Page:The Geologist, volume 5.djvu/70

52 have attempted to establish this is somewhat defective, but I have shown clearly that that on which it is denied is valueless.

I have already described the interpolation of masses of granite among the slate of Cape Town without displacement: this phenomenon obtains to a much greater extent in Namaqualand; great masses of granite, with little if any evidence of stratification, pass gradually into gneiss on either side, and, in fact, all round, without change of dip. These are called locally "bosses," and their scaling off is remarkable, giving them the rounded outline, whence their name. The same thing is seen in the change of hornblende-schist into greenstone or syenite, with large crystals of hornblende. Numerous instances of this occur; one of the most striking is between Klein Pella and Oomsdrift.

I have mentioned in a former Paper that the twists of the strata in which the copper-ore is deposited occur in gneiss, and when a section is seen on a hill-side no granite is visible, but when worked to any considerable depth, the rock loses its laminated character and becomes a felspathic granite or greenstone. A remarkable section was observed near Pella: a stream had worked a deep channel in the rocks; the edges of the ravine so formed were of well-marked gneiss, while the water ran ever a bed of granite without trace of lamination, the gneiss preserving the same dip on either side of the ravine. Indeed, it appeared to me as if metamorphosis of the rock into felspathic granite was the normal state below, while the gneissic lamination was a superficial indication of the old stratification-planes. While on this subject I will mention what appears to me to be a singular character of our palæozoic rocks here. The specimens I have sent home will show that all the Devonian fossils here lose every trace of their carbonate of lime. They are preserved, often very perfectly, in oxide of iron, but in my experience they are seen only on the exposed edges of the rocks, be these greatly inclined, as at Chatty and Hermansdorp, or only slightly so, as at Coxcomb and Jeffrey's bay. At Chatty I have seen a mass hollowed out in all directions by the decay of the encrinites on the edges, while tracing the same layer deeper in, it lost all trace of fossils. Frequent repetitions of this seemed to me to establish it as a rule that the fossils in the rock were only exposed by decomposition. Still it may be merely accidental. I should be glad to learn whether it is so or not.

I have stated that in the metallic twists, or saddles, I never saw granite in what I could consider the position of an intrusive rock. In one of the accessory twists which meet the metallic saddles at various angles, and which in section on a flat surface have the appearance of a feather, the shaft (a b) of the feather was composed of micaceous schist, with a few rather large crystals of felspar. I have frequently seen irregular-shaped patches of mica-schist following neither strike, nor any law that I could perceive, among the gneiss. Granite occurred in the same way in other spots.