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F. P. Mennell — Composition of Igneous Bocks. 263 



V. — The Average Composition of the Igneous Eooks.^ 

 By F. P. Mennell, F.G.S., Curator of the Ehodesia Museum, Bulawayo. 



HE average composition of the igneous rocks is a point of 

 considerable interest in its relation to the problem of their 

 differentiation, and several attempts have been made to solve it by 

 the collation of analyses. Thus, Mr. Clarke estimated the American 

 rocks to average 59*77 per cent, of silica (which may be taken as 

 representative), and Mr. Harker came to very similar conclusions 

 as regards the British rocks, obtaining 58*46 as his figure. The 

 process followed was to add up the results of all the obtainable 

 analyses and take their mean. If each class of rock analysed 

 occupied the same average amount of space — if, for example, the 

 basic intrusions were approximately equal in bulk to the acid ones — 

 such a process would give results of considerable value. As it is, 

 however, very little consideration will show that unless due weight 

 is attached to the relative abundance of the different classes, the 

 results will be very far removed from the truth. Even in Britain, 

 where the development of igneous rocks is comparatively insignificant 

 compared to the sedimentary ones, there are quite enough exposures 

 of the different types to demonstrate this fact. If a geologically 

 coloured map be examined, and the nature of the various patches 

 of igneous rock be enquired into, the immense preponderance of 

 granite becomes obvious, even though the basaltic lavas make 

 a great show on account of their horizontal extension. In fact, the 

 Dartmoor granite mass, if it be assumed to extend to a depth of only 

 one mile, would probably suffice to weigh down the scale against all 

 the other non-granitic igneous rocks combined. Yet, on the method 

 indicated above, the smallest dyke would be of something approaching 

 equal account, even if a number of analyses of the Dartmoor rock 

 were included. 



In other parts of the world where igneous rocks are far more 

 largely developed than in England, the predominance of granites 

 is even more striking. In Africa and Australia there are many 

 single granite masses which are exposed at the surface over areas 

 of not only hundreds but thousands of square miles. The Matopo 

 granite mass of Ehodesia, forming the hills now famous as the 

 burial-place of Mr. Khodes, covers a horseshoe-shaped tract of 

 country certainly not less than 5,000 square miles in extent ; in fact, 

 it may be two or three times as much, as only its northern and 

 north-eastern limits are yet known with certainty. And this is 

 only one of many ; in fact, out of the 250,000 square miles covered 

 by Southern Rhodesia and the adjacent territories, it is certainly 

 safe to say that 100,000 are granite, while there is scarcely any 

 other class of igneous rock with even a single outcrop large enough 

 to be visible if inserted in its true proportions on an ordinary map. 



The district immediately surrounding Bulawayo may be taken as 

 representative. I have mapped, in the course of nearly two years' 

 work, an area of 2,000 square miles with as near an approach to 



1 Read at the Southport Meeting of the British Association. 



