Part 3

The final phase, that of the minor intrusions, is admirably illustrated in Egypt, large areas of the desert consisting of low ridges formed of parallel bands or dykes of hard rocks, which have seamed the softer granite, the latter having subsequently been worn down by erosion. In other cases the dyke is of more basic composition and more easily denuded than the adjacent rocks.

When these evidences of past igneous activity are examined, they reveal the interplay of such varied and complicated chemical and physical effects that a large volume would be required to set forth the great body of facts observed. Of these only a bare statement of the most incomplete order must suffice. Igneous rocks are of very variable chemical composition, and for the purposes of discussion of their complex relationships have been subdivided into three series according to the amount of silica they contain, viz.:—The Acid Rocks with 50 to 70 per cent silica; The Intermediate Rocks with 50 to 60 per cent silica; The Basic Rocks under 50 per cent silica. As a whole, the igneous rocks are compounds of the silicates of alumina, magnesia, lime, potash and soda, with oxide of iron; the silicates of alumina, soda and potash are most abundant at the acid end of the scale, and those of lime, magnesia, with the oxides of iron and phosphates at the basic extreme. In consequence, the basic rocks are usually darker in colour and heavier than the more acid varieties. Not only have the igneous rocks a varying composition, but also a varying structure depending on the differences of origin and position. We may note their history as involving several stages, viz.:—(1) Solidification from the initial molten magma; (2) Deformation when under the influence of great earth pressures; (3) Transformation under the meteorological influences, at or near the earth’s surface. In addition there are the contact alterations produced near their junctions with the overlying strata, both within themselves and in the beds affected by the molten material.

A molten magma varies greatly in its behaviour according to its position, the solidification of that portion which remained deep-seated in the hidden reservoir beneath the earth’s crust taking place slowly and imperceptibly. A rock such as granite is not formed suddenly by instantaneous cooling, but step by step individual minerals are crystallized from the complex solution, and tend to develop as regular and often most beautiful crystals. It is a source of surprise to most to learn that the greater number of mineral substances, if free to develop from solution without external hindrances, tend to form solid bodies which are fashioned on geometrical principles, constituting one of the most interesting and mysterious problems with which scientific thought has to deal. The minerals, as a rule, tend to crystallize out directly in the order of their basicity, the oxides of iron with their eight-sided crystals being among the first, so that their octahedral outlines are usually well preserved. The other minerals of the rocks follow in succession; sometimes complicated intergrowths occur owing to two crystals of different composition starting their formation at the same time in the same portion of the solution. Finally, the last-formed minerals have to occupy the interspaces left by their consolidated neighbours, and in the more acid rocks this unenviable position is usually reserved for the free silica, forming _quartz_. These deep-seated masses, slowly solidified and entirely crystalline in their structure, have been called the _Plutonic_ rocks, the example familiar to all dwellers in Egypt being the red Aswan, or monumental granite. In it, the felspars and micas or hornblendes present tend towards their true crystal form, while the quartz occupies the interspaces.

Intermediate between the extruded volcanic rocks, and the highly crystalline varieties of the internal reservoirs of molten material, are the dyke or hypabyssal rocks, which as narrow vertical or nearly vertical intrusions pass in all probability from the deep-seated source towards the surface of the earth, when they reach it giving rise to volcanic effects. The dykes have a tendency to be parallel to one another, and in some regions are the most conspicuous features, if harder than the surrounding rock giving rise to marked ridges separated in many cases by comparatively shallow valleys. Not a few of the main summits in the Red Sea hills and Sinai have assumed their present form and outline owing to the unequal denudation of one of these hard bands, while many an ascent, which otherwise would have been by no means free from danger, is simplified by the formation of gullies due to the wearing away of the softer and more basic members of this series.

In the hypabyssal varieties, as well as in surface flows, an examination of hand-specimens often reveals the presence of larger crystals (usually termed porphyritic crystals) scattered in a ground-mass in which no definite structure is visible except under the microscope. These may in part have been formed before the molten magma of the rock had begun its upward ascent, but in general they and the finer-grained base, have crystallized under similar conditions. Examinations of the ground-mass microscopically also shows it to be entirely crystalline in its nature, but the individual components are of very small size.

