462 J. I). Dana — Subdivisions in Archaan History. 



The Huronian period covered, probably, much of Archeo- 

 zoic time ; and this is all in the way of correlation that can be 

 said. It is well to note here that if the Eozoon is really 

 animal in origin, the " Laurentian " rocks of Canada in which 

 it occurs must be Huronian, or the later of Archaean terranes. 



Respecting the Oceanic period it is observed above, " com- 

 mencing ivith the ocean in its place ." It a2)pears to be almost 

 a physical necessity that the oceanic depression should have 

 been made in the first forming of the solid crust, if the globe 

 cooled to the surface from the center outward ; that is, unless 

 a liquid layer remained long afterward beneath the crust. 



The depression was certainly made long before the close of 

 Archaean time. For the enormous amount of rock-making of the 

 Archaean over the continent implies the existence of emerged 

 rocks within reach of the decomposing, eroding and denuding 

 agencies of the atmosphere and atmospheric and oceanic waters. 

 A submergence in the ocean of 50 feet is almost a complete 

 protection against mechanical and chemical wear. Moreover 

 North America has its Archaean lands not only in the great 

 nucleal mass, 2.000,000 square miles in area, but also in the- 

 series of Archaean ranges parallel to the outlines of the 

 nucleus, which extend eastward to the eastern limit of New- 

 foundland, and westward to the Pacific. And it has corre- 

 spondingly shallow-water Cambrian deposits lying between 

 these ranges from eastern Newfoundland and the coast-region 

 of New Brunswick and Massachusetts, westward across the 

 continent about most of the Archaean outcrops, to within 300 

 to 400 miles of the Pacific Ocean, as shown by Walcott. 



There is hence reason for the .conclusion that, at the close 

 of Archaean time, the continent of North America was present 

 not merely in outline, but also in general features, and at shal- 

 low depths where not emerged. 



This fact with reference to North America means much. 

 It means that by the end of Archaean time, the continents 

 generally were essentially in a like condition — outlined and at 

 shallow depths where not emerged ; that, therefore, the oceanic 

 depression was then large and deep enough to hold the ocean. 

 Further, this last fact indicates, if the mean level of the conti- 

 nents was coincident with the water's surface, that the oceanic 

 depression had already a depth of 12,000 feet, or that of the 

 present mean depth of the waters ; and that, the lowering, 

 through later time, of the bed 1500 feet on an average (or 

 2000 feet according to other estimates) would give the conti- 

 nents their present mean height. And it is a fact of deep 

 geogenic significance, that nearly 1000 feet of this mean' 

 height was received after the beginning of the Tertiary. 



[To be continued.] 



