(S94 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



(leiice to be discussed indicate that these revolutions were temporary con- 

 ditions separated b}^ long periods of quiet which have left almost no record. 



The first line of evidence bearing on this conclusion is derived from 

 the quantity of salt in the sea. It has been seen that the sodium would 

 be derived from the weathering and erosion of a shell of average igneous 

 rock about 2,300 feet thick, if enveloping the whole earth, or about 6,800 

 feet thick if derived from an area equal to the present continental plat- 

 forms. It does not appear possible to materially modify these estimates. 

 The greater part of present erosion consists in the reworking of sedi- 

 mentary material, but since the opening of the Paleozoic it would appear 

 from the large volume of the strata deposited that at least a third of this 

 mile and a half shell of igneous rock must have yielded up its sodium. 

 The implication is that Precambrian erosion was not by any means so 

 profound, on the average, as the broad exposure at the surface of rocks 

 originating in the zone of flow would suggest. It would seem that at 

 times of great batholithic invasions regional metamorphism must have 

 gone forward freely under a load of a mile or two of cover, the recrystal- 

 lization being due to the great quantities of gas at high temperatures 

 penetrating upward from underlying magmas. It has been found, in fact, 

 for a number of far later batholithic invasions that they have approached 

 so near to the surface as to be covered only by a slightly older mantle of 

 tuffs and lavas of their own genesis ; yet their texture is granitic and the 

 cover rocks show extensive contact metamorphism. 



If Precambrian erosion took place over an area greater than the present 

 continental platforms, the mean depth of erosion of the igneous rock over 

 this larger area was less than 6,800 feet. If the early erosive processes 

 were less effective in weathering than in later times, then it would take a 

 slightly thicker mantle to give rise to the oceanic salt. This factor tends 

 to offset the one due to larger area of Precambrian lands, but is probably 

 of much less importance. We may conclude, therefore, that the mean 

 depth of erosion of average igneous rocks through all geological time has 

 been less rather than over 6,800 feet. Let us grant the maximum avail- 

 al)le for Precambrian time, as this is leaning backward from the direction 

 of the argument. Allow only one-fifth for later geological time, and the 

 mean depth for the Precambrian erosion of igneous rock is still not more 

 than a mile as a maximum figure. 



The second line of evidence bearing on the problem is found in the 

 peneplanation which occurred repeatedly, so that each Precambrian sys- 

 tem was laid down on an almost level floor of older crystalline rocks. A^ 

 compared to the scale of the initial relief following each revolution, the 

 peneplain, even if hummocky, represents erosion to practical complete- 