In the extruded volcanic rocks, on the other hand, the porphyritic crystals are probably of much earlier date than the ground-mass in which they are present, the latter being solidified above-ground, where cooling was rapid, pressure suddenly reduced, and many of the gaseous constituents were free to escape readily. As crystals require a certain temperature and a slow liberation of the heat for their formation, in many volcanic rocks the rapid cooling results in the production of a glass instead of a crystalline aggregate, so that the presence of a glassy matrix is in itself evidence of a quick loss of heat.

VII.—THE FORMATION OF METAMORPHIC ROCKS.

All materials, both sedimentary and igneous, composing the earth’s crust, are liable to be involved in movements of deformation, becoming subject to pressures and chemical agencies, which are often intense in character and productive of far-reaching internal changes. The result is the formation of a series of rocks, which may be of divers origins, but which having lost their original characteristics, may be grouped as metamorphic. By the action of compression and dislocation not only are the major masses folded or broken, but their individual constituents are brought into new relations both as regards their external conditions and internal molecular arrangement. Where lateral pressure has been active as regards the finer-grained rocks, such as clays, the individual particles arrange themselves with their longer axes at right angles to the pressure, the result being the production of a fissile structure or cleavage, totally distinct from the original bedding-planes of the strata.

Clays under such circumstances pass into slates, in which no very definite mineral banding is observed, or into schists, in which there is a tendency for the separate minerals, such as mica and quartz, to have a pseudo-stratified appearance, though the individual layers, if closely examined, are found to be lenticular, not parallel in shape. These schists and slates are developed on a large scale in the Red Sea hills and Sinai.

The extent to which compression and tension has been carried in these ancient sedimentary strata is well indicated in Sinai, where rounded pebbles of quartz have been fractured, the two broken parts somewhat separated, and the whole re-cemented in a schistose matrix.

In many instances, besides mechanical stress, another factor of metamorphism of rocks has to be invoked to explain the phenomena observed. Mineral changes take place in these rocks which are due to the superadding of thermal action, consequent on their direct contact with molten masses of granitic and other plutonic rocks. In Sinai, for instance, new minerals are called into existence by these contact effects, members of the _andalusite_ group, which are formed at high temperatures, being produced near the junctions, garnets being present in the mica-schists in a zone somewhat further removed from the contact area, while beyond these is a belt where the slaty strata show only traces of the formation of the new minerals in the presence of ill-defined knots, or segregations forming the so-called “Knotenschiefer” of the Germans.

It is possible to trace the influence of these contact and dynamic effects, not only in the sedimentary rocks, but also in those of plutonic origin. An excellent example of such changes, known as foliation, can be observed in the Dal Cataract, where a granite is present containing large crystals of red felspar. It is possible to trace the change of this rock into a variety which instead of being massive, is banded, the bands containing the larger red felspars all tending to lie in one plane. These are not parallel, but lenticular, certain of the minerals, especially the felspars, remaining comparatively unaffected as “eyes,” round which the other minerals appear to sweep. The quartz in many instances is found to be crushed into a mosaic of small grains, and the plates of mica have been dragged out and separated from one another by the shearing. These rocks, having a general composition resembling the granites, have been termed gneisses, and in some cases, similar effects have been produced by the intrusion of veins of granite along the horizontal planes of a more basic rock already banded. This result is well seen in the Third Cataract, where a foliated rock composed of a white felspar and mica has been pierced by veins of highly quartzose granite, which has then spread along the laminæ of the older member.

Similarly in Sinai, the granite, where in contact with the schistose sedimentary beds, has in certain favourable localities penetrated between the schistose layers, producing a banded rock which approaches a gneiss in character.

In Egypt, we are not at present acquainted with such gigantic overfolding, fracture, and pushing of whole masses of strata one over the other, as have been studied, for example, in the North-West Highlands of Scotland, but there is in Egypt itself, a vast series of gneisses, crystalline schists, marbles and dolomites whose origin is but dimly understood, and which may represent changes long anterior those exhibited by the ancient rocks of which we have been speaking. Developed in the untrodden wastes of the Sudan, in the wilder stretches of the Cataracts, in the desolate regions of the Etbai Desert of Egypt, their study is attended with difficulty and all our hypotheses must depend on the comparison of isolated specimens with similar rocks from more favoured regions.

