450 Chemical and Geological History of the Atmosphere. 



by the formation o£ hydrated silicates, and so, in taking a 

 general survey of geological history, we shall find that there 

 has been a fixation, not only of much carbonic acid in the 

 crust of the earth, but of a very large quantity of water as 

 well. This quantity is so great as to represent a very 

 considerable proportion of the total water existing on the 

 earth. Taken along with the moisture or uncombined 

 (hygroscopic) water present in the rocks it may amount to 

 as much as ^ or ^ of the quantity of water present in the 

 ocean itself. Mr. F. W. Clarke in his recent estimate of 

 the average composition of the rocks forming the crust of 

 the earth gives the water at 1*51 per cent., which for a ten 

 miles' thickness of the earth's crust would give a total of 

 about OS x 10 18 tons water. As this estimate seems not to 

 include hygroscopic water, it may be well also to note his 

 earlier estimate (which does include hygroscopic water), 

 viz., 1"96 per cent., a percentage which in a ten miles' 

 thickness of the earth's crust would give about 04 x 10 18 tons 

 water, or nearly one third of the water present in the ocean 

 (say 1*3 x LO 18 tons). It is obvious that if this water present 

 in the rocks has been derived from the atmosphere and the 

 ocean, the ocean must have contained considerably more 

 water in ancient times than it does now, and probably covered 

 a greater area of the earth. It is also worth noting that the 

 combined and hygroscopic water present in rocks may be 

 the real or the principal source of the steam which is evolved 

 in such large quantities during volcanic eruptions. 



A question of some interest which is naturally suggested 

 here is that regarding the ratio of the quantities of terrestrial 

 hydrogen and carbon to each other. It would be of very 

 considerable interest if the quantities should be found to bear 

 a ratio to each other that could be expressed by such a 

 formula as CH 4 , or that of some other hydrocarbon compound. 

 However, if we accept Dr. Sterry Hunt's estimate of the 

 quantity of carbonates on the earth, we shall find that there is 

 not enough carbon to form methnne (GH 4 the hydrocarbon 

 which contains the largest proportion of hydrogen) with the 

 hydrogen of sea-water alone. It is no doubt possible that 

 carbon might be present in large quantities in the internal 

 parts of the earth combined with iron and other metals, but 

 of this we have no definite information, and in any case we 

 would naturally expect, as already stated, that a large pro- 

 portion of the hydrogen and the carbon would be present on 

 the earth in the oxidized condition from an indefinitely early 

 period. 



In conclusion we may sum up the results of this paper by 



