24 Tj-aiisacfions. 



stands near it, especially when the supposition of upheaval based on the 

 occurrence of the volcano is contradicted by the occurrence of embayments 

 in the shore-line of another island not far away. 



The chief characteristics of crustal subsidences, as well as of crustal 

 upheavals, are that the submergences or emergences they produce may 

 vary from place to place in rate, amount, and date ; and in these significant 

 respects they will differ from the submergences or emergences due to other 

 causes, which must involve universal changes of ocean-level, everywhere 

 the same in date, amount, and rate, except where they are complicated 

 by contemporaneous local crustal movements. Evidently, then, it is 

 important to examine all structural and physiographic features of coral 

 reefs and of their encircled islands from which inferences may be made 

 as to the rate, amount, and date of the changes of level that they have 

 suffered, in order to learn how far they are everywhere alike, or how far 

 they vary from place to place. 



Extravagant Deformation is demanded by Large Changes of Ocean-level. — 

 A few examples of results already gained in this direction will be given 

 below. But let it first be noted that in order to produce the submergence 

 or upheaval of an island by 1,000 ft. a local subsidence or upheaval of the 

 island by that amount in an ocean of essentially constant level is a much 

 more economical movement than the vast crustal deformations involved in 

 a rise or fall of the ocean-sarface by the same amount around a still-standing 

 island ; for such a change of ocean-level can be brought about only by a 

 change of the same measure in the entire ocean-floor (except around the 

 still-standing island), or by a ten times greater cliange in a tenth of the 

 ocean-floor. 



Indeed, if a change of ocean-floor level over a tenth of its area involve 

 roughly compensatory changes of a similar area elsewhere, then in order 

 to cause a rise or fall of the ocean-surface by 1,000 ft. the failure of 

 compensation must be of the order of 10.000 ft. ; and, great as these 

 movements are, their whole measure must be accomplished in the same 

 period of time as that required for the much smaller measure of local 

 upheaval or subsidence of the island under discussion. It thus appears 

 that in seeking to account for a local submergence or emergence of 1,000 ft. 

 an economv of vertical movements in a reef-encircled island involves an 

 extravagance of movements elsewhere. Hence while small, slow, wide- 

 spread, and synchronous changes in the relative level of land and sea 

 mav be plausibly ascribed to changes in the level of the ocean as a result 

 of ocean-floor deformation, large, rapid, and local changes are best 

 accounted for by movements of the island or coast where they are 

 recorded. 



Nevertheless, some students of coral reefs have attempted to throw 

 the responsibility for large submergences or emergences of the islands that 

 thev have described upon other unspecified parts of the world. Thus 

 C. W. Andrews says, regarding the emergence of Christmas Island, a little- 

 dissected high-standing atoll, 1,200 ft. in altitude, in the eastern Indian 

 Ocean, " It seems very probable that it is the general level of the surface 

 of the sea that has iDeen altered, and not merely a local upheaval of a 

 limited land area that has taken place." Inasmuch as in this case all 

 islands and all continental shores that did not suffer emergence at the 

 same time must have subsided with the ocean, an enormous terrestrial 

 disturbance is involved in this method of accounting for the recently gained 

 altitude of a single small island. 



