UNDER LAPLACIAN HYPOTHESIS 309 



Until recently, the great granitoid areas of the Archean system 

 (tin- oldest known rocks) were thought to possess these obvious 

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

 of them were intruded into rocks which had previously been formed 

 on an older stirfaceby (i) 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 further investigation shall finally 

 exclude all accessible rocks from an original crust, the molten theory 

 will have lost its observational support. 



Relative to the primitive atmosphere. Under the Laplacian 

 hypothesis, the primitive atmosphere has been held to have been 

 vast, hot, and heavy, and to have contained (i) 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 oxida- 

 tion, as well as (4) that portion of all these constituents which is 

 now found in the atmosphere and in organic tissues. The assump- 

 tion back of this seems to be that heat always promotes the expulsion 

 of gases from rock; if so, the exclusion of the gases from the rock should 

 have been most complete in the white-hot primitive globe. The 

 conception that the rocks after cooling re-absorb the atmospheric 

 gases is expressed in the view, once prevalent, that the former 

 atmosphere and hydrosphere of the moon have been absorbed into 

 it, and in the familiar prophecies of a similar doom for the atmo- 

 sphere and hydrosphere of the earth. 



Adverse evidence. So great an atmosphere with so much carbon 

 dioxide and water-vapor should have given the earth a warm and 

 equable climate. Such climates indeed seem to have prevailed at 

 certain times during the earlier parts of the earth's history, as 

 during the later; but the studies of the past two decades have shown 

 that there was extensive glaciation on the very borders of the tropics, 

 as early as the close of the Paleozoic, and that there was glaciation in 

 northwestern Europe, in China in Lat. 31, in Australia, and perhaps 

 in South Africa, at the very beginning of the Paleozoic. It is even 

 claimed that there was glaciation in the early part of the Proterozoic 

 long before the Paleozoic, and this claim seems likely to be made 

 good. There seem to have been, even in very early times, much the 

 same alternations of uniform with diversified climates that have 

 marked later eras. The air-breathing animals of early ages, and the 

 devices that protected the leaves of plants against too intense sun- 



