i9o6] IN CHEMICAL GEOLOGY. 19 



A. B. C. D. E. F. 



Clarke. Clarke. Clarke. Clarke. Marker. Washington. 



SiO, 59.97 61.22 61.12 61.71 60.36 58.96 



AI2O3 15-39 15-75 15-77 15-48 16.07 15.99 



FcaOs 4.03 2.71 2.69 2.68 5.48 3-37 



FeO 3-56 3-53 3-6o 3-50 2.46 3-93 



MgO 4.60 4.51 4-46 4-i8 4.20 3.89 



CaO 5-41 4-93 S-02 4-94 5-i2 5-28 



NaoO 3-28 3-69 3-63 3-49 3-34 3-96 



KoO 2.97 2.90 2.87 3.02 2.83 3-20 



TiOo 56 .54 -61 -74 -12 I. OS 



P0O5 23 .22 .23 .26 .02 .37 



100.00 100.00 100.00 100.00 100.00 100.00 



Of these averages, only " D " and '' F " need be considered any 

 further, for they include the largest masses of trustworthy data. 

 " A " was only a preliminary computation, " B " and " C " are in- 

 cluded under " D " ; Marker's average contains too many imperfect 

 analyses. " D " and " F," however, are not strictly equivalent. 

 Washington's average relates only to analyses which were nominally 

 complete, and made in many laboratories by very diverse methods. 

 My average represents the homogeneous work of one laboratory, 

 and includes, moreover, many partial determinations. For the 

 simpler salic rocks determinations of silica, lime and alkalies are 

 generally all that is needed for petrographic purposes. The femic 

 rocks are mineralogically more complex, and for them full analyses 

 are necessary. The partial analyses, therefore, chiefly represent 

 salic rocks, and their inclusion in the average tends to raise the per- 

 centage of silica and to lower the proportions of other elements. 

 The salic rocks, however, are more abundant than those of the other 

 class, and so the higher figure for silica seems more probable. This 

 conclusion is in line with a criticism by Mennell,^ who thinks that 

 the femic rocks received excessive weight in my earlier averages. 

 Mennell has studied the rocks of southern Africa, where granitic 

 types are predominant ; and he believes that the true average should 

 approximate to the composition of a granite. A wider range of 



^ Geol Mag., ser. 5, vol. i, p. 263. For other discussions of the data given in 

 my former papers, see De Launay, Revue Gen. des Sciences, April 30, 1904; and 

 Ochsenius, Zeitsch, prakt. Geo!., May, 1898. 