SUMMARY.

1.—Every rock originates either from the solidification of molten material or of sediments, these constituting the Igneous and Sedimentary Rocks respectively.

2.—These may undergo deformation due to movements of the earth’s crust, such movements according to their intensity determining the re-arrangement of the constituents with the development of new structures, producing the Metamorphic Rocks.

3.—These varied rocks, when exposed at the earth’s surface, are brought under the influence of the agents of transformation, such as frost, contraction and expansion due to variations of temperature, the mechanical action of rain, sand borne by wind, and running water in the form of streams. In addition to these purely physical activities are others of a chemical nature, either proceeding from changes outside the terrestrial surface (carbonic acid dissolved in rain, etc.), or from processes operating within the earth’s crust (production of petroleum, coal, mineral veins, etc.).

4.—The chemical and physical activities deep within the earth’s crust result in the formation of the plutonic rocks solidified from molten solutions, intimately associated with them being the external manifestations of the working of these hidden forces as revealed in volcanoes and their associated phenomena.

5.—By the surface changes on the consolidated igneous and other rocks, the materials are supplied for the formation of some of the members of the sedimentary series, the sands and clays, while the limestones in large measure owe their origin to the property possessed by living animals of constructing shells from the carbonate of lime brought in solution to the oceans, etc., these shells, after the death of the animals, contributing to the formation of the calcareous rocks.

FOOTNOTES:

[Footnote 1: In geology, the term _rock_ is applied alike to the soft and hard materials composing the earth’s crust, as sands may pass into solid sandstones, soft muds become the most tenacious of clays, and hard basalt originate as a molten lava.]

[Footnote 2: This statement may not absolutely apply to Cairo itself, seeing that in the disturbed region of Abu Roash sandstone is actually at the surface, and the clays are less conspicuously developed than is the case to the south.]

[Footnote 3: Figs. 1, 2 and 4 are based on photographs of localities in Great Britain and Ireland taken for the Geologists’ Association collection of photographs.]

[Footnote 4: Farafra is an exception to this rule, the depression being due to the wearing away of soft Cretaceous beds higher than the sandstone.]

[Footnote 5: The well-known Petrified Forests.]

[Footnote 6: From information given by Mr. Harding King.]

[Footnote 7: At Abu Radham a dark layer of purple iron ochre seems to have been the cause of the investigation.]

[Footnote 8: For further details, see my paper “Petroleum: Its occurrence and origin,” Cairo Scientific Journal, No. 48, pp. 205-218.]

[Footnote 9: “The Natural History of Igneous Rocks,” p. 23.]

SHORT CATALOGUE OF THE =MAPS, PLANS, AND PUBLICATIONS= ISSUED BY THE SURVEY DEPARTMENT, MINISTRY OF FINANCE, EGYPT. * * * * *

=MAPS AND PLANS.= * * * * *

The following is a general list of the maps and plans offered for sale by the Survey Department. A booklet giving details of all sheets printed may be obtained free, on application either personally or by letter at the Headquarters of the Department, Giza (Mudiria), or at the Geological Museum, Public Works Ministry Gardens, Cairo, where all maps and plans are for sale, or through any bookseller.

Except where specially stated, the price of each map-sheet is 50 milliemes on paper, and 65 milliemes on cloth, and they are sent post free by the Department.

The reference marks denote: (*) map is in Arabic only; (†) map is in English only; (*†) map bears place-names both in Arabic and English; (*) (†) map can be obtained either in Arabic or English.

=Town Maps.=

The following list gives particulars of the maps published. The map of Alexandria, on the scale of 1:1,000 will be completed during 1911. The survey of Cairo on the scale of 1:1,000 is in progress.

Cairo (*†), 30 sheets, scale 1:1,000 (in preparation).

Alexandria (*†), 147 sheets, scale 1:1,000.

