 processes going on over the whole earth, in the shape of earthquakes, 

 landslides, fissures, subsidences of land, and the like? And as the 

 Alps were lifted up, and the plain of the Rhine was depressed be- 

 tween the Vosges and the Black Forest, may not mightier disloca- 

 tions, breaches, and destruction occur? Why may not the processes 

 which took place in the earlier epochs of the earth's history and were 

 so powerful in the more recent Tertiary be still going on? All this 

 seems so plausible that, with a few exceptions, the theory has been 

 almost universally agreed in. 



I briefly mention here Falb's theory, which, accepting the earlier 

 views, ascribes earthquakes to periodical swellings of the fiery fluid 

 interior of the earth, only because of the effect it has had on the pub- 

 lic in connection with some wholly unscientific predictions. More 

 worthy of consideration is the theory of Daubree, the late distin- 

 guished master of French and especially Alsatian geology, who did 

 not attribute the similar phenomena of volcanic and nonvolcanic 

 earthquakes to different causes, but maintained that all earthquakes 

 were produced by superheated steam issuing from surface waters. But 

 this theory needs no refutation. There are, however, some serious ob- 

 jections to the tectonic theory of earthquakes, plausible as it may 

 seem. In order to weigh them as we ought, we must as briefly as pos- 

 sible construct a picture of the constitution of the earth's interior. 



The average distance from the earth's surface to its center is sixty- 

 three hundred and seventy kilometres. The temperature of the earth 

 increases with the depth, at the rate, on a moderate estimate, of about 

 one degree centigrade for every forty metres. Hence, at a depth 

 of one thousand kilometres we would have a temperature of 25,000° 



