272 



THE POPULAR SCIENCE MONTHLY. 



Guinea in the Polynesian seas. In this area are 204 islands, very few 

 of which are high or with land still above the sea. Southward of this 

 area are many mountainous islands, or with highlands, surrounded with 

 reefs, evidently beyond the line of greatest subsidence. 



The evidence is satisfactory that the depressed area has gone down, 

 in comparatively recent geologic time, many thousands of feet, and yet 

 the subsidence may have been less than the elevation of lands else- 

 where ; for we have the elevation of the Rocky Mountains, Andes, 

 Alps, and Himalayas, modern events in geologic history. It is more 

 than probable that great subsidence in one section is correlated by ele- 

 vations elsewhere. And the depression of the Pacific area may corre- 

 spond to the elevation of northern lands which probably caused the 

 cold and glaciers of the glacial age. The movement w r as one of tho?e 

 great secular changes of the earth's crust which dates far back in its 

 history. 



There is evidence that the Pacific subsidence has ceased, or nearly 

 so, and that local elevations have I0112; since commenced. In about 40 

 instances, Pacific coral-islands have been elevated since reefs were 

 formed upon them. Many of these elevations are a few feet only, oth- 

 "ers, a few hundred feet ; as many as 600 feet in one or two instances. 

 These elevated masses of coral-rock have the perpendicular walls and 

 configuration before described. Metia, or Aurora Island (Fig. 15), is 

 one of the Panmotu group; its walls of coral-limestone are 250 feet 

 high, and resemble the Palisades on the Hudson. 



Fig. 15. 





'''''^'h^^i-ip^^^^lS^i 



Metia, or Aurora Island. 



Along the outer margins of the elevated islands are deep caverns, 

 showing, by their contour, the wearing and wasting action of waves. 

 The Bermudas are remarkable for their caverns ; the coral-made land 

 being in places 260 feet above the level of the sea. On the island of 

 Oahu, the Rev. John "Williams entered one by a descent of 20 feet, and 

 wandered a mile in one of its branches. Innumerable openings pre- 

 sented themselves on all sides. The roof, a superb stratum of coral- 

 rock, 15 feet thick, was supported by stalactitic columns, and thickly 

 hung with stalactites. 



