50 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. ^"'^voY.xTx, 



These three mountain masses were parts of a single undulating sheet of abnormal thickness. 

 Erosion has severed the continuity of the body, and the "domes" between them have been 

 removed, so that these relics represent the lower basin-shaped parts of this great basin intrusion, 

 which originally covered not less than 700 square miles. The volume of the gabbroidal matter 

 involved probably exceeded 300 cubic miles. The lofty chain of Nolangeni (fig. 15c) marks the 

 fissure up which the bulk of the eruptive matter ascended. (Du Toit '20.) 



Immediately beneath the gabbro of the Insizwa mass the hornfels is full of little veins and 

 stringers of granite and quartz-diorite, mostly vertical, and sometimes several inches in width; 

 the boundaries of these are ill defined in the hornfels fading away into the body of the latter, 

 while patches characterized by abundant feldspar occur near the contact showing the extreme 

 impregnation and veining of the hornfels by the igneous material, which also formed small gran- 

 itic veins in the base of the gabbro. The sulphide-minerals of the gabbro also occur in these 

 and in veins in the hornfels (cf. p. 55, and Tolman and Rogers '16) and Goodchild ('18)). The 

 olivine-free rocks toward the upper edge of the sheet become somewhat coarser in character, 

 with a tendency to carry coarsely crystalline and pegmatitic veins associated with quartz- 

 feldspar-micro-pegmatite. 



This vast assemblage of doleritic or gabbroidal sheets totaling originally 100,000 cubic 

 miles is found in various horizons in the Karroo system, being distributed through a range of 

 over 10,000 feet of strata. Du Toit ('20) considers that the higher masses were injected first, 

 and were followed successively by those lower and lower in the series. The differentiation is 

 described as due to "fractional crystallization combined with sinking of crystals so formed in a 

 body cooling from above downwards." As crystallization en masse extended downward from 

 the roof, the cake of nearly solid gabbro would be resting on a "lens" of partly fluid ultrabasic 

 matter, the consolidation of which had been deferred by its higher temperature, the presence 

 of mineralisers, and interstitial sulphides. From this the residuum was squeezed out and we 

 may note that the dioritic and granitic veins and lenticles which are absent in the higher levels 

 are most characteristic of the basal zone, and even penetrate the underlying floor. With these 

 are associated the sulphide-ores, the formation of which is described by Goodchild ('18). The 

 production of the gneissic structure in the ultrabasic rocks is explained in the manner suggested 

 by Bo wen ('19). 



We may perhaps class with intrusive basic rocks the series of volcanic pipes filled with 

 "kimberlite" which is so marked a feature of South African geology. There has been a great 

 deal of discussion concerning these, some authorities holding that the rock is a porphyritic 

 consolidation from a peculiar type of ultrabasic magma, like our "alkaline peridotite" but the 

 official geologists in South Africa are in accord with the view put forward by Dr. Bonney ('99), 

 that the rock is a breccia composed of ultrabasic constituents, though holding that the rounding 

 of the fragments is due to attrition in the vent (Rogers '05) . Du Toit considers that the breccia 

 consists of material of a threefold origin. Its minerals and rock-fragments are derived from (a) 

 gneisses and schists; (b) gabbros and ultrabasic rocks, also eclogites; and (c) ultrabasic rocks, 

 possibly limburgite, which as a lava brought up the fragments and became itself consolidated 

 and completely brecciated in the process. In addition fragments of Mesozoic rocks that have 

 fallen from higher levels are found in the pipes (Du Toit '06). Carvill Lewis's ('97) view of the 

 original presence of a magma allied to melilite-basalt, the brecciated products of which are 

 incorporated in kimberlite is supported by the occurrence in a pipe at Sutherland Commonage 

 of massive melilite-basalt with xenocrysts and xenoliths of the same characters as in kimberlite 

 (Rogers '05). Near Prieska dikes of lamprophyres allied to monchiquite, melilite-basalt, 

 (alnoite) and perhaps camptonite occur (Du Toit '08). Lacroix ('98) found nepheline in place 

 of melilite in one instance. The necks or dikes of kimberlite-breccia were formed in probably 

 Cretaceous times, apparently without any accompanying folding or faulting, except the mar- 

 ginal subsidences around the great continental block of South Africa. Possibly we may com- 



