Invasion to Regional Met amor phism. 185 



sodium to be retained in the sediments in insoluble form, 

 as shown by their average composition, the salt of the sea 

 requires the erosion of a shell of igneous rock 2,200 feet 

 thick enveloping the globe. If the erosion be regarded as 

 restricted to the continents, and their area be taken as 28 

 per cent of the area of the earth, the equation would re- 

 quire the erosion of 7,900 feet of rock. The actual aver- 

 age erosion from the present land areas has probably 

 been between these two figures, since ancient lands such 

 as Appalachia, which have supplied much sediment, are 

 now in part submerged. 



The erosion, however, has been very unequally distrib- 

 uted. No igneous rock has been removed from the broad 

 interior of the United States since pre-Cambrian times, 

 and the same is true of other wide areas. Therefore 

 there is room for the removal of many miles of igneous 

 rock from mountain axes, not counting the erosion and 

 re-erosion of the sedimentary formations. But allow for 

 the demands of Paleozoic, Mesozoic, and Cenozoic erosion 

 of igneous rocks, and the balance left to be assigned to 

 pre-Cambrian erosion is materially reduced. Under the 

 most favorable assumptions it can hardly be much over a 

 mile in thickness, and if allowances are made for later 

 continental fragmentation, it may not be much over half 

 a mile. 



Now the metamorphism of the basement complex is 

 universal, and the batholithic exposures cover enormous 

 areas. From such regions the older lavas have been re- 

 moved, and erosion has bitten deeply into the underlying 

 granites. But how deeply? For any particular region 

 it may be five miles, ten miles, or even more, but consider- 

 ing the limitations placed by the salt in the sea, it would 

 seem conclusive that the average erosion which has ex- 

 posed the Archean zone of anamorphism and rock flow 

 has not been more than a mile. Even the metasediments 

 of the ancient cover are counted into this estimate, for 

 although their removal would not add a fresh supply of 

 salt, their original formation had given it, and by just so 

 much had diminished the measure of. later erosion. It 

 would appear fairly certain that the profound metamor- 

 phism of the Archean complex does not imply the neces- 

 sity of formation beneath from fiYQ to ten miles of cover. 



This line of evidence, although indirect, is perhaps one 

 of the strongest of many which go to show that the con- 

 ditions of regional metamorphism are bound up with 



