The Quarterly journal of the Geological Society of London . map, was found a small area of an amyg-daloidal rock, containing many large amygdales, often tubular inshape, chiefly filled with quartz. The nature of the junctionbetween this rock and the normal diabase is indefinite, and wasdifficult to ascertain; but the impression gained was that thediabase suddenly became amygdaloidal. The case of the quartzite (Wl) in contact with the diabase nearthe northern boundary of the farm Tweefontein is interesting :it dips comparatively gently away from the granite, in contra-distinction to the overtil
The Quarterly journal of the Geological Society of London . map, was found a small area of an amyg-daloidal rock, containing many large amygdales, often tubular inshape, chiefly filled with quartz. The nature of the junctionbetween this rock and the normal diabase is indefinite, and wasdifficult to ascertain; but the impression gained was that thediabase suddenly became amygdaloidal. The case of the quartzite (Wl) in contact with the diabase nearthe northern boundary of the farm Tweefontein is interesting :it dips comparatively gently away from the granite, in contra-distinction to the overtilted or nearly vertical dip which all thesedimentary beds invariably have hereabouts. This is explained asbeing due to a subsidiary granite intrusion into the diabase, the topof which comes to light in the middle of the diabase to the westof the quartzite-ridge; this must have forced the sedimentarybeds outwards from the granite, thereby giving to its inner side a Fig. 1. -Quartzite-contaci with diabase near the northernboundary of the farm Tweefontein? Sedimentaryf Rocks curvature (fig. 1). When traced northwards the sedimentary bedsgradually assume a vertical, and then an overtilted position. This basic intrusion is a definite addition to the thickness of thestrata above the granite, since no evidence of absorption of thesediments by it on any large scale was discovered. However, theremarkable regularity of the boundaries which the slaty members ofthe series exhibit towards the diabase, as opposed to the constantlyfaulted outcrops of the associated quartzite-bands, is suggestive ofa certain amount of absorption by them. As a general rule, themoderate-sized intercalations do not even produce metamorphismvisible to the naked eye in the slates adjacent to them. Theextreme extent and nature of the metamorphism which they effectcan be well studied in the masses of slate enclosed in the largerintrusions remote from the granite, as, for instance, on the farmTweefontein. While sti
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Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
