GEOGRAPHICAL EVOLUTION 355 



ited near the land. For it must be borne in mind that 

 every mass of land as soon as it appeared above water was 

 at once attacked by the ceaseless erosion of moving water 

 and atmospheric influences, and immediately began to fur- 

 nish materials for the construction of future lands to be 

 afterwards raised out of the sea. 



Each great period of contraction elevated anew the much- 

 worn land, and at the same time brought the consolidated 

 marine sediments above water as parts of a new terrestrial 

 surface. Again a long interval would ensue, marked per- 

 haps by a slow subsidence both of the land and sea-bottom. 

 Meanwhile the surface of the land was channelled and 

 lowered, and its detritus was spread over the sea-floor, until 

 another era of disturbance raised it once more with a por- 

 tion of the surrounding ocean-bed. These successive up- 

 ward and downward movements explain why the sedi- 

 mentary formations do not occur as a continuous series, but 

 often lie each upon the upturned and worn edges of its 

 predecessors. 



Returning now to the chronological sequence indicated 

 by the organic remains preserved among the sedimentary 

 rocks, we see how it may be possible to determine the 

 relative order of the successive upheavals of a continent. 

 If, for example, a group of rocks, which as before may be 

 called A, were found to have been upturned and covered 

 over by undisturbed beds C, the disturbance could be 

 affirmed to have occurred at some part of the epoch repre- 

 sented elsewhere by the missing series B. If, again, the 

 group C were observed to have been subsequently tilted, and 

 to pass under gently-inclined or horizontal strata E, a 

 second period of disturbance would be proved to have 

 occurred between the time of C and E. 



I have referred to the unceasing destruction of its sur- 

 face which the land undergoes from the time when it 

 emerges out of the sea. As a rule, our conceptions of the 

 rate of this degradation are exceedingly vague. Yet they 

 may easily be made more definite by a consideration of 

 present changes on the surface of the land. Every river 

 carries yearly to the sea an immense amount of sand and 

 mud. But this amount is capable of measurement. It 



