Under the heading of diabase are included plagioclase-augite rocks, with or without olivine, of a character intermediate between gabbro and basalt. They differ from the gabbros in their finer grain, in the general absence of diallagic structures in the augite, and in the more or less porphyritic nature of their felspars, which are often ophitically intergrown with the augite. They differ from basalts, on the other hand, in being of coarser grain, and typically containing no glassy matter. The diabases of South-Eastern Egypt are more closely allied to the volcanic division of the basic rocks (basalts) than to the plutonic (gabbros), and many of the rocks here classed as diabases would be called dolerites by some English writers.
Fig. 36.—Diabase from under the Nubian sandstone, Rod el Nagi [10,417], × 40. pl, plagioclase felspar, ophitically intergrown with a, augite; l, limonite. The rock also contains large porphyritic felspars, not shown in the figure.
Perhaps the most interesting occurrence of diabase in this part of Egypt is a thick bed underlying the Nubian sandstone and exposed along the feet of its eastern scarps between latitudes 24° 30′ and 25°. Owing to the north-westerly trend of the scarp, it is cut obliquely by the meridian of 34°, and consequently only the southern portion of the deposit is shown on the geological map of [Plate XX.] The bed is well seen on the west of the road leading from Baramia to Dungash mines; this road follows the foot of the scarp for a long distance. The thickness of the diabase sheet as seen on this road is at least ten metres, and is very uniform for several kilometres. The rock is crushed and decomposed to such a degree that it is difficult to get a fair-sized specimen. Below the diabase, at a few points along the road, there are exposures of an ancient conglomerate of dark colour and extreme hardness. The relations of the bed to the sandstone are not absolutely certain, but the impression I got, when I passed the exposure on the two occasions of my outward and return marches, was that the diabase was an extensive outflow over the old conglomerate (thus indurating the latter), the sandstone being subsequently laid down on the undenuded diabase. The diabase is thus probably Cretaceous in age. A hand specimen [10,417] from one of the least decomposed portions of the sheet, in the Rod el Nagi, about fifteen kilometres west of Gebel Muelih, shows white to glassy porphyritic felspar crystals, frequently in lath-shaped forms measuring up to eight millimetres in length, in a fine-grained grey ground mass (see [Plate XXIV]). The rock contains many rusty specks, and occasionally as one turns the specimen about in the hand one can catch a rather dull flash from a crystal of some dark mineral. The sp. gr. of the rock is 2·85. Under the microscope the porphyritic felspars, which are very clear, are seen to be mostly plagioclase, though some of the crystals show simple twinning and may be orthoclase. The ground mass is holocrystalline, composed of felspar, augite, and limonite. The felspars of the ground mass are in the form of thin laths, with repeated twinning. The augite is nearly colourless, with a very pale brownish or greenish tinge, generally clouded by brown dusty matter and small irregular cracks. It forms irregular grains which are cut up in all directions by the ophitically intergrown felspars. The limonite is extremely abundant, scattered through the ground mass in rather large irregular grains which frequently show a tendency to square or hexagonal outlines; it is mostly opaque, but in some parts it is translucent, with a deep brown colour. The limonite is doubtless an alteration product of other minerals, probably magnetite and biotite, of which, however, no distinct traces now remain.
Besides forming the sheet under the Nubian sandstone, diabase occurs fairly abundantly in schists at various points, where it appears to form intrusions. A characteristic of these occurrences is its weathering into rusty looking brown “cannon balls.” Altered forms of diabase are associated with peridotites and other ultra basic rocks in the serpentine mass of Gebel Gerf (see [p. 328]). Diabase also forms veins, frequently magnetic, penetrating granites and schists, and at one point, near the foot of Gebel Awamtib, a dyke of diabase is even found penetrating the Nubian sandstone.
It is probable that these diabases are of very different ages at different points. While the occurrences associated with the sandstone are certainly of Cretaceous age, it seems difficult to imagine that those associated with the schists and serpentines are not vastly older; for though they are obviously younger than the schists, and possibly younger than the peridotites, they frequently show metamorphism to a degree which it is difficult to reconcile with a Cretaceous age, and which suggests that they antedated the folding which formed the ancient mountain chain on the flanks of which the Cretaceous strata were laid down.
Fig. 37.—Diabase, Gebel Abu Hamamid [10,400], × 40. a, augite; aa, augite, altering with separation of iron oxide; pl, plagioclase, often ophitically intergrown with the augite; s, green serpentinous mineral; m, magnetite.
As an example of a diabasic intrusion with “cannon ball” weathering, we may take the rock [10,400], which occurs in schists on the flanks of Gebel Abu Hamamid. When one of the rusty looking “cannon balls” is broken, the interior is seen to be a dark grey rock of very fine grain. The sp. gr. is 2·93. The microscopic slide shows the rock to be holocrystalline, consisting mainly of augite and plagioclase, ophitically intergrown, with some rather large grains and strings of a nearly isotropic clear yellow-green mineral, and scattered grains of iron oxides. The augite is of a pale brown colour; some of the crystals are fairly clear, while others show strong clouding and separation of iron oxide. The nature of the yellow-green mineral is somewhat uncertain; it polarises generally in very low colours, as an aggregate of fibres and tiny plates, sometimes showing a spherulitic structure between crossed nicols. Occasionally it includes granules of a highly refracting colourless mineral, possibly olivine; but there is a remarkable absence of the separated iron oxide which is usual with altering olivine, and the granules exhibit only very fine irregular hairlike cracks; if it is serpentine resulting from alteration of olivine, the olivine must have been a variety poor in iron.
