The topography and geology of the Fayum province of Egypt

In the neighbourhood of Qasr el Sagha and westwards for a considerable distance, small strike-faults are of common occurrence in the beds of the Qasr el Sagha series. As a rule these faults do not extend more than a few hundred metres in length, while the down-throw seldom exceeds two or three metres and in almost every case observed is to the north. The hade may be 65° or more. Fig. 1 shows an example near Qasr el Sagha. The most marked of these strike faults is seen to the east of Garat el Esh; commencing a little to the north-east of that hill it runs in a nearly due easterly direction till it cuts the cliffs of the Qasr el Sagha series after some five kilometres. Its down-throw is to the north and never exceeds a few metres; this small throw is however sufficient to cause a marked displacement of the highest bed of limestone forming the dip-slope surface of the plateau at the summit of the Middle Eocene beds.

At first sight it might be suspected that the very irregular trend of the different escarpments throughout the Fayûm was determined or influenced by fault lines; an extended examination of the cliffs however gave negative results, with one exception; the long narrow hill-mass to the north east of Gar el Gahannem is bounded by faults on both sides and that on the west can be easily traced for seven or eight kilometres northwards, and throughout its length its influence on the topography is very conspicuous.

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PART III.

GEOLOGY.

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Section IX.—GENERAL AND CLASSIFICATION OF STRATA.

The geology of the area[33] under consideration is almost entirely stratigraphical, the only igneous rocks being more or less local lava flows. The sedimentary rocks of the district have yielded an abundant fauna, both invertebrate and vertebrate; the latter is of unique interest, including as it does a number of highly interesting animal types quite new to science. An extended examination in the field, and comparisons with the stratigraphical succession in other parts of Egypt, checked by the determinations of the fossil molluscan fauna, make it possible to form a very fair estimate of the approximate age of the different rock-stages, although this may necessarily be subject to modification when the specific determinations of the entire collection of organic remains have been completed, and the development of vertebrate life has been correlated and compared with that in other parts of the world.

The depression is cut out in a great series of sedimentary rocks of Middle Eocene, Upper Eocene, and Oligocene age, and one of the features of the stratigraphy of the region is the constancy of many beds over wide areas. The dip of the beds throughout the area is nearly due north and at a very low angle, averaging 2° or 3°, but varying from 1° to 5°; this low dip is very constantly maintained except when locally affected by small faults. The structural geology and tectonics have already been discussed at some length in the previous sections.

The oldest beds found in the depression are the clays, marls, and limestones with Nummulites gizehensis, of Middle Eocene age. These are succeeded by a group of white marly limestones and gypseous clays, which largely underlie the cultivated alluvium of the Fayûm. They are followed by a series consisting of clays, sandstones, and calcareous grits, some beds of which are characterized by the abundance of small nummulites and Operculina. The latter series is followed by the uppermost truly marine Eocene beds, a group of alternating clays, sandstones and limestones, the “Qasr el Sagha Series” (or Carolia beds), characterized by an abundant invertebrate and vertebrate fauna, and equivalent to the Upper Mokattam beds of Cairo.

Above the Qasr el Sagha series, and well marked off from them both lithologically and palæontologically, is found a great thickness of variegated sands, sandstones, clays and marls, the “Fluvio-marine Series” (Jebel el Qatrani beds), divided near the summit by one or more thick intercalated lava sheets, the latter forming a convenient junction line. This series of variegated beds is of Upper Eocene—Lower Oligocene age.

No Miocene strata have been recognized within the area, but further north, as at Mogara, Lower Miocene deposits occur;[34] and it is probable that there is a continuous series of lithologically similar beds from the summit of the Fayûm escarpments (Lower Oligocene) to the Mogara Miocene.

The Pliocene is probably represented by the great terraces of gravel—raised beaches—which are such a marked feature in the geology of the district. Fossiliferous Pliocene deposits have also been recorded from the south part of the area by Schweinfurth.[35]

Pleistocene and Recent are abundantly represented by lacustrine clays, both ancient and modern, alluvial land and blown sand, the formation of which deposits is continuing at the present time.

