206 N. S. WHALER — FORMATION OF CONTINENTS. 



the level of the sea. When the process of uprismg brought the fold into 

 the realm of erosion, the irregular wearing of the mass began those 

 changes of burden to which we have learned to look for the immediate 

 determination of mountain-building action. Thenceforth the organiza- 

 tion of the land as regards its shape appears to have depended mainly 

 in the localization of the migrations of earth-matter brought about by the 

 orogenic forces. It is evident, however, that this migration began with 

 the subaqueous history of the fold, and that the land distortions of strata 

 has served the important end of determining the supermarine form of 

 the area. 



The rise of submarine folds from the depths of the sea to the air is 

 indicated in the geologic features of this and other continents. Instances 

 of this nature are seen in the peninsulas of Florida and Yucatan, in the 

 Antilles, and in the old ridge known as the Cincinnati axis. In the first 

 mentioned district a fold of the sea-bottom having a length of about 600 

 miles appears to have risen from a depth of about 5,000 feet. As yet the 

 process of erosion has not determined distinct axes of fracture on the 

 crest of this ridge, though there are some signs of such displacements. 

 The peninsula of Yucatan appears to represent a similar process of upward 

 growth of a fold which has not yet been mountain-built. The Antilles 

 seem to afford an instance of a lofty, narrow ridge which has risen from 

 a great depth and to have developed on its crest an extended mountain 

 system. The Cincinnati axis, a low ridge having about the length of 

 Florida, was formed in the middle Paleozoic seas, beginning to attain 

 the water's surface in the early stages of the Cincinnati group. The crest 

 of this broad elevation bears the marks of the mountain-building forces, 

 but the age of these dislocations is not certain. 



Conclusions. — It is true the cases in which we note the unlifting of folds 

 of the ocean floor which have attained the sealevel involve relatively small 

 areas as com23ared with the continental fields, yet when we trace the his- 

 tory of this and other continents back to the time when they were be- 

 ginning to take shape, we find that the conditions indicate the masses to 

 have been relatively much smaller than they are at present. It seems 

 that, given a case where an elevation of the sea-floor attained the surface, 

 the facts warrant us in supposing that the process of mountain-building 

 would begin as soon as locally intensified erosion and deposition occurred ; 

 this would induce a further migration of interior materials toward the 

 fold, thus favoring its greater elevation. If this be the case the earth folds 

 may, as regards their history, be divided into two groups : those which 

 fail to attain the surface of the sea and are not subjected to the stimulus 

 to further growth which mountain-building seems to bring about, and 

 those which, attaining the condition of land, are by the process of erosion 

 led to a rapid upward development. 



