THE MINERAL MATTER OF THE SEA 481 



present rate of river erosion is probably well above the average for 

 the earth's history, since the lands are now relatively high. 



5. There is still another way of approaching this problem. The 

 sedimentary rocks of the continental areas are estimated to have a 

 thickness, on the average, of something like a mile. No estimate 

 of their average thickness in the sea is possible, but it is probably 

 much less. If it be assumed to be one-fourth- as great, the volume 

 of sediment for the whole earth would be about 4 times that of the 

 rock in all the lands of the earth. These figures, it will be seen, are 

 near the least of those derived by the other modes of calculation 

 (see p. 479). 



Summary. — It appears, then, that we are to think of the decay 

 and removal to the sea of an amount of rock equal to at least several 

 times all that is now above the sea, during the course of the earth's 

 history. The results of the several lines of calculation place the 

 amount at 3 to 21 times all that is now above sea-level. The truth 

 may lie between these extremes. So uncertain is the nature of the 

 data, however, that we must recognize that the truth may lie outside 

 of either, so far as present knowledge goes. 



Renewal 0} the continents. — It is, of course, not to be inferred that 

 the continents were ever large enough to include all the material 

 which has been worn away from them, in addition to that which they 

 now contain. They may never have been much larger than now, 

 and they have certainly often been smaller. As their masses were 

 reduced by erosion, they were renewed, either (1) by the sinking of 

 the sea bottom, which drew the water off the areas which had been 

 covered by shallow water only, or (2) by the rise of the continental 

 areas. The former was probably the more common. 



The renewal of the lands has not always kept pace with their 

 reduction, so that the area of the lands and the amount of reck which 

 they have contained, have fluctuated notably from time to time. 



Effect of preceding changes on areas 0} sea and land. — Though 

 great weight is not to be attached to the figures worked out on the 

 basis of the assumptions made, it is believed not only that they are 

 suggestive of the amounts of rock which have been worked over in 

 the earth's history, but that they suggest lines of quantitative study 

 which are worthy of attention. If, for example, a given amount of 



