The Bed of the Ocean. 



too obvious to require further elucidation. There remains yet 

 another question which the historian of our planet may be ex- 

 pected to answer, viz., the probable cause of the formation of 

 ridges or mountain ranges, and of the creation of centres of vol- 

 canic activity. 



Starting with Humboldt's and Sir Charles Lyell's definition 

 of volcanic action as " the influence exerted by the heated interior 

 of the earth on its external covering," we are led to inquire — 

 What is the origin of this internal heat } The answer usually 

 given is, that it proceeds from a primarily heated and fluid 

 nucleus, to the gradual cooling of which we must attribute the 

 formation of the solid external covering called the earth's crust. 

 Observation has proved that the temperature of the earth-crust 

 increases from the surface downwards, but the greatest depth at 

 which it has been ascertained in mines and artesian wells does 

 not exceed 360 fathoms (where it is found to remain constant at 

 75^ F., or about 24° C). On the other hand, the existence of a 

 heated and fluid nucleus has been shown by recent calculations 

 to be open to grave doubts, if not altogetlier impossible. 



If in the absence of this cause of internal heat we proceed to 

 look for another, we may possibly find it in an element which 

 has been found invariably associated with volcanic action, and in 

 a cause of heat the effects of which come under daily obser- 

 vation. This element is the ocean, and the cause of heat, 

 pressure — namely, the pressure of superincumbent strata, both 

 fluid and solid. Pressure, as an important factor in the struc- 

 tural development of the earth-crust, has not escaped the atten- 

 tion of the geologist, but the enormous pressure which the water 

 contained in an oceanic basin must exert upon the bottom and 

 the sides of the basin — a pressure roughly calculated to amount 

 to one ton to the square inch for every mile of depth — has not 

 been sufficiently insisted upon as an adequate cause of heat in 

 the solid strata gradually accumulating at the bottom of the sea, 



