264 F. P. Mennell — Composition of Igneous Rocks. 



accuracy as can well be attained with the present imperfect topo- 

 graphical maps. By dividing the map into small squares an estimate 

 is readily made of the areas covered by the different rocks, with the 

 following result : — 



Square miles. 

 Sandstones, probably Tertiary (including some lava-flows) ... 215 



Metamorphic rocks ... ... ... ... ... ... 730 



Plutonic igneous rocks ... ... ... ... ... ... 1,055 



2,000 

 It will be seen that the plutonic rocks outbalance all the other rooks 

 put together. They comprise portions of four masses, of which 

 one is chiefly syenite (with 63 per cent, of silica) covering 15 square 

 miles. The others, with a combined area of 1,040 square miles, 

 are granite with a silica percentage probably averaging about 70 per 

 cent. It must not be thought that basic rocks are absent ; they are, 

 on the contrary, well represented by numerous dykes of dolerite and 

 basaltic flows. We shall, however, be making a generous allowance 

 for them if we suppose there are 1,000 dykes a mile long and five 

 feet wide, with 10 square miles of basalt 20 feet thick. We will 

 further suppose that these rocks contain about 50 per cent, of silica- 

 There are a few intrusions of porphyrite and orthophyre, but they 

 are of little importance and may be reckoned as allowed for amongst 

 the dolerites. 



Now let us see what results these figures lead to. We will 

 assume that each dyke and plutonic mass extends vertically to sea- 

 level, that is to say, goes down about a mile. (Bulawayo stands 

 at an elevation of 4,500 feet above the sea, and much of the district 

 is higher.) We have therefore : — 



Total 1,056 



Multiplying each silica percentage by the volume of the respective 

 rock, adding up the products and dividing by the total volume, we 

 obtain an average of 69'88 for the whole. Whatever composition is 

 assigned to the granites, the general average will, in fact, approach it 

 within a few parts per thousand. We thus arrive at the conclusion 

 that if all the other rocks of the area were to be fused into the 

 granite masses the difference they would make would be quite 

 imperceptible lithologically, and scarcely noticeable in a chemical 

 analysis. Such a result would, I believe, hold good for the entire 

 African continent and certainly for the whole of Ehodesia. There 

 is nothing to indicate that a different conclusion would be reached 

 in any other extensive area where the plutonic rocks are adequately 

 represented, and there is accordingly reason to believe that granite 

 substantially represents the magma from which even the most basic 

 rocks have been developed by some process of differentiation. 



