240 EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



crust was slowly sinking and thus continually renewing the conditions 

 of sedimentation. Why did it subside? At the end of the Cretaceous 

 the same area began to rise. What change of conditions caused it now 

 to rise? It has continued to rise until the present time, and is still 

 rising. The whole amount of rise can not be less than 20,000 feet; for 

 if all the strata which have been removed by erosion were again 

 restored, the highest portion of the arch which was sea bottom at the 

 end of the Cretaceous would now be 20,000 feet high. This, however, 

 is only the last oscillation of this area, for beneath the Carboniferous 

 there are several unconformities showing several oscillations of the 

 same kind in earlier periods. During the Devonian the area was laud, 

 for the Carboniferous rests uncomformably on the Silurian. During the 

 Silurian it was sea bottom, receiving sediments of that time. Beneath 

 the Silurian there are two other unconformities showing similar oscil- 

 lations. These earlier oscillations were probably as great as the one 

 now going on, but we can not measure them as we can the last. 



Another striking example, still more recent and widespread, is the 

 enormous oscillations of the Glacial period. It can not be doubted that 

 over very wide areas — several millions of square miles — there were at 

 that time upward and downward movements of several thousand feet, 

 and therefore producing enormous changes in physical geography and 

 climate. What was the cause of these movements? They were doubt- 

 less modified, as will be shown later, by other movements superimposed 

 on them; but the causes of the latter must not be confounded with that 

 of the former. 



We have given only two striking examples, but they are really the 

 commonest of all crust movements. They are everywhere marked by 

 unconformities of the strata; they are everywhere going on at the pres- 

 ent time. In some places the sea is advancing on a subsiding land; in 

 others a rising land is advancing on the sea. These movements are 

 more conspicuous along coastlines, because the sea is a datum level by 

 which to measure them, but they affect equally the interior of conti- 

 nents, as shown by the behavior of the rivers, which seek their base 

 level by erosion in a rising and by sedimentation in a sinking country. 



Many theories have been advanced to explain these movements, 

 especially of certain very local shore-line movements. In volcanic 

 regions they have been attributed to rise or recession of the volcanic 

 heat and consequent columnar expansion or contraction of the crust. 

 On nonvolcanic sedimentary shore lines elevation has been attributed 

 by some to the rise of the interior heat of the earth and consequent 

 expansion of the crust produced by the blanketing effect of sedimen- 

 tary deposit; while others, with more reason, think that regions of 

 heavy sedimentation sink under the increasing load of accumulating 

 sediments; but it is evident that, while such theories may explain some 

 local examples in volcanic regions and along some shore lines, they can 

 not explain subsidences in the interior of continents, much less the 



