FRAGMENTATION BAREELL, 301 



subject to excessive erosion, and thus arise the long-enduring breaks 

 in the marine sedimentary records. These are also the times of most 

 marked mountain making and of greatest progress in continental 

 fragmentation. Then follow periods of higher ocean levels in which 

 parts of the continents are again covered by the sea. Here then we 

 see the cycle of the eras illustrated by the most marked changes of 

 s;ea level, which distinguish Paleozoic, Mesozoic, and Cenozoic. What 

 has been the still longer trend of geologic events? 



The revolutions of the Archeozoic, accompanied by batholithic in- 

 vasions of the continents on a world-wide scale, were followed by 

 erosion of the continents down to the levels of crystalline rocks. 

 Hence the ocean level was then low relative to the continents. Dur- 

 ing the Animikian (Proterozoic), however, the seas appear to have 

 spread widely over the deeply eroded continents, but in the closing 

 Proterozoic revolution there was a marked rising of basic magmas 

 and a falling sea level, as no late Proterozoic marine records are 

 known. This time of no or very scanty marine record Walcott has 

 named the Lipalian interval. During the Paleozoic, on the other 

 hand, a high sea level was the rule, and the Permian revolution, 

 although it caused a general withdrawal of the seas, did not greatly 

 change their mean level, as is shown by the lack of deep erosion of 

 the previously deposited Paleozoic sediments. It is true that, on 

 Ihe whole, wider lands followed in the first half of the Mesozoic, 

 but during the Cretaceous the oceans again invaded the continents in 

 one of their widest floods. The Cenozoic, however, has witnessed 

 an oscillating rise of the lands until now they appear to have a 

 greater mean elevation than at any time since the Lipalian interval. 



Smoothing out the crustal oscillations connected with periods and 

 eras, it appears that a great cycle of progress has run through earth 

 history. High and wide lands marked the Archeozoic and Protero- 

 zoic revolutions; fragmentation Avas apparently widening the earliest 

 ocean basins and lowering the ocean levels, but the juvenile waters 

 from the accompanying igneous intrusions reelevated them. Then 

 came a long time, from the early Paleozoic to the close of the De- 

 vonian, during which the oceans rose and repeatedly spread over the 

 lands. Since the later Paleozoic, however, the ocean level had tended 

 to sink, and fragmentation appears to have gained on the accessions 

 of juvenile water. The resulting wider, higher, and more diversified 

 continental areas have favored the evolution of land vertebrates. 

 Fortunate indeed has been tlie summation of these processes, for 

 without this change in the general trend of the ocean Jevel it is 

 doubtful if the evolution of man could ever have been attained. 



But the lands have been kept high above the increasing waters 

 only by the breaking down of portions of their areas into ocean 

 basjns, a sacrifice which can not go forward forever if there is to be 



