ARCHAEAN TIME. 147 



tion ; and then, in the second place, and probably long afterward, a 

 crust was formed outside from cooling ; and until finally, in the third 

 place, the vapors of the atmosphere were mostly condensed, and an 

 envelope of waters, nearly or quite universal, was thus made. De- 

 pressions for special oceanic basins would have been early begun, over 

 the cooling and contracting sphere ; and it is probable, as elsewhere 

 shown (pp. 160, 728), that the existing continental areas were defined 

 in general contour in this first-formed crust, and that within their con- 

 fines appeared the first dry land. This crust has since, through all 

 time, continued cooling and increasing in thickness. 



III. A third era, or a continuation of the preceding, carrying for- 

 ward the cooling to 80° or 100° C. (175° to 212° F.), or to a tempera- 

 ture admitting of the existence of the simplest forms of vegetable life. 

 Through this era, the crust, by its contraction from cooling, which 

 was in unceasing progress, must have been slowly varying and aug- 

 menting its surface reliefs. 



At the same time, the wear of the rocks of the crust, wherever they 

 were exposed to the ocean's waves or currents, aided by their disinte- 

 gration where above the waters, would have resulted in the formation 

 of stratified deposits out of the detritus ; and so have begun the series 

 of formations over the surface that makes up the earth's supercrust 

 — the only part of the earth's structure which is within the reach of 

 direct investigation. 



At first, the beds of detritus formed in the hot waters (a powerful 

 chemical agent through their heat, and the silica and other materials in 

 solution) would have been consolidating and crystallizing beneath, 

 while accumulation was going on above ; and this may have continued 

 to be true throughout the age, and in fact long after the waters had 

 passed the temperature-limit of 100° C. The rocks of this era should 

 therefore be much like those that resulted from the original cooling, 

 because made chiefly out of the latter by reconsolidation and recrystal- 

 lization, except that schistose and quartzose rocks would have been 

 more common in the new formations. 



These Archaean rocks are the only universal formation. They ex- 

 tend over the whole globe, and were the floor of the ocean and the 

 material of all emerged land, when life first began to exist. The 

 thickness which they acquired during the long era from the time of the 

 first-formed crust can never be known. 



Professor Helmholtz has calculated, from the rate of cooling of lavas, 

 that the earth, in passing from 2,000° to '200° C, must have taken three 

 hundred and fifty millions of years. But the temperature when the 

 Archaean ended was probably not over 38° C. (100° F,), to reach 

 which many more scores of millions of years must have been passed. 

 The era was Ions- 



