428 GEOLOGY 



specific gravity. The solid crust should have been similarly light 

 and homogeneous, and it should have formed a universal stratum 

 susceptible of identification. Except at the very surface, it should 

 have been completely crystallized, for the cooling must have been 

 very slow, a condition favorable for the growth of crystals. No 

 very large amount of fragmental volcanic material can be assumed 

 to have covered the original crust if we entertain the view that the 

 primitive atmosphere contained all the water of the future hydro- 

 sphere, for that leaves no adequate explosive agency in the molten 

 globe to produce abundant volcanic fragments. Such material 

 cannot well be supposed to have concealed the original crust per- 

 manently, for many thousands of feet of rock have been eroded 

 from the surface of the oldest known areas. It is equally improb- 

 able that the original crust has been concealed everywhere beneath 

 sedimeats derived from itself. There should therefore be areas 

 of the original crust exposed at the surface and they should 

 presumably be large. 



Until recently, the great granitoid areas of the Archean system 

 (the oldest known rocks) were thought to answer these obvious 

 characteristics of an original crust; but it has been found that most 

 of these great granitoid masses are intrusive in rocks which had 

 previously been formed on an older surface by (1) lava outflows, 

 (2) volcanic explosions, and (3) sedimentation. This reduces to 

 an unknown, and apparently to a vanishing quantity, the rocks 

 that can be referred plausibly to a supposed original crust. If the 

 trend of further investigation shall follow the present tendency 

 and finally exclude all the accessible rocks from an original crust, 

 the molten theory will have lost its observational support, at least 

 in its original form. 



2. Relative to the Primitive Atmosphere 



The primitive atmosphere has usually been held heretofore to 

 have been vast, hot, and heavy, and to have contained (1) all the 

 water of the globe, (2) all the carbon dioxide now in carbonated 

 rocks, (3) that portion of the oxygen which has been added to the 

 rocks by oxidation, as well as (4) that portion of all these constit- 

 uents which is now found in the atmosphere and in organic tissues. 



