THE PROTEROZOIC ERA 199 



seen that the duration of the Proterozoic era was exceedingly great. If 

 length of time may be estimated from thicknesses of sedimentary beds, 

 the duration of the era is comparable to all succeeding time. 1 It is at 

 present impossible to state the length of geological periods in numer- 

 ical terms, but it would appear that the duration of the Proterozoic 

 era should be spoken of in terms of millions or tens of millions of years, 

 rather than in terms of a lesser denomination. 



The sources of the sediments. — It should also be noted that these 

 extraordinarily thick beds of sediment mean the destruction of a still 

 larger amount of some older rock. The volume of rock destroyed 

 must have exceeded the fragmental product deposited by the amount 

 of mineral matter which remained in solution in the sea at the close 

 of the period, together with such amount as had been carried down by 

 ground water to levels below the surface, and there deposited. 



It is important to note further that a large portion of the sedi- 

 ments was produced by mature decomposition of older rocks, and 

 this implies that they were not derived by rapid mechanical abrasion 

 such as that which accompanies and follows great elevation and exces- 

 sive precipitation. The great series of quartzites were derived from 

 the complete decomposition of quartz-bearing rocks, and involved 

 the almost complete separation of the quartz grains from other con- 

 stituents, while the thick beds of shale arose from the complementary 

 clayey products of decomposition, from which most of the basic oxides 

 had been removed by carbonation. It is scarcely too much to say 

 that the materials of the larger part of these great series first became 

 soils on the surface of the parent areas, and were only removed at 

 a rate that permitted the renewal of the soil beneath as fast as it was 

 washed away above. This mode of derivation is perhaps the most 

 important distinction between the Proterozoic series, as a whole, and 

 the Archean. 



The sediments involved in the Huronian formations were derived 

 from the Archean; those involved in the Animikean, from the Archean 

 and Huronian. It follows that the Animikean system had its ultimate 

 origin in the Archean, except in so far as it was derived from igneous 

 rock intruded into the Huronian during or after the formation of that 

 system. If the Archean lands in the vicinity of Lake Superior were 

 high enough at any one time to give origin to the thick sediments 



^Van Hise, Bull. 86, U. S. Geol. Surv., p. 491. 



