SUBMARINE MOUNTAINS 55 



earthquake shocks have pulled the trigger and so given the 

 final impulse to catastrophic collapse. The reality of the slid- 

 ing process was impressed on the writer while mapping vol- 

 canic islands in both Atlantic and Pacific oceans. 



For one illustration we visit eastern Samoa of the southwest 

 Pacific. There Ofu and Olosega, twin basaltic islands, rise pre- 

 cipitously out of the water, with shapes that immediately sug- 

 gest a geological drama. See Figure 27. The two islands are 



i OLOSEGA ISLAND 



OFU ISLAND^ 



FIGURE 27. MAP OF OFU AND OLOSEGA ISLANDS, SAMOA. FRINGING CORAL 

 REEFS HAVE GROWN OUT SINCE THE COLLAPSE OF THE ORIGINAL VOLCANO. 



separated by only a few hundreds of feet of water. Magnificent 

 exposures of the constituent, layered lavas, outcropping in the 

 high cliffs, make it clear that the twin islands are residuals of 

 a single island whose area was much larger than the combined 

 areas of Ofu and Olosega. The chart shows the ground-plan 

 of the existing cliffs composed into two great curves, one con- 

 cave to the north and the other concave to the south. Field 

 observations, which need not here be detailed, led to the con- 

 clusion that this peculiar topography resulted from two geolog- 

 ically recent slides on opposite sides of the original island, and 

 that the displaced masses of rock are now buried under the 

 ocean. Accordingly the high, arcuate cliffs are the inner limits 

 of gigantic scars left by the landslides. In principle the curved 

 crest-lines are like those seen at the heads of scars made by the 

 slipping of rock masses on the continents. In the Ofu-Olosega 

 case the scars only can be seen. The stated theory of origin 



