THE PROTEROZOIC ERA 467 



sedimentary rocks of the Proterozoic systems. The accumulation 

 of so much sediment would in itself mean a vast lapse of time, and 

 when it is remembered that the four systems are separated from 

 one another by unconformities, each of which may represent as much 

 time as that involved in the accumulation of a system, it will be 

 seen that the duration of the Proterozoic era was exceedingly long, 

 possibly comparable to all succeeding time. The duration of the 

 era cannot be stated in numerical terms, but it would appear that 

 it should be spoken of in terms of tens of millions of years, rather 

 than in terms of a lesser denomination. 



Destruction of rock implied. Thick beds of sediment mean the 

 destruction of a still larger volume of older rock, for much of the 

 more soluble part of the rock destroyed does not appear in the 

 sedimentary formations. It is important to note that a large part 

 of the Proterozoic sediments were produced by the mature decom- 

 position of older rocks. It is scarcely too much to infer that the 

 materials of the larger part of the great formations of quartzite, 

 slate, and shale first became soils on the surface of the parent 

 areas, and that their removal was at a rate comparable to that of 

 their renewal by rock decay. This point is pf significance because 

 it implies the existence of climatic conditions and geologic pro- 

 cesses comparable to those of the present time. 



If the Archean lands in the vicinity of Lake Superior were high 

 enough at any time to furnish the thick sediments of the Proterozoic, 

 their height would perhaps have surpassed any existing elevation, 

 but is is not probable that such elevations existed at any time. It 

 is more probable that as erosion proceeded, the land reacted by 

 rising slowly, or that the sea bottom sank, drawing off the waters 

 and leaving the land relatively higher. In this way, degradation 

 and elevation may have been in progress at the same time, and the 

 one process may never have got far ahead of the other. It is be- 

 lieved by some geologists that the removal of sediments in large 

 quantities from the land would result in its rise, and that their 

 deposition on the sea bottom would cause that to sink. The 

 doctrine that the surface of the lithosphere sinks and rises under 

 increase and decrease of load is one phase of the general theory 

 of isostasy. 