General map of Alexandria Municipality (French and Arabic), 10 sheets, scale 1:6,000.

Mit Ghamr (*†), 4 sheets, scale 1:1,000.

Mansura (*†), 16 sheets, scale 1:1,000.

Suez (*†), 20 sheets, scale 1:1,000.

Suez (*†), 1 sheet, scale 1:2,500.

Sohag (*†), 6 sheets, 1:1,000.

Tanta (*†), 15 sheets, scale 1:1,000.

Girga (*†), 6 sheets, scale 1:1,000.

Aswan (*†), 23 sheets, scale 1:1,000.

Port Said (in French), 1 sheet, scale 1:5,000.

Zagazig (*†), 20 sheets, scale 1:1,000.

Damanhur (*†), 14 sheets, scale 1:1,000.

Benha (*†), 25 sheets, scale 1:1,000.

=Cadastral Maps.=

These are maps of the villages showing each _hod_ and plot of land. They are printed in Arabic only. In ordering, the name of the village and the numbers of _hod_ and plot should be given. The following list gives the particulars of the maps for each mudiria (province):—

Beheira mudiria (*), 3,300 sheets, under survey, scale 1:2,500.

Gharbia mudiria (*), 3,460 sheets, scale 1:4,000 and 1:2,500.

Daqahlia mudiria (*), 2,237 sheets, scale 1:2,500.

Sharqia mudiria (*), 2,974 sheets, scale 1:2,500.

Menufia mudiria (*), 2,173 sheets, scale 1:4,000 and 1:2,500.

Qaliubia mudiria (*), 778 sheets, scale 1:2,500.

Giza mudiria (*), 766 sheets, scale 1:4,000.

Fayum mudiria (*), 2,263 sheets, scale 1:2,500.

Beni Suef mudiria (*), 942 sheets, scale 1:2,500.

Minia mudiria (*), 1,635 sheets, scale 1:2,500.

Assiut mudiria, including Kharga Oasis (*), 2,273 sheets, scale 1:2,500.

Girga mudiria (*), 1,313 sheets, scale 1:2,500.

Qena mudiria (*), 1,568 sheets, scale 1:2,500.

Aswan mudiria (*), 1,076 sheets, scale 1:2,500.

=Topographical Maps.=

Scale 1:10,000 (10 cm. = 1 kilometre; 6·3 inches = 1 mile).—The names on these maps are in most cases in Arabic and English. The following table shows the number of sheets published:—

Beheira mudiria (*), 260 sheets.

Gharbia mudiria (*†), 213 sheets.

Sharqia mudiria (*†), 29 sheets.

Daqahlia mudiria (*†), 11 sheets.

Menufia mudiria (*†), 73 sheets.

Qaliubia mudiria (*†), 65 sheets.

Giza mudiria (*†), 90 sheets.

Fayum mudiria (*†), 126 sheets.

Beni Suef mudiria (*†), 21 sheets.

Assiut mudiria, including Kharga Oasis (*†), 72 sheets.

Aswan mudiria (*†), 63 sheets.

Aswan or First Cataract (†), 6 sheets.

The Nile Valley from Aswan to Korosko(†), 36 sheets (paper only, 25 milliemes each).

Scale 1:25,000 (4 cm. = 1 kilometre; 2·5 inches = 1 mile).—A provisional map of Northern Gharbia has been published on this scale, pending the publication of the 1:10,000 sheets of this area. There are 91 sheets.

Scale 1:50,000 (2 cm. = 1 kilometre; 1·3 inches = 1 mile).—These maps are printed in three colours. Names are given in English, and as a rule in Arabic as well. This series is completed for the whole of the cultivated area of the Nile Valley and Delta. There are 145 sheets.

Scale 1:1,000,000 (1 cm. = 10 kilometres; 1 inch = 16 miles).—The six sheets of this map, covering the whole of Egypt, have now been published. The names are in English. The price of each sheet is 50 and 65 milliemes for paper and cloth editions respectively, or the whole can be obtained mounted on cloth, varnished, and fitted with rollers for 550 milliemes.