The following table will show the sequence of strata and the classification adopted in the present memoir:—

Table showing Succession and Classification of Strata in the Fayum.

			Approximate average thickness in metres, north part of Fayum.
RECENT AND PLEISTOCENE				Alluvial soil, clays, sands, etc.
				Blown sand.
				Lacustrine clays, extending to about 23 metres above sea-level.
(MIDDLE?) PLIOCENE				Gravel Terraces (? Pleistocene).
				Shell-borings on rock surfaces.
			50	Fossiliferous deposits of Sidmant.
LOWER OLIGOCENE	Tongrian			Fluvio-marine Series (Jebel el Qatrani beds).
			30	Sandstones and sandstone-grits with silicified trees and
				Basalt sheets, interbedded and contemporaneous.
UPPER EOCENE	Bartonian.		250	Variegated sands, sandstones, clays and marls, with limestone-grits and thin bands of limestone. The upper beds contain Unio sp., Lanistes bartonianus, Blanck., Turritella pharaonica, Cossm., Potamides scalaroides, Desh., P. tristriatus, Lam., Pleurotoma ingens, May.-Eym. In the lower beds are large numbers of silicified trees associated with vertebrate remains including Arsinoitherium Zitteli, Beadn., A. Andrewsii, Lankester, Palæomastodon Beadnelli, Andr., P. minor, Andr., Mœritherium Lyonsi, Andr., M. trigodon, Andr., Megalohyrax eocænus, Andr., M. minor Andr., Saghatherium antiquum, Andr. and Beadn., S. minus, Andr. and Beadn., S. magnum, Andr., Ancodus Gorringei, Andr. and Beadn., Geniohyus mirus, Andr., G. fayumensis, Andr., G. major, Andr., Phiomia serridens, Andr. and Beadn., Pterodon africanus, Andr., P. macrognathus, Andr., Eremopezus libycus, Andr., Testudo Ammon, Andr., and frequent crocodilian and chelonian remains.
MIDDLE EOCENE	Parisian.	Upper Mokattam	155	Qasr el Sagha Series (Carolia beds).
				Alternating limestones, marls, clays and sandstones with Qerunia (Hydractinia) cornuta, May.-Eym., Astrohelia similis, Felix., Echinolampas Crameri, Loriol., Ostrea Reili, Fraas, Ostrea elegans, Desh., Alectryonia Clot-Beyi, Bellardi, Exogyra Fraasi, May.-Eym., Carolia placunoides, Cantr., Cardita fajumensis, Oppenh., Macrosolen Hollowaysi, Sowerby, Turritella pharaonica, Cossm., T. carinifera, Desh., Mesalia fasciata, Lam., Rimella rimosa, Sol. The vertebrate remains include Mœritherium Lyonsi, Andr., M. gracilis, Andr., Barytherium grave, Andr., Eosiren libyca, Andr., Zeuglodon Osiris, Dames, Gigantophis Garstini, Andr., Pterosphenus Schweinfurthi, Andr., Psephophorus eocænus, Andr., Thallassochelys libyca, Andr., Podocnemis antiqua, Andr., P. Stromeri, v. Rein., Stereogenys Cromeri, Andr., S. podocnemioides, v. Rein., Tomistoma africanum, Andr., with siluroids and Propristis Schweinfurthi, Dames.
		Lower Mokattam	50	Birket el Qurûn Series (Operculina-Nummulite beds).
				Sandstones and clays, with sandy limestones, and one or more well marked concretionary sandstones weathering into large globular masses.
				Nummulites Fraasi, de la Harpe, N. Beaumonti, Operculina discoidea, Schwag., Qerunia cornuta, May.-Eym., Plicatula polymorpha, Bell., Pectunculus pseudopulvinatus, Orb., Cardita Viquesneli, d’Arch., Cardium Schweinfurthi, May.-Eym., Venus plicatella, May.-Eym., Macrosolen Hollowaysi, Sow., Lucina pharaonis, Bell., Tellina scalaroides, Lam., Clavellithes longævus, Sol., Voluta arabica, May.-Eym., Turritella pharaonica, Cossm., T. carinifera, Desh., with Zeuglodon Osiris, Dames, and Z. Isis, Beadn.
				Ravine Beds.
			70	White marls and marly limestones with gypseous clays; Nucularia sp. Leda sp., Corbula aff. pixidicula, Desh., Lucina sp. (? pharaonis), Tellina tenuistriata, Desh., Zeuglodon Isis, Beadn., and scales and teeth of fish.
				Wadi Rayan Series (Nummulites gizehensis beds).
			130	Limestones, marls, clays, etc., with Nummulites gizehensis, Ehrbg., N. curvispira, Carolia placunoides, Cantr.

