A short Description of the Geology of part of the Eastern Province of the Colony of the Cape of Good Hope
Authors: Pinchin R. 
Published: October 2009

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this paper, which was illustrated by maps and sections, the author gave the results of Dr. Rubidge’s and his own observations on the geology of the above region. The two principal sections described were from Cape Saint Francis across the Great Winterhoek and Langeberg ranges to the lacustrine Triassic rocks near Jansenville, and from Port Elizabeth to Somerset. The lowest rock in the first section is the quartzite of the Great Winterhoek, which is immediately overlain to the northward by shales and sandstones containing Devonian fossils. Beds with similar fossils occur at the Kromme river, Cape St. Francis, and near Uitenhage. A well-known patch of horizontal Secondary strata stretches west along the Gamtoos river, overlying the Enon conglomerate, as in the case of the Jurassic strata of other parts of Uitenhage. The northern ranges, Langeberg, Klein Winterhoek, and Zuurbergen, are regarded by the author and others as formed of rocks belonging to the Carboniferous series, although closely resembling those of the Great Winterhoek in lithological character, except that among them are bands of the peculiar rock described by Bain as “Claystone-porphyry,” by Wyley as a “Trap-conglomerate or hardened Trap-ash,” by Jones as a “Trap-breccia,” by Atherstonc as an “intrusive Trap,” and by Sutherland as a “Boulder-clay.” Rubidge regarded it as a metamorphic rock; and this view is adopted by the author, who describes it as underlying and overlying the clay–shales, which always separate it from the quartzite, and as passing imperceptibly into the shales.

He says:—“ A glance at the map (P1. IV. fig. 1)shows that this rock does not form a continuous band, as stated by Bain (Geol. Trans. 2nd ser. vol. vii. p. 185), and more or less acquiesced in by geologists since his time—but that it consists of a number of longitudinal patches or strips, more or less parallel with each other, and more or less parallel with the line of strike of the rocks with which it is in all cases interstratified. These longitudinal ships die out as mere threads in some places, and swell to a mile or two miles in width in others. They die out altogether, and are succeeded by other parallel strips at some little distance; and they appear split up, as it were, into double or triple nearly parallel strips, ultimately either dying out or uniting together again, as the case may be. Their interstratification is shown by, and will be best understood upon reference to, the Section, fig. 2.

“In no case is this metamorphic rock contiguous to the quartzite, which is itself more or less metamorphosed, but is invariably separated from it by the clay-shales. It appears underlying and overlying the clay-shales* in synclinal troughs, in anticlinal axes, in every imaginable position, in and within the clay–shale series; but it never touches the quartzite.

“A careful consideration of all of these facts forced the conviction upon my mind that this rock must belong to the clay–shale series, with which it is interstratified, and that it was in all probability metamorphosed since its upheaval and consequent dip. I am the more inclined to this belief, since it is difficult to decide where the one rock ends and the other begins, so imperceptibly do they pass into each other. On the plans and sections (P1. IV.) I have drawn a hard line of demarcation between these rocks, for the sake of distinctness; but no such abrupt line of division occurs in nature.”

Mr. Pinchin expressed his belief that even “the fragments and pebbles of quartz, quartzite, and binary granite” in the great metamorphic band may have been originally such nodules as are now found in the clay–shales, and that the substance of the latter has been converted into the dolerite–like matrix of the former.

The mottled beds of “Ecca rock” are referred by the author to the Carboniferous series. The author also referred to the Post–tertiary and recent rocks containing remains of Mollusca identical with species now living in the adjacent seas, lying unconformably upon the Devonian, and conformably upon the Secondary rocks, at various places near the coast.