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On the Metamorphosis of Rocks in the Cape Town District, South Africa
Published online by Cambridge University Press: 17 March 2016
Extract
I gave, in a paper in the February number of the ‘Geologist,’ a general view of the facts in the geology of this country which have led me to believe that the metamorphosis of rocks is due to a slow and gradual change in their constituents ; of which change water is one of the chief agents, and the internal heat of the earth not a necessary adjunct.
I now propose to describe more particularly those relations of the quartzite with the palæozoic rocks, a careful examination of which has rendered necessary an entire reconstruction of the geological map of the country. That map, published in the Transactions of the Geological Society, was the work of an able man, and the evidence on which the Devonian (Upper Silurian, Bain) was separated from the Clay-slate formation was (so far as I have been able to verify it), I believe, such, that he would not have been justified in coming to another conclusion by any generally admitted principle of the science; for this reason, I invite the criticism of European geologists on my facts and inferences, and their aid in solving many difficulties which still remain unexplained.
I stated in my former Paper that the plains and lower hills and valleys of the coast region, extending from Cape Town to the mouth of the Fish River, were formed of blue slaty and sandy rocks. These were all referred to one formation by Lichtenstein.
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References
page 367 note * The strike is too near perpendicular to the range of mountains, and the two ranges should have diverged at right angles.
page 368 note * I have borrowed the pencil of a friend, Mr. R. Miller, to represent these relations more clearly to the eye. It is doubtful if the relation of the strike will be understood.
page 369 note * In explanation of the great desire I have always felt for the opinions of European geologists of note on the subject of these relations of our rock, I must remark, that I am quite a self-taught geologist, and have had no experience in any country but this. Moreover, all the colonial geologists, while admitting the relations described, see nothing inexplicable by admitted theories in them. Dr. Atherstone, for instance, believes that the quartzose sandstones were originally deposited in the positions mentioned, interstra-tified with the slates. Mr. Bain believed them of different and unconformable formation: so does Krauss. Since he has seen the Devonian fossils, Mr. Bain is inclined to think the slates and quartzites conformable, and that he has made a mistake in the boundary of the formation in the east, while he strongly affirms the accuracy of his sectiou in the west.
It will be seen by reference to former Papers, that on my belief in the truth of Mr. Bain's section I founded the prediction that the clay-slate and Devonian would be proved one formation. When I use the word Devonian as applying to all our strata, I would explain that I make no pretension to settle the question of their age on my own authority, or to deny the possibility of there being strata as old 01 older than the Cambrian or older rocks in Britain. I simply mean that I have seen no reason for believing in any older rock unconformable with the Devonian; and I hope I have shown that the position of the quartzite, with reference to the latter, is the same as it is to the clay-slate, which Bain and Wylie believe so much older.
I ask for the aid I mention because I think the opinion of high authority would be of great value to us by showing what is regarded as credible, and what is not. The assistant-secretary of the Geological Society, some years ago, told me that the story of quartzite metamorphosis was rejected in toto. I hope the labours of Messrs. Sorby, Daubree, Hunt, and others, have somewhat modified opinions. At the period in question my belief in this assimilation of rocks of different ages had not been confirmed by those discoveries which rendered the map published by the Society entirely obsolete as to some of the principal formations of the Colony. It was quite natural that under such circumstances the fulfilment of my early prediction should have been regarded as the confirmation of lucky guesses; but when I pointed out the fact that as to the Carbouiferous and clay-slate formations, the result of my researches was, as I have said, to render the map obsolete, I think that in taking no notice of my communications the Society lost an opportunity.
page 370 note * The discovery of Devonian fossils at the northern foot of the Coxcomb, in rocks with but a slight inclination, and that of the same species close to the Van Stadensberg, in beds with a dip of 46°, with a clear section connecting them with the schists at the De Stade's River mines, is singular in connection with this remark of my friend, and would seem to indicate that he has been misled as to the relative ages of formations in the east, in the same manner as Mr. Bain has further west.
page 371 note * Mr. Wylie, in his section of this part of the Colony, which, though not published, is placed in the Town Hall, in Cape Town, makes the shales above spoken of conformable with the sandstones.