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HISTORICAL GEOLOGY 



be of Archaeozoic age. Graphite, which may be metamorphic organic 

 matter, indicates the presence of plants. Graphite, however, may be 

 of inorganic origin, derived perhaps from petroleum. No remains 

 that can be positively identified as fossils have been found in the 

 Archaeozoic rocks. 



It has been suggested (Daly) that the Pre-Cambrian limestones were entirely prod- 

 ucts of chemical precipitation. This is based on the assumption that the land areas 

 were at first relatively small, and that the abundance of decaying, soft-bodied organ- 

 isms on the sea floor produced ammonium carbonate, which led to a continuous precipi- 

 tation of such lime as was available. Hence the ocean was limeless, and it was not 

 until the lands became more extended that a sufficient quantity of lime salts was 

 brought in by rivers to counterbalance that thrown down by the ammonium carbon- 

 ate and sodium carbonate on the sea floor. If this be true, the earlier organisms 

 could not form calcareous shells or skeletons. The fact that Pre-Cambrian and 

 Cambrian limestones, even when unaltered, show no signs of having originated from 

 shell remains is offered in proof. 



Duration. — An immense but unknown duration is assigned to the 

 Archaeozoic era, an era so vast that even if it were possible to state 

 the duration in terms of years the number would be so large as to 

 convey little meaning to the human mind. If millions of years were 

 consumed by the later eras, tens of millions must be ascribed to this 

 era. In fact, it is possible that the Archaeozoic may have been longer 

 than all the subsequent eras taken together. 



Bearing upon the Theories of the Earth's Origin. — (i) According to the theory that 

 the earth was originally a molten globe (nebular hypothesis) which upon cooling first 

 formed a crust, we should expect to find the earliest sedimentary rocks underlain by 

 an igneous or metamorphic-igneous floor, provided that igneous activity was slight 

 after the crust became cool enough to permit the operation of the agencies of erosion 

 and the weather. It seems more probable, however, that in these early stages igneous 

 activity would be unusually prevalent, with the result that lava flows, volcanic prod- 

 ucts of enormous thickness, as well as great intrusions might completely hide the 

 original crust, if indeed it were not remelted. 



(2) According to the planetesimal theory, the matter gathered in from space became 

 bo hot at the (a) center that it recrystallized to form an essentially igneous core, (b) A 

 thick zone outside of the central core, made up largely of planetesimal matter, partly 

 of igneous rock erupted from below, theoretically underlies the (c) next, and relatively 

 thin zone which is composed largely of extrusive igneous rock, with smaller amounts 

 of sediments and of planetesimal matter gathered from space. This is the zone which, 

 according to the planetesimal theory, appears at the surface and is known as the 

 An haeozoic. 



It will be Been from the above that according to either the planetesimal or the 

 modified nebular hypothesis the "crust" of the earth would have practically the 

 lame characters and that, therefore, even though fundamentally different, no means 

 is afforded of testing the two theories. 



