Graphite-schists.
A considerable mass of graphite-schist occurs associated with diorite rocks near some ancient gold workings at Gebel Allawi. The extent of the graphite-schist has not been mapped, but it occurs on the north slope of the mountain, and as seen from the summit it appears to run out westward for miles as a broad black band. The rock [10,378], which has a sp. gr. of 2·70, consists of a mixture of graphite with calcareous and earthy matter. The parts richest in graphite are readily cut with a knife, and mark paper easily. Analysis shows that the graphite, though sufficiently abundant to appear in the hand specimen the dominant mineral of the rock, is nevertheless present in surprisingly small proportion; a sample tested by Mr. Pollard was found to contain only a trifle over one per cent. of carbon.
A similar graphite-schist occurs associated with mica-schist in one of the emerald mines of Gebel Sikait, and it may be remarked that graphitic schists have also been observed by Dr. Hume near the lodes in the gold mines of Baramia and Um Garaiart, both of which localities, however, lie outside the limits of the area here described.
The origin of the graphite-schists is uncertain. Perhaps the hypothesis presenting the least difficulty is that which supposes them to have been formed from ancient sedimentary rocks containing the remains of plants; on this view the close association of dioritic rocks suggests that the great of igneous intrusion may have played a part in the distillation of the more volatile matters of the plant remains, while the schistose structure shows that pressure has also been active in the formation of the rock. But the absence of any associated rocks which bear distinct signs of having originally been ancient sediments makes one hesitate to refer the graphite to an organic origin, and possibly the carbonaceous matter may have been produced from quite another source than that of plant life.