EMERGENCES 505 



lutions. These may be compared to the crests of great and slow moving 

 waves recorded in the major changes of the strand-lines, and on these 

 great waves are superimposed minor waves of more rapid movement, as 

 seen in the various emergences establishing the periods or systems. These 

 are the "secular vibrations" or "oscillations" of Dana. 131 There is still 

 another order of waves, the wavelets of varying degree which delimit the 

 formations and cause all local strand-lines to be decidedly oscillatory. 



The emergences of the first and second order are seemingly due to the 

 periodic shrinkage of the earth's mass, being the times when the earth's 

 circumference is diminished. The lands as well as the oceans shrink; as, 

 however, the latter areas subside relatively more and have nearly three 

 times the areal extent of the former, it may be readily seen why the waters 

 retreat more rapidly than they transgress the continents. During times 

 of shrinkage parts of continents may also become down-faulted, thus 

 adding greater area to the oceans and accentuating an already negative 

 strand-line. Suess states that these subsidences "surpass all others in 

 importance," while Chamberlin and Salisbury, in their Geology, affirm 

 that "the master movements are the sinkings of the ocean basins" (I, 1904, 

 520). 



The shrinkage of the globe at fairly regular intervals must also cause it 

 periodically to revolve a little faster, the increased spinning probably add- 

 ing a new factor in the more rapid and greater emergence of the lands 

 toward the poles of both northern and southern hemispheres. At each 

 one of the periods of the earth's shrinkage it is thought an equatorial 

 protuberance appears, toward which some of the oceanic waters are like- 

 wise drawn. By equatorial protuberance it is not intended to convey the 

 idea that an elevation of land or water, or both, in every case completely 

 encircled the earth along this belt, but rather that somewhere in this 

 region there was a compensating protuberance. It is postulated further 

 that these elevations were very low, and that a heaping of 10 feet more 

 of water above the normal in most cases, and in none more than 100 

 feet, would be sufficient to withdraw the average continental transgressions 

 of the oceans. As the earth is a "failing structure," the protuberances 

 slowly flatten down, and during this process the oceanic waters again 

 gradually return toward the poles, thus adding their quota to the renewed 

 transgression. Most of the North American inundations have come from 

 the south, these floods being certainly more persistent in that part of the 

 continent. This fact is made clearer by contrasting the percentages of 

 the water-covered areas of North America with those of the United States 



131 Dana : American Journal of Science, vol. 22, 1856, pp. 342-347. 



