ranges of quartzite, often unconformable with them, which quartzite is continuous with like quartzite conformable with the Devonian beds. Whence I infer that the rocks of a tract of country may he so altered by molecular changes common to all (probably in the instance of our rocks the infiltration of silica), that beds of widely different ages may present the same lithological character, and that when horizontal quartzose (or calcareous[1] or felspathic?) rocks are continuous with inclined rocks of the same kind it is not always safe to infer that beds resting conformably on the latter are much newer formation than those on which the sandstones rest unconformably, that the beds a, b, are very far older than c, d, for instance.
Fig. 2.
It is my conviction then (though I admit that my evidence is not quite conclusive) that the inclined slaty rocks of this Colony, west as well as east, all belong one formation, which geologists at home have, on the evidence of fossils, pronounced to be Devonian; and that the quartzite is a rock which has undergone a superficial change, and may therefore be called metamorphic. This siliceous metamorphosis is associated with other changes. The clay-slaty beds are often converted into ochry, micaceous, and chloritic schists.
There is not in the Eastern Province much evidence of ordinary metamorphic action, except in the claystone-porphyry of Bain, which I regard as a product of metamorphic action, as I shall more fully explain hereafter. At the Matland mines, about twenty miles west of Port Elizabeth, are slates like those which at Chatty contain Devonian fossils. Some of these have been converted into chloritic, hornblendic, and micaceous schists, without any evidence of the proximity of eruptive rocks. In the planes of bedding of these schists are veins of quartz, and occasionally carbonate of lime, not very rich in copper-pyrites. I regard the hard blue crystalline limestone of the same locality, in which lead and zinc ore occurs, as partially, at least, metamorphic. At George and other places intermediate between Cape Town and here, granite occurs, but as I have had no opportunity of examining it, I shall trace the evidences of metamorphic action from Cape Town northward.
At Cape Town I found granite-veins varying from one to three feet to as many lines diameter running parallel with the strike of the clay-slate rocks without displacing them, showing, I think, that they had been changed in situ. Other veins crossed the strike. Again,
- ↑ I think I saw calcareous beds of which all I have asserted of the quartzite might be predicated.
