METHODS OF CORRELATION 1141 



7. Correlation by diastropJiism. 



The recognition of the widespread character of hiatuses or gaps 

 in the sedimentary series, noted in preceding sections, suggests 

 that the causes of these hreaks are of more or less universal extent. 

 These causes are diastrophic, or deformative of the earth's crust, 

 for every hiatus signifies a retreat of the sea, followed by a read- 

 vance, indicating, thus, a relative rise of the land-mass, which re- 

 sults in the emergence of large tracts, followed by a relative de- 

 pression, resulting in submergence. If these movements affect in- 

 dividual land blocks only, the regression and transgression are 

 chiefly confined to this block, while, at the same time, as has been 

 shown in Chapter I, a reverse change in relative level of sea and 

 land may be noted on the stationary blocks, this being brought about 

 by the readjustment of the entire sea-level, necessitated by the 

 partial displacement of it in one region. Thus, if one land block 

 rises independently of the others, its shores will suffer a negative 

 or retreatal movement, and its margins will emerge. As a result 

 of this, however, there will be a partial elevation of the sea-level 

 as a whole, due to the displaced water, and this will affect all 

 blocks, including the emerging one. In the latter it will tend 

 merely to reduce somewhat the total amount of sea-retreat which 

 the elevation of the block would bring about, but in the stationary 

 blocks there would be a universal advance of the sea over their 

 margins. Thus retreatal movements in one continent would be 

 correlated with transgressive movements in the other, and, since 

 emergence is followed by erosion, and submergence by deposition, 

 base-leveling of one continent would go on, with deposition beyond 

 its margins, at the same time that deposition over the submerged 

 margins of the other continents and a reduction in the erosive ac- 

 tivities over the unsubmerged portions of those continents would 

 occur. 



If the movement affects the suboceanic crustal block, however, 

 a universal sinking or rising of the sea-level will result, in con- 

 formity with the lowering or elevation of this block. This will be 

 manifested in a universal retreat and readvance of the sea along all 

 continental margins, with the production of a widespread hiatus 

 in the succession of formations. If the interval between the two 

 movements is a large one, with comparative stability of the land, the 

 base-leveling processes will tend to wear the country down to a 

 nearly uniform level, while the resultant marginal deposits will tend 

 further to raise the sea-level, and thus set the gravitative movement 



