part 2] BEOLOGl OF THE MAEBLE DELTA. 135 



formed pyroxene (as shown by the peripheral development of 

 hornblende) seems to be in process of resorption. 



The account recently given by me l of an instance of the de- 

 silication of originally acid dykes cutting serpentine, whereby 

 silica, potash, boron, and fluorine have passed out of the latter, 

 leaving the magma with an excess of alumina that has separated 

 as corundum, has a hearing upon the action under discussion. 



In addition, a concentration of titanium-oxide and sometimes 

 of sulphides is brought about, and sphene, rutile, and pvrrhotite 

 appear, mostly in the dyke-margin, but also just within the 

 country-rock. 



Scales of graphite are to be found in some of the granite- 

 injections: for example in a thin sill traversing the fourth quarry 

 on The Glen, where the mineral was fairly abundant in the mar- 

 ginal portion, while in a second vein the graphite was abundant 

 only at the very apex of the intrusion and yet hardly represented 

 in the marble adjoining thereto; mention has already been made 

 of this mineral in a dyke on La Joncquet. 



The occurrence of graphite in nepheline-syenite cutting graphite- 

 bearing limestones on the Botogolski Golez in Siberia- may be 

 cited as a parallel, while the mineral has also been noticed in 

 pegmatite near Ampe in Ceylon by F. Griinling and at Ticonderoga 

 (New York County) by J. F. Kemp. It was discovered also 

 in a peculiar calcite -bearing diopside- hornblende -granulite on 

 Xdongini and La Joncquet — possibly a recrystallized basic sill — 

 close to the horizon upon which the marbles carry graphite, as 

 mentioned earlier. 



The origin of the graphite in these Natal occurrences may be 

 ascribed either to small amounts of hydrocarbons set free during 

 the metamorphism of the marbles, or to the absorption of such 

 (carbonaceous) country-rock, rather than to the reduction of 

 carbon dioxide by hydrogen or hydrofluoric acid gases. 



Coming finally to the problem of calcitization, we note an 

 analogous occurrence in The Dolomite, near Potgietersrust in the 

 Transvaal, at the contact with intrusive pyroxenite ; but, in this 

 case, Dr. B. B. Young 8 has shown that after dedolomitization the 

 serpentine pseudomorphs were removed, and the cavities so formed 

 filled in with secondary calcite from above, presumably by the 

 action of surface-water-. 



At the Marble Delta the following deductions can be drawn. 

 First, the rock thus altered was a nearly pure magnesian lime- 

 stone to start witli : secondly, calcitization was only effected along 

 certain of the contacts ; thirdly, men' temperature could not have 

 been an important factor, since, as we have seen, the alteration 

 in one case is asymmetrical with regard to the dyke: fourthly, 

 dolerite- dykes not improbably have no such effect, for one at 



1 Trans. Geol. Sue. s. Africa, \>>\. wi (1918) p. •">:!. 



2 L. Jaczcwski. Neues Jahrb. 1901, p. 7t: sec also 0. Stntzer, Zeitsohrift 

 f. Praia. Geol. 1910, p. 1<>. 



:t Trans. Geol, Soc. S. Africa, vol. xix (l!»l<;i p. :»7. 