Section X.—MIDDLE EOCENE (PARISIAN).

A.—Wadi Rayan Series.—(Nummulites Gizehensis Beds).

(A.I.e. Schweinfurth, I.b. Mayer-Eymar,[36] Lower Mokattam of Cairo).

Beds of this group are chiefly found in the south of the depression. The wadis Rayan and Muêla, as already shown by Schweinfurth and Mayer-Eymar[37], are cut out in clays and limestones of Lower Mokattam age; the upper beds of limestone, containing among other fossil[38] numerous examples of the large Nummulites gizehensis, form the greater part of the floor of the depression west of the Fayûm cultivation, stretching from Jebel Rayan to the foot of Gar el Gehannem,[39] 28 kilometres west of the western end of the Birket el Qurûn (Section XX). Near the latter hill examples of N. gizehensis of inordinately large size occur.[40]

At the conical hill at the southern entrance to Wadi Muêla the following beds were noticed:—

Top of hill.
1.	Hard white limestone with small nummulites, Lucina, Callianassa, and echinids. Salt occurs in thin deposits along joint-planes. The lower part of this bed is largely composed of small nummulites and bryozoa. This generally white limestone passes down into
2.	Brown, usually sandy, limestone with oysters and small nummulites. In it are intercalated thin beds of greenish brown sandstone and clayey sand with impressions of bryozoa. Some of the brown sandy limestones are full of small nummulites. Ostrea and Carolia numerous. The beds are not constant, the clayey sandstones passing insensibly into sandy limestones.
3.	Softer beds with large nummulites, corals, Ostrea, Nautilus.
4.	Soft green and brown clays, with occasional oyster-beds.

At the corner of the cliff 7½ kilometres N.N.W. of the monastery of Der el Galamûn, in Wadi Muêla, occur about 80 metres of hard white nummulitic limestones, with beds of argillaceous sandstone and sandy clays. Fossils are numerous and include nummulites of several species (N. gizehensis, etc.), Carolia placunoides, different species of Ostrea, with gastropods (among others Terebellum sopitum), bryozoa, etc. It is very noticeable that the nummulites, especially the small species, occur in remarkable profusion not only in the limestones but often in the clays.

The following section will give a good idea of the general alternations found in this area; it was measured at Jebel Rayan,[41] 24 kilometres west of the western end of the cultivation of Gharaq basin.

Top of plateau.		Metres.
1.	Hard snow-white limestone with occasional nummulites passing down into hard highly nummulitic limestone; N. gizehensis, Ostrea sp., Lucina sp., Mitra sp., and Carolia placunoides occur among others	31
2.	Vertical-faced bed of greenish clayey sands and sandy clays (glauconitic) with Carolia, Ostrea and Nummulites. Near top of bed there is much gypsum. The nummulites in this bed are often collected together so as to form hard concretionary masses; these masses, by becoming more numerous, finally form a hard bed of nummulitic limestone intercalated in the clays near the top. The junction of the clays with the limestone of Bed No. 1 is very irregular	16
3.	Greenish shelly sands and sandy clayey bands, interbedded with impure chalky nummulitic limestones with N. gizehensis, N. curvispira, and a third smaller species; Ostrea sp. This bed is much obscured by debris	11
4.	Hard slate-blue shales, weathering to paper-shales	2
5.	Brownish marls passing up into clays	2
	Limestone band largely made up of small and large nummulites and echinids	1
	Glauconitic (?) and clayey sands and sandy clays, with Ostrea, Carolia, and nummulites, weathering with a vertical face. In some bands large numbers of small and large nummulites lie embedded in every position, as if tossed about by currents during the process of becoming buried by sediment. Gypsum occurs in thin veins and often encloses the nummulites	36
6.	Hard markedly-white nummulitic limestone full of N. gizehensis and other species (N. curvispira, etc.); the rock usually has a dark brown colour when freshly fractured. A shelly band rich in corals occurs nine metres from the top. The upper part is more marly and less nummulitic than the rest of the bed. Base invisible	30
	Total thickness of beds in the above section	129

