O'Eeilly — On Gaseous Products of Great Eruptions. 19 



been demonstrated by Atkin, and the wondrous organic life of the 

 ocean is essentially due to the presence of gases in relatively small 

 quantity. Now, no class of agents in nature more easily escape 

 attention or baffle investigation than gases, unless they present 

 themselves physically or chemically fixed, so as to allow of their 

 determination and measurement. The reason of this is obvious. 

 Almost all the gases acting at the surface of the earth have densi- 

 ties less than that of air ; consequently, unless restrained or brought 

 into combination, they tend after emission to rise in the air, and 

 becoming mixed with the atmosphere, pass to a very great extent 

 beyond our observation and our control. 



That the geologist should therefore attribute to them a rather 

 subordinate and ill-defined part in the series of phenomena which he 

 is called on to study can be understood. Brought, as he is, face to 

 face with the rock masses forming the crust of the earth, or with 

 the water masses which cover three- fourths of its surface, he natu- 

 rally attaches importance to them, rather than to the gases which 

 have ever acted, and are acting continually, from the interior or at 

 the surface of that crust, but which by their very nature escape his 

 attention even while still active agents, and which are so difficult 

 of determination and measurement. There is, therefore, some justi- 

 fication for my calling attention, in this respect, to those earlier 

 phases of the earth's development, which are usually treated as 

 either purely of the domain of astronomy, or are not admitted 

 as being tangible for the geologist. 



Whatever the hypotheses which may be accepted as to the con= 

 ditions of development of the earth, it is generally taken for granted 

 that the successive phases of its existence have been similar in 

 nature, if not in degree, to those which Science has been led to 

 attribute to the other heavenly bodies. Thus we are led to believe 

 that it has passed through all the phases observable in one or other 

 of these heavenly bodies : from that of a nebula, becoming more 

 and more condensed, to that of a sun; and from that of a sun 

 through successive stages to the condition of things with which 

 geology usually commences, that is, of a globe, having a crust or 

 solid exterior, and therefore in a relatively cooled state, and ca- 

 pable of allowing the condensation of water on its surface and the 

 existence of organic life thereon. Now spectroscopy and observa- 

 tion have shown that in the nebulae, as in the comets and as in the 



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