=Special Maps on Various Scales.=

Map of the Delta (†), 4 sheets, scale 1:200,000. Price, 75 milliemes per sheet, or the complete map mounted on cloth, varnished and fitted with rollers, 700 milliemes.

Lower Egypt and the Fayum, 1904 (latest edition) (†), 1 sheet, scale 1:500,000.

Lower Egypt, showing lines of communication (†), 1 sheet, scale 1:500,000.

Northern Gharbia (*†), 1 sheet, scale 1:200,000.

Kharga Oasis (†), 1 sheet, scale 1:500,000.

Dakhla Oasis (†), 1 sheet, scale 1:500,000.

Baharia Oasis (†), 1 sheet, scale 1:500,000.

Farafra and Iddalia Oases (†), 1 sheet, scale 1:500,000.

Provisional map of the Eastern Desert of Egypt, East Qena-Aswan to Red Sea (†), 20 sheets, scale 1:100,000.

Provisional map of the Eastern Desert of Egypt, between Qus, Sayala and Red Sea (†), 2 sheets, scale 1:500,000.

Provisional map of a part of the Eastern Desert Oilfield (†), 1 sheet, scale 1:100,000. Price, 100 milliemes on paper and 150 milliemes on cloth.

Provisional map of a part of the Eastern Desert Oilfield, showing registered prospecting areas (†), 1 sheet, scale 1:100,000. Price, 100 milliemes on paper and 150 milliemes on cloth.

Red Sea and Sinai Oilfield, showing registered prospecting areas (†), 1 sheet, scale 1:316,800. Price, 100 milliemes on paper and 150 milliemes on cloth.

Jemsa Oil Zone (†), 1 sheet, scale 1:75,000 and 1:250,000. Price, 50 milliemes.

Mersa Matruh chart (†), 1 sheet, scale 1:4,500.

Mersa Matruh topographical map (†), 1 sheet, scale 1:10,000.

Mersa Matruh and Ras Allam Rum (†), 2 sheets, scale 1:25,000.

Aqaba-Rafa, 1906 (*†), 3 sheets, scale 1:100,000.

Aqaba-Rafa, 1906 (*) (†), 1 sheet, scale 1:500,000 (paper, 25 milliemes; cloth, 40 milliemes).

The Nile Valley from Aswan to Sudan boundary (†), 1 sheet, scale 1:250,000.

Port d’Alexandrie (French), 3 sheets, scale 1:4,000.

=Wall-Maps, for use in Schools.=

The price of each map is 700 milliemes, except that of the Mediterranean Basin, which is 500 milliemes. Each map is mounted on cloth, varnished, and fitted with rollers. The following list gives the maps published and in preparation:—

Africa (physical) (*), scale 1:6,000,000.

Africa (political) (*), scale 1:6,000,000.

The Nile Basin (*) (†), scale 1:2,500,000.

Egypt (*) (†), scale 1:750,000.

The Delta and the Fayum (*), scale 1:200,000.

Mediterranean Basin (*), scale 1:3,000,000.

Western Europe and the British Isles (*), scale 1:1,500,000.

Asia (physical) (*), scale 1:6,000,000.

Asia (political) (*), scale 1:6,000,000.

Europe (physical) (*), scale 1:3,000,000.

Europe (political) (*), scale 1:3,000,000.

The World (*) (Mercator’s projection).

The Hemispheres (Eastern and Western) (Airy’s projection)

British Isles, scale 1:750,000.

North America (*) (physical) 1:6,000,000.

North America (*) (political) 1:6,000,000.

IN PREPARATION:—

South America (physical) 1:6,000,000.

South America (political) 1:6,000,000.

=Geological Maps.=

Geological map of Egypt, scale 1:1,000,000. English. Six sheets, 70 × 58 cm. Price, 100 milliemes per sheet. Complete map, mounted on cloth, varnished and fitted with rollers, 850 milliemes.

Geological map of Egypt, scale 1:2,000,000. English. One sheet, 68½ × 67 cm. Price, 200 milliemes on paper, and 300 milliemes mounted on cloth and fitted with rollers.

A number of maps have been published in the various Geological reports. Further information may be obtained under the respective headings in the list of Geological Reports, pp. V and VI.

