PRIMEVAL ROCKS. 171 



as to lead to the belief that they pass over into one another Is certain. It 

 is also maintained by many geologists (erroneously, as I believe) that cases 

 occur in which a series of transitions exists from granite to glassy lavas. If 

 both these propositions were correct, it would follow that a transformation 

 of sediments into lavas would be possible under certain conditions, but it 

 would not follow that this is the usual history of lavas or even that it is 

 the history of a single lava. Neither does it follow that because some 

 granites are metamorphosed sediments all granites are of this class. 



Possible character of primeval rocks. TllC oldcst SedimCUtary rOclcS COmpOSe tllO 



Archaean wholly or in part. These rocks are also much more uniform in 

 composition than later stratified rocks. They must have been derived in 

 great part from the primeval rocks, which therefore possessed the same 

 mean composition as the schists. This composition is substantially iden- 

 tical with that of granite. Hence, a rock chemically similar to granite 

 formed the primeval surface. This rock must also have formed at high 

 temperatures, very slowlv, and under great pressure. It must inevitably 

 have been chiefly crystalline, and all analogy and experiment lead to the 

 belief that it can have contained no glass. It must have been a holocrys- 

 talline porpliyry or a granular rock. The atmospliere previous to the 

 solidification of the surface of the globe must have contained at least as 

 much water as the ocean now holds, as well as most of the carbon now present 

 in limestones, coal beds, etc. The pressure of this atmosphere must have 

 been at least three or four thousand pounds per square inch and the boiling 

 point of water nmst have been correspondingly high. When, or soon after, 

 the temperature at the surface sank to the critical point of water (580'^ 

 C, \Mendek^"eflP), and tiierefore while the surface was still red-hot, water 

 must have condensed upon it. Judging from what is known experiment- 

 ally of igneo-aqueous fusion, conditions more favorable to this process could 

 not be imagined. Now there is much reason to suppose that granite has 

 been produced by igneo-aqueous fusion. It is therefore in the highest 

 degree probable that the terrestrial surface when the earth first ceased 

 to glow was granite, very probably accompanied to some extent by allied 

 plagio clastic rocks. It is far fi-om impossible tliat portions of it may have 

 had a g-neissoid structure. 



