MODERN SEISMOLOGY — HODGSON 349 



gold mine was begun in 1834. In 1924 it had reached a depth of 

 6,726 feet. The bottom of this mine is not, however, the point nearest 

 the center of the earth to which man has penetrated. Tlie Calumet 

 and Hecla mines in Michigan, though only about 6,000 feet deep, 

 reach a horizon 4,600 feet below sea level, said to be the point nearest 

 the center of the earth on which man has been able to tread. The 

 South American mines being in the mountains, their greater depth 

 of shaft does not penetrate so far below sea level. Representing the 

 radius of the earth by a line 120 miles in length — the distance from 

 Montreal to Ottawa — the bottom of the Michigan mines lies at a 

 point only 138 feet away. The number of deep mines or deep bores 

 is small ; their cost is enormous. To what depth have you personally 

 inspected the interior of the earth? 



If physics deals with generalities, geology studies details, but 

 practically only surface details, as we have said. True, time has 

 broken and uptilted parts of the earth's crust, exposing at the sur- 

 face that which must once have lain at considerable depth. True, 

 also, volcanoes bring up materials from below, though from what 

 depths these materials come is not so well defined as we could wish. 

 The geologist, having studied the earth's surface features, admits 

 that, so far as he can learn, the average density of the surface crust 

 is only about half as great as that of the earth as a whole. We leave 

 him as he argues from the known to the unknown, from the ob- 

 served to the conjectured, from the fact that near the surface the 

 temperature increases about 1° F. for each 90 feet in depth to the 

 possibility that at a depth of 40 miles the temperature may be about 

 2,300° F. or the "white heat of the blacksmith's forge," « from 

 measures of the elasticity and compressibility of rocks under high 

 pressures in the laboratory to conjectures as to their properties 

 at great depths within the earth.'^ We turn to seismology for the 

 decisive tests of any theory as to the structure of the earth on which 

 we live and move and have our being, from which we get our means 

 of livelihood, within which we find our final resting place; the 

 fortress of mystery which man has, at last, completely surrounded, 

 but which he has never penetrated, nor ever shall. 



For some, an earthquake is a rare phenomenon of passing, if 

 for the moment absorbing, interest; for others it is a dreaded 

 nightmare of horror. To the engineer, it is a factor in his problems 

 of design; to the insurance agent and the financier alike, it is a risk; 

 to the newspaper man, it means business; to the geologist, it is a 

 tectonic agent; to the seismologist, it is all of these and more. 



• Daly, U. A., Our mobile earth, 342 pp., Charles Scribner's Sous, New York, 1920. 

 ' Daly, R. A., The outer shells of the earth, Amer. Journ. Scl., vol. 15, pp. 108-135, 

 February, New Haven, 1928. 



