The sections of the Zeurberg, of Yan Zonder's Plain (given above) on the Graff Reinet road, together with a somewhat imperfect recollection of that of Graham's Town, had led me to believe that the relation of the porphyry to the slate was constant. It is now evident that I was in error here. The fact of the masses of the porphyry crossing the slate without disturbing it seems to me greatly to strengthen my views as to its metamorphic origin by rendering the hypothesis of Messrs. Bain and Wylie untenable. The former gentleman supposed that the porphyry had been poured over the surface of the strata as liquid lava. Mr. Wylie referred its origin to volcanic action, producing ash, which was deposited at the bottom of the ocean, and formed this igneous-like rock with its contained granite pebbles arid fragments of rock. The fact of the direction of its masses being at an angle of 30° with the strike seems to me to be incompatible with either of these hypotheses. The ranges of porphyry, like those of quartzite, die out and reappear. The normal position of the porphyry appears to be as in fig. 4, but I have seen it placed as in figs. 5 and 6:—
Slate. Porphyry. Slate.
Fig. 4. |
Slate. Porphyry. Slate.
Fig. 5. |
Slate. Porphyry. Slate.
Fig. 6. |
The character by which we all agree to recognize this rock is the presence of masses of quartz and granite of various sizes with occasional fragments of slate and other rocks. Sometimes these masses are as much as fifty pounds in weight, at others they are so minute as to be scarcely recognizable by the naked eye. In a recent journey to Paardenpoort I met with a mass of this porphyry which terminated in a vein about a foot thick, with very minute crystals. Now what is the character of the rock among which this porphyry is interposed or interstratified? It is such that no one acquainted with the two would pretend to diagnose them, save by the presence of the crystals above mentioned. Nor would the blowpipe, or even more careful analysis, so far as I am aware, enable him to do so. If then the base of the rocks differ so little, and there is evidence that no displacement has taken place in any known section (see Bain and Wylie), is it not clear that this rock has originated in slow conversion? Yet I believe whatever may be predicated of it may be equally so of granite; for it contains granitic masses in great numbers, and often of large size: besides granite, veins occupy precisely the same position among rocks which I have given reasons for believing to be the same strata in the Western Province.
But it will be seen above that I am not disposed to admit that the evident displacement of strata is at all times due to eruptive agency. I have given instances, on a very small scale, in which I feel sure it is not so. I hope ere long to be able to show that the infiltration of quartz from above has produced this- effect, but my evidence on this
