OSCILLATORY MOVEMENTS. 121 



of sediments. During the whole of this time the area of the earth's 

 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 cannot be less than 20,000 feet; for 

 if all the strata which has 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 sev- 

 eral unconformities showing several oscillations of the same kind in 

 earlier periods. During the Devonian the area was land, for the Car- 

 boniferous rests unconformably 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 oscillations. These 

 earlier oscillations were probably as great as the one now going on, but 

 we cannot measure them as we can the last. 



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

 enormous oscillations of the Glacial period. It cannot be doubted that 

 ove* 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 present 

 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 con- 

 spicuous along coastlines, because the sea is a datum-level by which to 

 measure them, but they affect equally the interior of continonts, 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, espe- 

 cially of certain very local shoreline 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 non- 

 volcanic sedimentary shorelines 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 sedimentary deposit, 



XVIII— Bull. Geol. Soc. Am., Vol. 8, 1896 



