HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 91 



IV. The primitive volcanic eon. — Prodigious volcanic action, closely 

 following the solidification of the crust, during which the forcing out 

 of lavas from below by the progressive crystallization of the remain- 

 ing magma, or by plutonic stresses, was the leading event, the result 

 being great beds of lava and tuffs on the surface, and later great intru- 

 sions of other lavas into and through these. The surface flows were 

 formed largely of the more fluent lavas, and hence were generally of 

 the basic or neutral class, and the pyroclastics were predominantly 

 of the same type, while the intrusions were more largely, though not 

 wholly, of the stiff acidic type. Later, under metamorphic action, 

 the basic lavas became schists in part, and the acidic gneisses, 

 neither class, however, excluding the other. Contemporaneously with 

 this volcanic action, but quite subordinate to it, atmospheric and 

 hydrospheric action is postulated, resulting in sedimentary deposits 

 interlaid with the volcanic, but in greatly inferior quantity. The 

 presence of life is implied by carbonaceous elements in the sedimen- 

 taries. This period corresponds to the Archeozoic eon, as limited 

 by those geologists who separate the Archeozoic from the Proterozoic 

 (Algonkian) system. 



V. The sedimentary eon. — The remaining time, reaching to the 

 present, is characterized by the dominance of atmospheric and hydro- 

 spheric action over volcanic, and is recorded by the ordinary 

 elastics, sandstones, mudstones, etc., derived from well-decomposed 

 rocks, and by limestone. It begins with the Proterozoic sedimentaries 

 and extends to the present time, embracing the Paleozoic, Mesozoic, 

 and Cenozoic systems. 



Under this scheme, the accessible record is made to begin with 

 some stage of the volcanic eon, but probably not the earliest. 



III. STAGES OF GROWTH UNDER THE ACCRETION (PLANET- 

 ESIMAL) HYPOTHESIS. 



It is theoretically possible that the earth may have grown up by 

 accessions in ways other than those sketched under the planetesimal 

 hypothesis, as set forth in this work, and so it is necessary to recog- 

 nize that accretionary evolution is not necessarily identical with planet- 

 esimal evolution, but the latter is taken as a type form and furnishes 

 the basis for the following sketch of evolutionary stages. 



