For purposes of classification, the igneous rocks of South-Eastern Egypt may be divided into five main groups, based on silica-percentage:— 1. _Ultra-acid rocks_, containing over 80 per cent of silica. 2. _Acid rocks_, containing from 65 to 80 per cent of silica. 3. _Intermediate rocks_:— (_a_) _Sub-acid rocks_, with from 60 to 65 per cent of silica. (_b_) _Sub-basic rocks_, with from 55 to 60 per cent of silica. 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.