446 Mr. J. Stevenson on the Chemical and 



In addition to the difficulties which have just been 

 discussed, there is one to which reference was made in 

 connexion with the argument drawn from the composition of 

 basic and acidic rocks. We stated that if the acidic rocks 

 of the earth's crust, containing as they do 60 per cent, or 

 more of silica, have been derived from a primordial basic 

 rock-mass containing only 45 per cent, (or less) of silica, it 

 is not easy to see what has become of all the missing bases. 

 It should be noted that the quantity of missing bases is even 

 greater than one may at first be apt to infer from the above 

 percentage of silica. 



In basic rocks containing 45 per cent, of silica and 55 per 

 cent, of bases, there will be 122 parts by weight of bases to 

 100 parts of silica, while in acidic rocks containing 60 per 

 cent, of silica and 40 per cent, of bases, there are only 66 

 parts by weight of bases for 100 parts of silica, or not much 

 more than half the proportion of base relatively to silica 

 found in the basic rocks. Acidic rocks would thus seem to 

 have lost about half the quantity of base which was originally 

 present. If then they have been derived from primordial 

 basic rocks we should expect to find a very large quantity of 

 carbonates, sulphates, chlorides, and hydrated oxides on the 

 earth, or else we should infer that a very large quantity of 

 oxide of iron has been removed, perhaps several times more 

 than the 6 per cent, already postulated. As regards the 

 amount of carbonates &c. we have already remarked that there 

 seems not to be as much limestone (which is the principal 

 carbonate present) as would be equivalent to the sandstone 

 or free silica alone, while the sulphates, chlorides, and 

 hydrated oxides are comparatively small in quantity. There 

 might no doubt be large quantities of carbonates in the rocks 

 underlying the great oceans and these might be accompanied 

 by a comparatively small quantity of sandstone. It is quite 

 reasonable at any rate to infer that considerable quantities of 

 carbonates exist below the great oceans from observations 

 made regarding the evolution of carbonic acid at the bottom 

 of the sea, or at least the presence of carbonic acid in varying 

 proportions at different places in the sea (see Prof. Dittmar's 

 figures on this subject in vol. i. p. 219 et seq. (Physics and 

 Chemistry) of the Reports issued in connexion with the 

 'Challenger' Expedition); and at the same time we should 

 reasonably expect that there should not be much sandstone 

 underlying the great oceans, or at any rate the abysmal regions 

 of the great oceans, on account of the very long period during 

 which they are supposed to have been in existence. 



Also, as regards the quantity of oxide of iron removed from 



