O'Eeilly — On Gaseous Products of Great Eruptions. 21 



having a crust relatively thinner and very differently constituted 

 from what now exists; and, since we suppose an ocean of some 

 depth and an erosive action, we are led to admit the formation of 

 strata under conditions, in some sort, corresponding to those of the 

 present state of things. Therefore we should represent to ourselves, 

 at that period, an atmosphere having a direct relation, both as re- 

 gards quantity and constitution, with the then phase of cooling and 

 contraction of the earth. Moreover, we must suppose surface rocks, 

 more or less altered, fissured, and penetrable, in which became re- 

 tained chemically and physically a certain amount of gases which 

 previously existed as atmosphere ; and, finally, we have to picture 

 to ourselves an ocean in which, as at present were retained in solu- 

 tion gases, also part of the then atmosphere, in quantity and qua- 

 lity relative to the temperature and constitution of the then ocean 

 mass, and relatively to the prevailing atmospheric pressure. These 

 conditions have continued to prevail up to the present time, but in 

 degree and in proportions which must have depended, and must 

 continue to depend, upon the successive phases of contraction and 

 the surface changes of the earth. We can even imagine a last 

 stage when contraction will tend to cease, when, therefore, the 

 emission of gases will consequently become less frequent and more 

 and more diminished, when the atmosphere as well as the ocean 

 will become more and more chemically and physically retained by 

 the rocks forming the crust, and when finally our earth will cease 

 to have either an atmosphere or an ocean. 



If I have thus ventured so far back into time, it is in order to 

 distinctly establish the sequence of relation and the dependency 

 which I conceive to have existed at all times between the cooling 

 and contracting sphere and the atmosphere. And if we might 

 comprehend under that term the sum of the gases existing — 



(a) Free at the surface of the earth ; 



(b) In chemical combination with, and physically retained 



by, the rocks forming the crust ; and 



(c) The gases held in solution by the ocean and other waters ; 



then we might consider the sum (a + b + c) as representing, or as 

 being proportional to, the total amount of contraction effected 

 since the period of the commencement of formation of the crust. 



