ORGANIC A GENC1 A'.s. 



107 



these facts together, it seems probable that several hun- 

 dred thousand square miles of volcanic land have been 

 lost. Of this only a small fraction has been recovered by 

 the action of corals and waves. 



Amount of Vertical Subsidence. This may be 

 roughly estimated in many ways : 1. Soundings a little 

 way off barriers have reached 2,000 feet, and off atolls 

 7,000 feet. 2. The average slope of volcanic islands of 

 the Pacific is about 8, but, taking it even as low as 5, a 

 barrier ten miles from shore would indicate a subsidence 

 of 4,500 feet. (D B = A D . tan A.) But barriers are 

 found at much greater distances than ten miles. 3. The 

 average height of volcanic islands of the Pacific in non- 



FIG. 60. v, volcanic island. ; ^1, shore lino ; />, place of barrier ; AB^ slope of 



bottom, 5. 



subsiding areas is 6,000 to 10,000 feet. Now, every atoll 

 represents such an island, entirely submerged, and every 

 closed lagoon the same deeply submerged. But it is very 

 improbable that none of these reached the average of those 

 remaining. Taking all these facts together, it is probable 

 that the extreme subsidence is not less than 10,000 feet. 



Amount of Time Involved. It is evident that the 

 rate of sinking can not have been greater than the rate of 

 coral ground-rising ; otherwise the corals would have been 

 drowned. Again, the rate of ground-rising is far less than 

 the rate of coral-prong growth. If the annual growth of 

 all the prongs were taken, ground to powder, and strewed 

 over the area shaded by the coral branches, it would give 

 the annual rising of the ground. It is evident that this 

 would be very small in comparison with the growth of the 

 prongs. In addition to this, it must be remembered that 

 large spaces of a coral reef are bare. Taking all these 



