62 THE FLOOR OF THE OCEAN 



The comparison seems significant even if glassy basalt does not 

 now constitute a complete earth-shell. 



The Volcanic Mountain a Test of Crustal Strength. — Com- 

 pared to the width of an ocean basin, a thickness of 50 miles 

 for the crust is small, and the crust must be regarded as 

 relatively thin. On that skin of the earth the volcanic piles 

 have been built. Their constituent rock is between two and 

 three times as dense as the displaced water. The roughly 

 conical pile is two to six miles thick (high) and at base has 

 a minimum width of 100 to 200 miles. Each mountain is an 

 enormous mass resting on the original floor of the ocean, and 

 the way in which it is supported is at once seen to be of prime 

 importance in connection with the crust-substratum hypothesis. 

 We now face the second fundamental question: can the mem- 

 brane-like crust continue to bear these great superficial loads 

 of rock, if the substratum has little or no strength ? The answer 

 depends: first, on the degree to which the crust is stressed by 

 the loads; second, on the degree of stability actually character- 

 izing the volcanic mountain; third, on the degree of strength 

 possessed by crystalline rock when, under the controlled condi- 

 tions of the laboratory, it is subjected to the same conditions of 

 pressure and temperature as those affecting the assumed crust. 

 We shall briefly review recent researches bearing on all three 

 topics, which manifestly have vital relation to earth physics in 

 general. 



That the volcanic cones rising from deep water represent 

 great loads on the earth's crust has been proved by the gravity 

 pendulum. The first chapter contains a brief account of the 

 so-called "reductions" of observed values of gravity to the cor- 

 responding sealevel values. We learned, too, that at each oc- 

 cupied station the difference between the reduced value and 

 that expected from the general figure of the earth, or the stand- 

 ard spheroid, is called a gravity anomaly. Of the different 



