Geological History of the Atmosphere. 439 



place in very early times, and even roughly to estimate the 

 extent to which it may have taken place, or at least there are 

 certain observed facts which may be explained by means of 

 the foregoing theory. These data or facts have reference to 

 the chemical composition of rocks at varying depths of the 

 earth's crust, or perhaps more properly the composition of 

 verv ancient rocks as compared with that of those which are 

 regarded as being more modern. It is well known that the 

 oldest rocks, or those which are regarded as the oldest, are 

 much more basic than comparatively recent rocks, in fact 

 the ancient rocks are frequently called " basic," and the 

 more recent rocks " acidic/' There is, so far as I am aware, 

 no exact line of demarcation or exact definition of the terms, 

 but, generally speaking, rocks or minerals which, like the 

 supposed ancient rocks, contain about 55 per cent, of bases 

 and only 45 per cent, of silica are called " basic," while rocks 

 or minerals which, like the bulk of more recent rocks, contain 

 about 60 per cent, or more of silica and only 40 per cent, or 

 less of bases, are called " acidic. 5 ' Now this difference in the 

 composition of ancient and comparatively recent rocks may 

 quite well have been brought about in the course of geological 

 history. According to the usually accepted theory, the earth 

 was so hot in very early times as to be in the liquid condition, 

 and also according to accepted theories, the tides were 

 very high and energetic in those early times. Under these 

 circumstances, the materials which now constitute the crust 

 of the earth must have been well mixed, and therefore the 

 composition of the liquid mass (at any rate in its upper part, 

 containing as it probably did materials that were mutually 

 miscible or had sufficient chemical affinity to combine with 

 each other and produce compounds that were mutually 

 miscible) must have been fairly uniform throughout. Indeed 

 it should have been quite comparable, as regards uniformity 

 ,of composition, with the composition of the sea at the present 

 day. There would therefore be no distinction of basic and 

 acidic rocks at that period. The proportion of silica and of 

 bases would be uniform throughout the mass. But when the 

 earth cooled sufficiently to become solid, or to permit of the 

 formation of a hard crust on the surface, natural operations 

 might very well have brought about the great variety that is 

 now observed in the composition of rocks. The primitive 

 atmosphere, as we shall see in the course of this paper, most 

 likely contained very large quantities of carbonic acid. This 

 carbonic acid, acting along with water or aqueous vapour on 

 the primitive crust, would soon decompose the rocks, forming 

 carbonates, hydrates, and free silica or rock-forming materials 



