CONVERGENCE OF EVIDENCE 893 



and the rate of accumulation even in the geosyncline tends to be reduced, 

 though the rate in the geosynclines depends more largely on the rate of 

 subsidence. 



Fifth. The relation of high sealevel to the vertical forces tending to 

 maintain isostasy. When a continent is leveled with respect to a higher 

 sealevel, the erosion of the smaller areas involves less isostatic strain 

 toward rejuvenating uplift than if the whole continent were worn down 

 to a lower level. Longer times of quiet may elapse and the movements 

 toward equilibrium need not be so wide-spread nor violent as they must 

 be for a greater depth between the isostatic level and the continental 

 baselevel. 



Sixth. The stages of lost record due to breaks of various magnitudes 

 during sedimentation have already been mentioned. In addition to this, 

 for the more distant times the record tends to become more imperfect, 

 owing to the later events of geologic history. Some parts are uplifted and 

 destroyed; others are mantled over with later sediments. 



Seventh. It is possible that beside these factors which make for a low 

 apparent rate in the Paleozoic, a real change may have taken place in the 

 nature of the earth's internal forces. The later revolutions have been 

 less profound than the great convulsions of the Archean, but diastrophism 

 may make up for this by becoming more recurrent, tending to stimulate 

 in post-Paleozoic eras the mean rate of erosion and sedimentation. 



The stratigraphic record of the Precambrian is so fragmentary and 

 imperfect that the preceding method of calculation is quite inapplicable. 

 To gain some idea as to the mean rates of erosion and sedimentation 

 which prevailed, recourse may be had to another line of argument. The 

 Precambrian portion of geologic history shows regional metamorphism 

 acting more than once on a world-wide scale. Molten rock welled up 

 from the depths; part of it poured over the surface; other and greater 

 parts did not reach the surface, but as abyssal magmas melted in, broke 

 up, and engulfed the overlying crust. By aid of the escaping gases the 

 cover rocks were crystallized. Compressive forces contorted and mashed 

 them and led to recrystallizations, transforming both older and younger 

 rocks into an intricate structure — the basement complex. Erosion planed 

 sufficiently deep to expose these structures of the zone of rock flowage as 

 the dominant type of Precambrian structures. In later ages such internal 

 activities have been restricted to the belts of mountain systems. This 

 contrast would seem to suggest that the Precambrian was characterized 

 as a whole by more rapid, wide-spread, profound, and violent geologic 

 activities, associated with the youth of the earth; but other lines of evi- 