The following is a section of the beds exposed in Wadi Muêla compiled from a paper by Mayer-Eymar on this oasis:—

Top.					Metres.
Parisian.		Id.		White siliceous cavernous limestone with Lucina globulosa, Desh., Gisortia, Rostellaria, Eschara Duvali, Michelin., (Probably ≡ bed No. 1 of our J. Rayan section)	10
				Greyish-yellow marl, rich in places with Ostrea Gumbeli, Pecten mœlehensis, May.-Eym., Vulsella chamiformis, May.-Eym., Velates Schmiedeli, Chemnitz, Cerithium fodicatum, Pleurotoma, Borsonia, Fusus, Rostellaria, etc.	6
				Yellowish sandy marl, with small nummulites.
		Ic.		Yellowish marls, divided by one or two bands of red clay, with Nummulites gizehensis	7
				Hard bedded clay	1
				Vari-coloured gypseous marls	4
				(Probably ≡ beds 2, 3, 4 at J. Rayan).
		Ib.		Very hard, rich greenish-grey, siliceous limestone with N. gizehensis, Pecten corneus, J. Sow., and Lucina (L. consobrina, Desh., and L. Defrancei, Desh.).	4 to 5
				(Probably ≡ upper part of bed 5 at J. Rayan.)

There is a considerable difference in thicknesses between the above section and that of Jebel Rayan. Our heights agree closely with those of Schweinfurth, so that it is probable that Mayer-Eymar is in error, notwithstanding his challenge of Schweinfurth’s figures in the paper mentioned.

B.—Ravine Beds.

The beds of this series, consisting of gypseous clays, clayey marls, and white marly limestones, are met with bordering the cultivation on the east, west and north sides; they pass under the alluvial soil of the cultivated land and are frequently seen in the bottoms of canals, and especially in the deep ravines known as El Bats, and El Wadi (Plates III and V). The relation of these beds to the Rayan series below is well seen at the prominent outstanding hill Gar el Gehannem (Fig. 2); here the plain to the east and south is formed of the uppermost member of the Wadi Rayan series, a limestone full of Nummulites gizehensis. In the hill itself the latter is directly overlain by gypseous and glauconitic sandy clays and marls, with hard intervening beds of yellowish, often marly, limestone. The upper beds consist of alternating clays, sandy limestone and sandstone, at the top being a thick bed of the latter passing up gradually into the sandstones of the Birket el Qurûn series. The following is the detailed section:—

