THE QUIZZVHOTA LACCOLITE 89 
Mountain sandstone.t I do not, however, wish to include in this 
discussion the question of the granite bosses as I have no new 
studies on them to offer, but the evidence for the absorption of 
sediments is very plain, especially along the granite contact at 
Sea Point, which was used by Hutton in illustration of his principles, 
and at Robertson, where the granite is bordered by a rim of Jit- 
par-lit injection and other phenomena that I believe will afford 
positive evidence for the part these masses played in the general 
igneous injection of the country, when the material is worked up. 
The igneous rocks of Cape Colony are very simple—enormous 
tracts of dolerite above and granite below; there are none of the 
complicated varieties to obscure the main principles. There is a 
curious side-issue in respect to the injection of andalusite crystals 
or occasionally orthoclase crystals in the sediments round the 
granite.2_ I assume for the purpose that the evidence for the absorp- 
tion of sediments by the granite is as complete as for the dolerite, 
at least in Cape Colony, and that the original magma plus the 
material absorbed separated into dolerite and granite. We do not 
at present know what the proportions of basic and acid rock were, 
but we can suppose that they were equa]. If, now, we take the 
mean of any analyses of normal granite and dolerite and contrast 
it with the analysis of any non-calcareous slate, adding a little 
sandstone if the silica percentage is small, we shall find that the 
excess of material after the igneous rock has used up all it wanted, 
consists of silica, alumina, and potash, and naturally, if the igneous 
rock is locally rich in potash, then of silica and alumina only. That 
is to say, the very general injection of silicate of alumina in the 
form of andalusite, chiastolite, and sillimanite, and the fairly com- 
mon injection of orthoclase crystals, would be a result of the absorp- 
tion of the slates and the rejection by the granite-dolerite magma 
of waste material. I take as an illustration the analyses given in 
Professor Judd’s Students’ Lyell, which can easily be checked. 
A more suitable series of analyses would be the slates intruded by 
the granite, the granite and the dolerite dykes in the granite, which 
«See F. H. Hatch and G. S. Corstorphine, Geology of S. Africa, Figs. 2 and 3; 
pp. 37 and 41. 
2E. H.L. Schwarz, Ann. Rept. Geol. Comm., 1897, Cape Town (1899), 54. 
