ARCH^AN TIME. 441 



the emerged and emerging lands ; the large excess of carbonic 

 acid and oxygen in the air and water a source of rock-destruction ; 

 before the close of the era, the formation of limestones and iron- 

 carbonate by chemical methods, removing carbonic acid from the 

 air and so commencing its purification ; the accumulation of sedi- 

 ments without immediate crystallization or metamorphism, and 

 thereby the beginning of the earth's supercrust. 



III. The Archaeozoic geon. Life in its lowest forms in existence. 



1. The Era of the First Plants : Algse, and later of aquatic Fungi 



(Bacteria), commencing with the mean temperature of the ocean 

 at possibly 150° F., since plants now live in waters up to and even 

 above 180° F. Limestones formed from vegetable secretions, and 

 silica deposits from silica secretions ; iron-carbonate, and perhaps 

 iron oxides formed through the aid of the carbonic acid of the 

 atmosphere and water ; large sedimentary accumulations, where 

 conditions favored, thickening the supercrust. 



2. The Era of the First Animal Life : mean temperature at the 



beginning probably about 115° F., and at the end 90° F., or lower ; 

 limestones and silica deposits formed from animal secretions ; 

 deposits of iron-carbonate and iron-oxides continued ; large sedi- 

 mentary accumulations. 



The sedimentary or stratified beds of Archaean time are the oldest and 

 most obscured parts of the geological record. Sooner or later in the Arch- 

 seozoic era "dynamical metamorphism" began, or metamorphism dependent 

 on heat from a dynamical source, that is, heat generated by movements in 

 the thickening crust, aiding the heat still in the earth's mass, or statical 

 heat. Thereby, during a crisis of upturning, the thick accumulations of 

 sediment became metamorphic or crystalline ; but the statical heat was still 

 so great that the temperature was easily made that of fusion, and conse- 

 quently the fusing of fusible sedimentary beds took place and outflows 

 through openings or fissures of granite, syenyte, dioryte, gabbro, and other 

 like rocks, derived severally from granitic, syenytic, diorytic, and gabbronitic 

 or related sediments ; but deep-seated igneous effusions may not have been 

 common, for strains in a thin, rather hot supercrust might extend little 

 below it, and, moreover, igneous ejections from a deep-seated source and 

 through volcanoes reached their maximum in the later part of geological 

 time. 



Although these eras are not marked off in the rocks, there are facts 

 enough to prove that they represent, in a general way, tlie system of 

 progress in Archaean time. Millions of years must have elapsed during 

 the cooling from over 2500° F. to 500° F. ; a very long era during that 

 from 500° F. to 150° F. ; and another long era during that from 150° F. to 

 115° F. ; and still another during that from 115° F. to 90° F. Archaean time 

 was long, immensely long. 