Summit of Gar el Gehannem.			Thickness in metres.
1.	Hard yellow and white limestone crowded with shells, chiefly large individuals of Carolia placunoides and Ostrea Fraasi. Numerous nummulites in upper part		Lower beds of Qasr el Sagha Series (45 metres)	25
2.	Limestone full of Turritella carinifera, Ostrea Clot-Beyi			1
3.	Brown clays			6
4.	Shelly limestone with Carolia, Turritella, Ostrea, Cardita and Qerunia (Hydractinia)			1
5.	Greenish clays			6
6.	Nummulitic limestone with Carolia, Qerunia and four species of Turritella			1½
	Light blue clays			2
7.	Light green and brown sandstone with irregular concretions			2½
8.	Brown shelly limestone full of Carolia placunoides, Ostrea Reili, O. Fraasi, Turritella, Balanus and nummulites		Birket el Qurûn Series (50 metres)	2
9.	Yellow sandstone with bands of shelly limestone crowded with nummulites, oysters, etc. Near top casts of Cardita, Carolia; also Cerithium, Teredo, Ostrea, Pecten, Pinna, and echinids. Calcareous concretions near base			18
10.	Clays with much gypsum			6
11.	Yellow sandstone with Balanus. Bands crowded with two species of nummulites and occasional oysters. In places the foraminiferal bands become highly calcareous. Below similar, with hard compact grey bands and occasional fish-spines and teeth			24
12.	Similar to above, with numerous casts of Cardita, etc., and small Ostrea		Ravine Beds (10 metres)	24
	Argillaceous sandstone with thick stockwork of gypsum and calcareous nodules			6
13.	Light yellow, brown, and greyish gypseous clays			3
14.	Yellow-brown sandstones and sandy limestones, often argillaceous. Fish-scales.
	Brown clays
	Yellow-white marls and marly limestone			5
15.	Hard light yellow shelly limestone, in part marly, in part sandy			10
16.	Ochreous-yellow, grey, and white clays and marls with gypsum			9
17.	Hard yellow-white shaly marl with numerous shell-impressions; much gypsum			3
18.	Yellow marly clays; soft yellow and grey-brown clays, dark sandy glauconitic, yellow, and black, clays. Zeuglodon remains fairly common. Shell impressions. Much gypsum
	Fairly hard yellow-white glauconitic marl			10
Marly limestone with Nummulites gizehensis forming top of Rayan beds.

The clays, marls, and limestones of the Ravine beds are generally found to contain fairly numerous shell-impressions, including Nucularia sp., Leda sp., Cardita sp., Corbula aff. pixidicula, Lucina sp., Oudardia ovalis, Desh., Tellina tenuistriata,[42] numerous small fish-scales, and occasional large teeth of sharks; while the skeletons of the toothed-whale Zeuglodon Isis are fairly common, although usually in poor preservation.

In the ravine of El Bats, about one kilometre west of Sêla, these beds (5-6 metres thick) are seen unconformably overlaid by 12 metres of false-bedded gypseous sands and clays passing up into the superficial cultivated loam. The junction of these alluvial deposits and the underlying Eocene is distinctly unconformable and an intervening pebble-bed is occasionally present (Fig 3).

In the large ravine known as El Wadi, which traverses the west side of the cultivation of the Fayûm, these beds are frequently well exposed; their lithological characters remain very constant. Here, as in El Bats, they are unconformably overlain by a varying thickness of Pleistocene and Recent clays. Their surface, a plain of subaerial denudation, represents the original floor of the depression before the entry of the sediment-carrying water from the Nile Valley through the Lahûn gap; its irregularity is seen in Plate V.

The plain bordering the cultivation to the east of Sêla and Rubiat likewise consists of these same white marls with fish-scales, etc.; they pass regularly under the cultivated land. Shaly marls, gypseous clays, and chalky limestones of the same age are seen in, and to the south of, the railway crossing the desert between Sêla and Medum. Eastwards they stretch into the Nile Valley, being found exposed along the desert-edge bordering the cultivation at Medum, Nawamis and Masaret-Abusia.

The same beds are exposed immediately to the east of the village of Sersena, midway between Sêla and Tamia. They are again well seen in the ravine below the last named village, and forming the narrow strip of the desert projecting into the cultivation as far as the northern end of the Tamia lake; they also occur on the shore of the latter at El Tuba, about 2 kilometres south of the village. At Tamia their exposure measures 25 metres in thickness.

At various points along the north side of the Birket el Qurûn exposures of this series occur, the beds forming the lower sloping part of the cliffs overlooking the lake, as well as the base of the island “Geziret el Qorn,” although only the upper beds are visible above the water of the lake. Both here and along the northern shore of the lake they are for the most part hidden by the high level recent lacustrine clays, but where occasionally exposed their identity is certain, the characteristic small brown fish-scales being abundant, besides occasional teeth, with shell-impressions of the different genera enumerated above.

