LACCOLITES AM) BYSMALITES. 191 



form of andalusite, chiastolite and sillimanite in the rocks 

 around the granite accounts for this. The soda and potash 

 can be derived from the overlying basic magma, for though the 

 generalised analysis given above shows the soda and potash in 

 equilibrium in the slate and the dolerite, nevertheless the carbon- 

 dioxide and water in the slate, if abstracted, as they do not enter 

 into the interchange of substances, would render the available 

 percentage of soda and potash in the slate somewhat in excess 

 of that required by the dolerite. The figures quoted are inten- 

 tionally round numbers, but they express what is meant ; anyone 

 who objects to the theory could adduce analyses which would 

 disprove the whole argument, but the analyses selected are general 

 averages. What is contended, however, is that, in South Africa 

 at least, injection of basic rocks implies the simultaneous injection 

 of acid rocks deeper down, and that the spaces occupied by basic 

 and acid rocks are far in excess of what could have been created 

 bv simple thrusting aside of the sediments, and hence absorption 

 of the sediments has taken place in both cases. 



In the question of the supply dykes which connect the acid 

 and basic intrusions, these are frequently seen traversing the 

 granite, fine examples being shown in the Lion's Head and Wyn- 

 berg, in the Cape Peninsula. These, however, are simple dolerite 

 dykes. There should be somewhere, if the theory outlined above 

 is a true explanation, dykes of intermediate composition, that is 

 to say, sometimes consolidation should have taken place before 

 the acid and basic constituents had had time to separate out. In 

 the great laccolite regions of the Kei and Kentani and in Cradock 

 these intermediate dykes are quite frequent. In the neighbour- 

 hood of the great Quizzyhota laccolite, which is onlv one of many 

 gigantic lumps of dolerite in the neighbouring divisions, there 

 are two great parallel dykes of diorite which, from their easy 

 weathering, have given rise to a double furrow across the country 

 from the sea-level at the Komgha River to Cathcart, known as 

 the Transkei Gap. 



The Diffusion Dumb-hell. 



Taking the whole evidence which the granite bosses, the 

 dolerite dykes and the dolerite laccolites afford us in Cape Pro- 

 vince, one is led to conceive of a system of igneous injections of 

 a dumb-bell shape. Below we have the Malmesbury clay-slates, 

 above the Karroo sediments, in between the various silicious sedi- 

 ments of the Cape system, or sometimes, in the north, of the Pal- 

 Afric group. Kheis quartzites and Pretoria iron-bearing 

 quartzites. Below, the magma eats out great holes and fills them 

 with granite ; above the same magma eats out the holes and fills 

 them with basic rock. In between are thin dykes of communica- 

 tion, usually dolerite. but under certain conditions diorite. The 

 slates above and below became absorbed, and the material from 

 both was added to the general stock of magma. The average 

 magma remained fluid in this dumb-bell system for some time, 

 till, for some reason, the acid part concentrated in the lower part 



