4. [Basic rocks,] containing from 45 to 55 per cent of silica.
5. [Ultra-basic rocks,] with less than 45 per cent of silica.
The classification is based on silica percentage, for although up to the present it has not been found possible to undertake the chemical analysis of the rocks, the microscopic examination of thin sections itself gives a very fair guide to the chemical composition, and the specific gravities of the different rocks, which increase progressively from the acid to basic groups, afford a further guide in this direction. The table on the following page shows the different rocks comprised within the five main classes.
The five classes are not all of equal importance. As already mentioned, the acid and basic types are predominant. The ultra-acid and sub-acid rocks are present in relatively small quantity, and are in close relationship with those of the acid series which occur in great abundance; the rocks of the sub-basic and ultra-basic groups, on the other hand, though by no means insignificant in their distribution (diorites and serpentines cover very large areas), appear to be closely linked in the field with those of the basic group. The district is so large, and the investigation of it has been so limited, that it would be unwise to draw from the above remarks the deduction that the rocks have originated from two main magmas; the statements are merely those of the facts of observation, so far as observation has proceeded.
The scheme tabulated above differs from established classifications only in the inclusion of an ultra-acid class of rocks. This inclusion is necessitated by the presence in the district of huge masses of quartz-rocks which are almost certainly igneous in origin. These rocks are almost pure silica, and though they are properly regarded as an extreme form of pegmatite, their silica-percentage (over 95) is too high for them to be placed with their near relations the granites; and on grounds of symmetry of classification it appears justifiable to have an “ultra-acid” class corresponding with that of ultra-basic rocks.
Classification of Igneous Rocks.
| Ultra-acid. | Acid. | Intermediate. | Basic. | Ultra-basic. | |
|---|---|---|---|---|---|
| Sub-acid. | Sub-basic. | ||||
| Quartz-rock. | Granite. | Syenite. | Diorite. | Gabbro. | Pyroxenite. |
| Normal pink granite. | Augite-diorite. | Troctolite. | Amphibolite. | ||
| Red pegmatitic granite. | Mica-diorite. | Pyroxene-granulite. | Peridotite. | ||
| Aplite. | (Serpentine). | ||||
| Biotite-granite. | |||||
| Muscovite-granite. | |||||
| Hornblende-granite. | |||||
| Granite-porphyry. | Syenite-porphyry. | Diorite-porphyrite. | Diabase. | ||
| Augite-porphyrite. | Mica-diabase. | ||||
| Kersantite. | |||||
| Quartz-felsite. | Trachyte. | Andesite. | Basalt. | ||
ULTRA-ACID ROCKS.
Quartz-rock of igneous origin occurs in the form of intrusive bosses and veins penetrating older igneous and metamorphic rocks at Marwot Elemikan and elsewhere. The rock is practically pure vitreous quartz. Its dazzling white aspect in the sun renders it very conspicuous in the field. The shape of many of the masses (bosses), their close association with pegmatite and aplite in at least one locality, the absence from the veins of other minerals (calcite and various ores) such as usually accompany vein-quartz deposited from solution, and the occasional presence in the rock of mica flakes, all point to an igneous origin, and justify us in regarding this quartz-rock as the final product of differentiation of a granitic magma, representing its ultra-acid portion.
The most conspicuous occurrence of quartz-rock is in the three nearly conical hills called Marwot[125] Elemikan, which rise from the granitic plain at the head of Wadi Elemikan in latitude 23° 53′, and which from their snow-white colour form landmarks visible from great distances. The largest and most northerly hill of the three is a boss about 300 metres in diameter, rising sixty-eight metres above the plain, which itself has here an altitude of 580 metres above sea. The other two hills are somewhat smaller; they lie respectively two kilometres south-east, and two and half kilometres south, of the largest one; a black hill of about equal size rises from the plain mid-way between them. The highest of the three white hills, which was occupied as a triangulation station, is composed almost entirely of glassy quartz, the only other mineral seen in it being a white mica, of which a few flakes were visible in the outer portions. The mass is cracked in all directions, and the faces of the separated blocks frequently show a slight yellowish staining.