THE RHYTHM OF MOTION. 269 



similarly have their alternating periods of greater and less 

 intensity. 



There is evidence also that modifications in the Earth's 

 crust due to igneous action have a certain periodicity. Vol- 

 canic eruptions are not continuous but intermittent, and as 

 far as the data enable us to judge, have a certain average 

 rate of recurrence; which rate of recurrence is complicated 

 by rising into epochs of greater activity and falling into 

 epochs of comparative quiescence. So too is it with earth- 

 quakes and the elevations or depressions caused by them. 

 At the mouth of the Mississippi, the alternation of strata 

 gives decisive proof of successive sinkings of the surface, 

 that have taken place at tolerably equal intervals. Every- 

 where, in the extensive groups of conformable strata that 

 imply small subsidences recurring with a certain average 

 frequency, we see a rhythm in the action and reaction be- 

 tween the Earth's crust and its molten contents a rhythm 

 compounded with those slower ones shown in the termina- 

 tion of groups of strata, and the commencement of other 

 groups not conformable to them. There is even 



reason for suspecting a geological periodicity that is im- 

 mensely slower and far wider in its effects; namely, an al- 

 ternation of those vast upheavals and submergencies by 

 which contents are produced where there were oceans, and 

 oceans where there were continents. For supposing, as we 

 may fairly do, that the Earth's crust is throughout of toler- 

 ably equal thickness, it is manifest that such portions of it as 

 become most depressed below the average level, must have 

 their inner surfaces most exposed to the currents of molten 

 matter circulating within, and will therefore undergo a 

 larger amount of what may be called igneous denudation; 

 while, conversely, the withdrawal of the inner surfaces from 

 these currents where the Earth's crust is most elevated, will 

 cause a thickening more or less compensating the aqueous 

 denudation going on externally. Hence those depressed 

 areas over which the deepest oceans lie, being gradually 



