300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 192 8 



entiation, but the exposed intrusives on the continents are mostly 

 granite, and the basic portions represent a smaller volume. Great 

 intrusions of basic magmas rising from the deeper asthenosphere 

 would, under the principles of isostasy, produce subsidence. Regions 

 in which the evidence is strongest for fragmentation show, in a 

 number of cases, great outpourings of basic lavas reaching maximum 

 thicknesses on adjacent coasts. The evidence for fragmentation is 

 therefore paleontologic, physiographic, and structural. The strong- 

 est evidence, however, is paleontologic. If this evidence is strong, it 

 can not be denied by an appeal to isostasy, as a mechanism of igneous 

 activity, described in this topic, may be invoked which only requires 

 a larger scale of action than that observed to have occurred on the 

 surviving lands. 



PERIODIC AND SECULAR SINKING OF THE OCEAN LEVEL 



With the development of a solid crust came the origin of the 

 oceans, and with every rise of magmas the volume of water has been 

 increased. Therefore, throughout the geologic ages there has been a 

 contest between land and sea, between the volume of ocean water 

 and the capacity of the oceanic basins. From epoch to epoch the sea 

 level is oscillatory, and from era to era there are progressive secular 

 changes. 



The widely flooded condition of most of the continents during 

 the Paleozoic is shown by the accumulation of marine sedimentary 

 rocks upon their surfaces. This result of a high ocean level is in 

 contrast to the deep and widespread erosion of the highly emergent 

 continents just previous to the Paleozoic era, and is most probably 

 to be ascribed to the increase of water during the Paleozoic. On 

 the other hand, it also appears that during the time of revolutions 

 closing the eras, the ocean level sinks and the horstlike continents 

 become highly emergent. Higher and higher the continents have 

 risen during the later Tertiary until the mean elevation of the land 

 has become about 2,400 feet, probably 1,000 to 1,G00 feet higher than 

 the mean elevation during the Paleozoic era. 



Many geologists have assumed that the synchronous emergence 

 of the several continents is due to a deepening rather than widening 

 of the ocean basins. This, however, brings in difficulties with the 

 theory of isostasy. The simpler view appears to be that during the 

 times of revolution the ocean basins widen by continental fragmen- 

 tation rather than deepen as a whole. At the same time, individual 

 lands tend to warp up to offset isostatically the loss of rock previously 

 removed by erosion. 



The great revolutions are accompanied and followed by complete 

 or nearly complete withdrawals of the sea. The continents are then 



