SUBMARINE MOUNTAINS 65 



attraction of an indefinitely extended, uniform plate of granite 

 about 30 feet in thickness. Hence from the anomaly maps it is 

 a fairly conservative conclusion that each Hawaiian island 

 presses on the general sea floor with a weight like that of a 

 cylinder of granite a mile thick and having a horizontal 

 diameter as great as 200 miles, the width of one of these vol- 

 canic buildings. The excess loads at the Atlantic islands are 

 somewhat smaller, and yet they too must severely tax the 

 strength of the earth's crust, if this is no more than 50 miles in 

 thickness. 



Naturally the foregoing estimates of load cannot be used 

 for computing the strength of the crust until another step is 

 taken. We need to know whether the great volcanic loads can 

 be stably borne. There is, in fact, no reason to believe that the 

 sub-oceanic crust is yielding to the loads. Let us call to the 

 witness stand Bermuda; then some analogous islands of the 

 Pacific; and, finally, the large number of volcanoes capped by 

 living coral reefs that belong to the atoll and barrier types. 



A vertical bore-hole was sunk through the shell (-coral) 

 sand of Bermuda (Figure 34) to a depth of about 240 feet 

 below sealevel. There the bit of the boring machine entered 

 the wave-eroded surface of a volcano of basaltic habit. See 

 Figure 35. The penetrated lava was largely fragmental, as if 

 the material had been exploded out of an ancient vent. Inter- 

 spersed among the fragments are small shells of marine ani- 

 mals that lived and became extinct in Tertiary time, at least 

 ten million years ago. Already at that time Bermuda, a vol- 

 canic pile, had been truncated by the Atlantic waves and 

 reduced to a shoal. Ever since, the beveled cone, 100 miles 

 across at its base and more than 12,000 feet high, has been 

 pressing on its floor, and yet apparently the sub-Atlantic crust 

 has not yielded to the load. 



Similar paleontological proof of crustal stability, in the 



