I()0 LACCOLITES AND BYSMALITES. 



The bysmalite that I think most nearly approaches the idea 

 of a receptacle for the waste products from above is the granite 

 boss at Robertson. The strata intruded are steeply-folded 

 Malmesbury beds, clay slates, mica-schists, limestones and 

 carbonaceous shales. The sediments show a concentric arrange- 

 ment round the granite, and, to a certain extent, lateral thrust 

 has taken place ; but when a plug of a mile in width by two miles 

 in length is intruded into sediments there is hardly room to doubt 

 that the whole space now occupied by the granite has not been 

 created by thrusting aside the walls of a cavity. The argument 

 becomes still more pertinent with regard to the granite underlying 

 the Cape Peninsula and the mass surrounding Saldanha Bay, 

 which, are many miles across in either direction. In the Robert- 

 son granite, in addition, there is evidence of extensive lit-par-lit 

 injection on the southern side. The granite substance has been 

 thrust in between the laminae of the slates in such a way that 

 it is impossible to tell where the granite ends and the sediments 

 begin. One can trace the gradual absorption of the slates by 

 the igneous magma. At Sea Point the absorption was of a 

 different nature, due no doubt to the tougher nature of the day 

 slates, which are not easily split for the injection of thin veins 

 of granite-substance. The dykes that come off the granite pene- 

 trate the slates in numberless stout projections and frequently 

 enclose slabs of the slates in the granite. On the road above 

 many " basic segregations " on the granite were exposed in the 

 cutting, and these may be regarded as the slabs of slate almost 

 entirely absorbed. 



If one takes a mica-schist as the normal rock intruded by 

 granite, that is, a slate which has been altered by the abstraction 

 of the more soluble substances — iron. lime. etc. — and compares 

 it with a normal granite, we have the following figures : — 



Mica-schist Granite. 



Silica 67 per cent. Jj per cent. 



Alumina . 19 ., ,, 13 ., ,. 



Iron oxides 4 ,. 2 ,, .. 



Magnesia 2 



Lime — — 



Soda 2 per cent. 3 per cent. 



Potash 4 ,, ,. 5 ., „ 



The features of the absorption of the mica-schist by the 

 granite will, therefore, be that more silica is called for. alumina 

 is in excess, iron is to be got rid of, and soda and potash are 

 wanted. If we suppose there is an interchange of substances 

 going on between the basic magma on top with the acid magma 

 below through the channels of connecting dykes, then the silica 

 required will come down and satisfy the magma, the 

 alumina is sufficient above and the granite substance cannot 

 absorb it ; therefore it has to be got rid of somehow, and the 

 very general development of silicate of alumina crystals in the 



