92 G. Sohuchert — Problem of Continental Fracturing 



therefor lias been greatest in the oceanic basins, the areas of 

 greatest rock densities. 



Increase of water with. time. — If the oceans have progres- 

 sively enlarged and deepened, it is natural to ask, Has the 

 quantity of water increased with time, and if so, what was the 

 source of supply? Some geologists, and more especially some 

 petmlogists, have concluded that every active volcano and most 

 of the thermal springs are adding much new water — the so- 

 called "juvenile" water, of magmatic origin — to the old 

 accumulations of the hydrosphere. As yet we have but little 

 in the way of estimates based on field or laboratory experiments 

 to give us any adequate idea how much water an active volcano 

 liberates. Some years ago the writer hazarded the guess that 

 the increase of water since the beginning of Paleozoic time 

 may have been as much as 25 per cent. 



Decrease of water during glacial periods. — That the strand- 

 lines of the oceans are decidedly mobile is well known and 

 these oscillations are generally ascribed to the rising and sink- 

 ing of the continents. This conclusion is undoubtedly in large 

 measure true, but that the ocean bottoms also rise locally and 

 so displace water, resulting in rising strand-lines, is readily 

 deducible from the mere presence of oceanic islands and sub- 

 merged ridges, because these masses have risen above the mean 

 of the oceanic bottoms. On the other hand, it is known that 

 periodically the ocean bottoms subside, but apparently no more 

 than a few hundred feet at a time, and as a result of these 

 subsidences the strand-lines the world over are markedly lowered 

 when the overlapping marine waters are withdrawn and the 

 continents are most emergent as they are at present. The 

 mobility of the strand-lines is every now and then further 

 augmented during the glacial periods when the volume of 

 water in the oceans is decreased and the extracted quantity is 

 piled up on the land as ice. During the Pleistocene, Daly 1 

 states that the strand-line was thus repeatedly lowered in the 

 tropical areas, and estimates that the maximum was from 200 

 to 230 feet, during the times of greatest cold. At the same 

 time reef-corals were then almost non-existent, permitting the 

 ever active marine waves to cut down the protruding oceanic 

 islands within the tropics to a little below sea-level. These 

 truncations are now the submerged platforms that lie as a rule 

 between 180 and 300 feet beneath the present sea-level, and 

 on which the reefs have since grown, keeping pace with the 



1 R. A. Dalv, The glacial-control theory of coral reefs, Proc. Amer. Acad. 

 Arts Scj., .li, 157-251, 1915. . 



