440 Mr. J. Stevenson on the Chemical and 



containing a much higher proportion of silica than the 

 original rock-mass of the earth. There may also have been 

 a very considerable quantity of hydrochloric acid in the 

 primitive atmosphere or ocean, which of course would also 

 act on the crust with the formation of chlorides ; but as the 

 total quantity of chlorides on the earth is very small in compa- 

 rison with that of carbonates, they may for our present purpose 

 be left out of account. Now if we were to add to modern acidic 

 rocks the bases (chemically speaking) of the rocks composed of 

 carbonates, oxides, and hydrated oxides that are found inter- 

 spersed with them, or if we were simply to imagine them all to 

 be fused together, the resulting mass of silicate rocks would 

 obviously be less acidic and more basic than it was pre- 

 viously, though possibly it might not be so basic as the deep- 

 seated primordial rocks, which are technically called " basic." 

 Indeed, we may be quite sure that it would not, for it is well 

 known that sandstone or free silica is very much more 

 abundant than limestone, and even in the English Oolitic 

 system, which is comparatively rich in limestone, Prof. 

 Phillips estimated that there was three feet of san Istone for 

 one foot of limestone. If then the total limestone of the 

 earth bears such a small proportion to the total sandstone, it 

 is evident that if the materials composing the upper part of 

 the crust of the earth were all fused together the composition 

 of the resulting mass would still be acidic when compared 

 with that of the older or primordial rocks lying lower down. 

 Tiierefore, if the whole rock-mass of the earth was at one 

 time uniform in composition or homogeneous throughout, a 

 portion of the basic constituents must have been abstracted 

 from the materials which now constitute the upper part of 

 the crust. The principal bases in both basic and acidic rocks 

 are alumina, lime, magnesia, oxide of iron, potash, and soda. 

 Of these, oxide of iron is the base, the deficiency of which in 

 acidic rocks is most pronounced when compared with its 

 percentage in basic rocks. Indeed, it is possible that the 

 other bises b°ing so largely represented by carbonates, 

 chlorides, and sulphates may not be deficient at all, though 

 this is rather doubtful, as we shall see later on ; but in any 

 case there would be required a fuller knowledge of the 

 subject than we at present possess to enable us to answer 

 this part of the question deh'nitely. In the case of iron, 

 however, the difference is very marked. Basic rocks contain 

 from about 5 to 30 per cent, of oxide of iron the average 

 amount being probably at least 8 per cent., and that princi- 

 pally in the form of protoxide of iron. Acidic rocks, on the 

 other hand, contain on the average only some 2 or 3 per cent. 