=PUBLICATIONS.= * * * * *

The following is a general list of the publications of the Survey Department, and a few others which are for sale at the Headquarters of the Department, Giza (Mudiria), and at the Geological Museum, Public Works Ministry Gardens, Cairo. A booklet giving full details can be obtained, on application either personally or by letter.

Except where specially stated, the publications are 8vo, and in English, and are supplied post free by the Department. They can also be obtained through any bookseller.

=Archæology.=

ARCHÆOLOGICAL SURVEY OF NUBIA.

BULLETIN 1.—Dealing with the work (archæological and anatomical) from September 20 to November 30, 1907. English. 39 pp., 27 illustrations. (Out of print).

BULLETIN 2.—Dealing with the work (archæological and anatomical) from December 1, 1907, to March 31, 1908. English. 69 pp., 52 illustrations. Price, 100 milliemes.

BULLETIN 3.—Dealing with the work (archæological and anatomical) from October 1 to December 31, 1908. English. 52 pp., 5 illustrations. Price, 100 milliemes.

BULLETIN 4.—Dealing with the work (archæological and anatomical) from January 1 to March 31, 1909. English. 28 pp., 2 illustrations. Price, 100 milliemes.

BULLETIN 5.—Dealing with the work (archæological and anatomical) from October 1 to December 31, 1909. English. 35 pp., 5 illustrations. Price, 100 milliemes.

BULLETIN 6.—Dealing with the work (Archæological and anatomical) from January 1 to April 15, 1910. English. 30 pp., 8 illustrations. Price, 100 milliemes.

ANNUAL REPORT OF THE ARCHÆOLOGICAL SURVEY OF NUBIA, SEASON 1907-8. VOL. I: by GEORGE A. REISNER. Price, with volume of plates, L.E. 2.

ANNUAL REPORT OF THE ARCHÆOLOGICAL SURVEY OF NUBIA, SEASON 1907-8. VOL. II: Report on the Human Remains, by Dr. G. ELLIOT SMITH, F.R.S., and Dr. F. WOOD JONES. Price, with volume of plates, L.E. 2.

PHILÆ,—REPORT ON THE ISLAND AND TEMPLES OF, by CAPT. H. G. LYONS, with introductory note by W. E. GARSTIN. 1896. English. 67 pp., 78 illustrations. (Out of print).

PHILÆ,—REPORT ON THE ISLAND AND TEMPLES OF, by CAPT. H. G. LYONS. 1908. English. 4to, 32 pp., 14 illustrations. Price, 200 milliemes.

=Geography.=

RIVER NILE AND ITS BASIN—PHYSIOGRAPHY OF THE, by CAPT. H. G. LYONS. 1906. 411 pp., 14 maps, 34 illustrations. Price, 400 milliemes.

TURCO-EGYPTIAN BOUNDARY BETWEEN THE VILAYET OF THE HEJAZ AND THE PENINSULA OF SINAI—THE DELIMITATION OF THE, by E. B. H. WADE, together with additions by B. F. E. KEELING and J. I. CRAIG. 1906. (Survey Department Paper, No. 4). 89 pp., 2 maps. Price, 150 milliemes. See also Geology.

=Geology.=

ABU ROASH, NEAR THE PYRAMIDS OF GIZA—CRETACEOUS REGION OF, by H. J. L. BEADNELL. 1902. 48 pp., 2 maps, 19 illust. Price, 200 milliemes.

ARSINOITHERIUM ZITTELI (Beadnell), FROM THE UPPER EOCENE STRATA OF EGYPT—PRELIMINARY NOTE ON, by H. J. L. BEADNELL. 1902. 4 pp., 6 illustrations. Price, 50 milliemes.

ASWAN (FIRST) CATARACT OF THE NILE—DESCRIPTION OF, by DR. BALL. 1907. 121 pp., 5 maps, 28 illustrations. Price, 200 milliemes.

BAHARIA OASIS, ITS TOPOGRAPHY AND GEOLOGY, by DR. BALL and H. J. L. BEADNELL. 1903. 84 pp., 8 maps, 2 illust. Price, 200 milliemes.