220 



A. Smoker — British Igneous Rocks. 



VI. — On the Average Composition of British Igneous Eocks. 

 By Alfred Harkbb, M.A., F.G.S., Fellow of St. John's College, Cambridge. 



THE subject of the average composition of collections of rocks 

 has been dealt with in a paper published ten years ago by 

 Mr. F, W. Clarke on " The Eelative Abundance of the Chemical 

 Elements."^ The object of that paper was to arrive at an estimate 

 of the mean chemical composition of the earth's crust, but 

 incidentally the author shows a close correspondence between 

 the mean compositions of groups of miscellaneous rocks from 

 different regions. It appears from his figures that a moderate 

 number of analyses — less than a hundred — if taken without any 

 selection, is enough to give value to their mean. It may therefore 

 be of interest to apply the method to our own country. I have 

 accordingly collected published analyses of 397 British igneous 

 rocks, viz., 195 English, 31 Welsh, 79 Scottish, and 92 Irish, 

 and calculated their mean. By searching it would doubtless be 

 possible to find additional analyses, though probably not many, 

 and the number taken is quite sufficient for our purpose. I have 

 taken all the complete analyses that I found, except a very few 

 known to be faulty, since it is the essence of the statistical method 

 that no selection should be exercised. Partial analyses I have not 

 used, with the exception of 78 analyses of Malvern rocks by 

 Timins, in which the alkalies were not estimated. The mean 

 given by the 397 analyses (or in the case of the alkalies by 319) 

 appears in Column I. 





British Kocks 



SiOj 



58-75 



TiO, 



0-12 



AI3O3 



15-64 



Feo, O3 



5-34 



FeO 



2-40 



MnO 



0-15 



MgO 



4-09 



CaO 



4-98 



Na,0 



3-25 



K,0 



2-74 



H'oO 



2-23 



p;o5 



0-02 



11. 



III. 



British Rocks 



American ^ 



(corrected). 





68-46 



59-77 



0-69 



0-53 



15-13 



15-38 



5-34 



2-65 



2-40 



3-35 



0-40 



0-09 



3-84 



4-40 



4-98 



4-81 



3-25 



3-61 



2-74 



2-83 



2-23 



1-51 



0-25 



0-21 



99-71 99-71 9914 



These figures, however, need some corrections for various reasons. 

 In particular, certain constituents have not been determined in all 

 cases. Titanic acid was estimated in 30 analyses only and recorded 

 as 'traces' in 4 others. The mean for the 34 rocks is 1*39. This 

 is doubtless too high a figure to be inserted in the general average, 

 because titanic acid has usually been sought in such rocks (chiefly 

 of basic and ultra-basic composition) as were expected to yield it 

 in noteworthy quantity. Since some arbitrary assumption is 



1 Bull. Phil. Soc. Washington, vol. xi (1889), pp. 131-142; also in Chemical 

 Neivs for Jan. 17, 1890. 



