Febeuaey 26, 1897.] 



SCIENCE. 



327 



stratified series. Every line of uncon- 

 formity marks an old eroded land surface, 

 and every conformable series of strata a 

 sea bottom receiving sediments. We give 

 but two striking examples of such oscilla- 

 tions. 



The Colorado plateau was a sea bottom, 

 continuously or nearly so, from the begin- 

 ning of the Carboniferous to the end of the 

 Cretaceous, and during that time received 

 about 12,000 or 15,000 feet in thickness of 

 sediments. During the whole of this time 

 the area of the earth's crust was slowly sink- 

 ing and thus continually renewing the con- 

 ditions of sedimentation. Why did it sub- 

 side? 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 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 uncon- 

 formities 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 can- 

 not measure them as we can the last. 



Another striking example, still more re- 

 cent and widespread, is the enormous oscil- 

 lations of the Glacial period. It cannot be 

 doubted that over very wide areas— several 

 millions of square miles — there were at 

 that time upward and downward move- 

 ments of several thousand feet, and there- 



fore producing enormous changes in physical 

 geography and climate. What was the 

 cause of these movements? They were 

 doubtless 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 onljr two striking ex- 

 amples, but they are really the commonest 

 of all crust movements. They are every- 

 where 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 conspicuous 

 along coastlines, because the sea is a datum 

 level by which to measure them, but they 

 affect equally the interior of continents, 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 cer- 

 tain very local shoreline movements. In 

 volcanic regions they have been attributed 

 to rise or recession of the volcanic heat and 

 consequent columnar expansion or contrac- 

 tion of the crust. On non-volcanic sedi- 

 mentary shorelines elevation has been at- 

 tributed by some to the rise of the interior 

 heat of the earth and conseqent expansion 

 of the crust produced by the blanketing 

 effect of sedimentary deposit; while others, 

 with more reason, think that regions of 

 heavy sedimentation sink under the in- 

 creasing load of accumulating sediments ; 

 but it is evident that, while such theories 

 may explain some local examples in vol- 

 canic regions and along some shorelines, 

 they cannot explain subsidences in the in- 

 terior of continents, much less the wider 

 and more extensive movements spoken of 

 above. We must look for some more gen- 

 eral cause. What is it ? 



It must be confessed that the cause of 



