CONVERGENCE OP EVIDENCE 895 



iiess. The development of such peneplains implies that the crust was 

 stable and the earth rigid through long periods of time. Cooling, follow- 

 ing each period of magmatic invasion, must have gone on to a very con- 

 siderable depth, giving a temperature gradient perhaps lower than the 

 present, and the deeper body of the earth may then, as now, have had 

 throughout its present rigidity. These features alone show that Precam- 

 brian time was very long. The degree of evolution and differentiation of 

 all the invertebrate phyla by the opening of the Paleozoic suggests also 

 that the preceding time was as long as, or longer than, all subsequent time. 

 The evidence from the uranium minerals comes now to support that view. 



We are confronted, then, by several controlling conditions as to the 

 nature of the Precambrian ; the enormous duration of those ages, at least 

 700,000,000 years, perhaps 1,000,000,000 years; the wide-spread and re- 

 current igneous activity and regional metamorphism ; the average depth 

 for the erosion of igneous rock needed to reveal the Basement Complex 

 over the continents not over a mile, and the existence of long periods of 

 quiet and peneplanation. The long periods of quiet were comparable to 

 the Cambro-Ordovician save that the continents in the earlier time tended 

 to stand just above, rather than just below, sealevel. Instead of sheet 

 after sheet of sediments l)eing thinly spread, successive thin layers of rock 

 were planed from the land — now here, now there — in addition to the re- 

 moval of more limited tracts of higher plateaus and mountains. 



The difference in geologic history between the Precambrian and the 

 following eras was, consequently, largely due to the change in the meaii 

 sealevel; a slow change through a wider sweep of time than even those 

 age-long tides which divide earth history into periods ; a sweep so wide 

 that all of geologic time has brought to pass only one completed rhythm, 

 the continents now emerged from the Paleozoic seas suffering dominant 

 erosion as in the remote Precambrian. But in other aspects the change 

 is not rhythmic, but progressive ; for the present conditions are, in many 

 respects, not comparable : the ocean water has progressively increased in 

 volume, but the continents have broken down into ocean basins in larger 

 ratio. These causes are progressive and do not represent a return to pri- 

 mordial conditions, but the effects on the level of the sea have exhibited 

 such a returning phase. 



During the periods of plutonic overturn of the crust through the rise 

 of magmas the rate of erosion for the uplifted areas may have exceeded 

 the present rates for mountain regions, but during the long periods of 

 quiet and peneplanation erosion must have sunk to a negligible amount. 

 Slight oscillations downward would liave stopped it altogether and have 



LXVI — Bull. Geol, Soc. Am., Vol. 28, 1916 