At the western end of the lake the Ravine beds form the lower part of the cliff as well as the plain to the south; the underlying Nummulites gizehensis limestone not being exposed. The series consists of some 45 metres of white and grey shaly marls with harder bands of siliceous limestone intercalated throughout, one of which usually forms the uppermost bed. It is, in fact, the development in places of one or other of these hard beds of limestone near the top of the series that gives rise to the bold promontories, or horns, which occur on the north side of the Birket el Qurûn.

The greater part of the marls and clays met with from 18·5 to 112·5 metres below the surface in the boring at Medinet el Fayûm in all probability belong to the Ravine beds.

The maximum thickness of this series is 70 metres, measured at Gar el Gehannem.

C.—Birket el Qurun Series (Operculina-Nummulite Beds).

The above designation is convenient and applicable to these beds, which form the escarpment immediately overlooking the lake on the north side throughout its length.

The group includes all the beds between those last described and the well-marked Qasr el Sagha series, homotaxial with the Upper Mokattam (the brown beds) of Jebel Mokattam, near Cairo. It thus appears to be the equivalent of the upper part of the white beds (quarried limestones) of the Mokattam section, although the lithological characters are entirely different, the massive limestones of Jebel Mokattam being represented in the Fayûm by an arenaceous and argillaceous series, deposited probably in water of far less depth. Where the different members of this series are well exposed certain beds are found to be characterized by the abundance of two foraminifera, the one a small thin-shelled Operculina (O. discoidea)., and the other a small thick nummulite.[43] The tests of these foraminifera sometimes make up entire bands of rock. In addition, the series includes certain beds which at times become very fossiliferous, and contain a well-preserved molluscan fauna.

The series is well seen in the desert separating the Fayûm from the Nile Valley; on the south-east and east sides of the former; along the northern boundary of the cultivation and the Birket el Qurûn; and westwards in the cliffs to beyond the outlying hill-mass of Gar el Gehannem.

The following section was measured on the south-west of the Fayûm, from Ezba Qalamsha (on the confine of the cultivation) to the ridge summit 5 kilometres to the south-east.

Top.				Metres.
		Summit of ridge 5 kilometres south-east of Ezba Qalamsha.
		Pliocene Raised Beach with occasional Ostrea cucullata, Born., made up of gravels with blocks of limestone.
Birket el Qurun Series.		1.	Ochre-coloured calcareous sandstone and sandy limestone crowded with foraminifera (Nummulites Fraasi, etc.), Ostrea, etc.	38
		2.	Sandy limestone, largely made up of foraminifera (Operculina discoidea ?)	2
		3.	Sandy shale	2
		4.	Sandstone, partly calcareous, with much gypsum	3
		5.	Calcareous sandstone with concretionary weathering	17
		6.	Shale with gypsum	2
		7.	Calcareous sandstone	4
		8.	Shale with gypsum	2
		9.	Calcareous sandstone, hard and yellowish	2
		10.	Gypseous shale with numerous small shells (Tellina sp.) passing down into sandy limestone. (This bed is the uppermost member of the Ravine beds)	6
			Total thickness	78
Base, cultivation level.

To the north of the Lahûn pyramid the beds agree generally with the above. The following are the chief divisions here:—

Top of Hills.		Metres.
	Gravel Terrace (Pliocene) 22 metres thick.
1.	Calcareous sandstone and sandy limestones full of nummulites; also Ostrea, etc.	31
2.	Ochre-coloured calcareous sandstone or sandy limestone, often crowded with Operculina discoidea and some Nummulites Fraasi, etc.	12
3.	Sandy limestone with small foraminifera at top and some shells. The upper part of this bed has been quarried	20
4.	Shales and shaly limestone; gypsum	—
	Total thickness	63

The foraminiferal sandy limestones of this series are seen at points in the desert bounding the eastern margin of the cultivation, notably east of Sersena and at the top of the hill 15 kilometres north-east of Rubiat.

The following section was measured at the prominent hills 17 kilometres 28° N. of E. (magn.) of Tamia:—

			Metres.
1.	Greyish laminated sandy clays with gypsum; Ostrea band near top		7
2.	White sandy limestone with numerous badly preserved Ostrea, Pecten, and other lamellibranchs		1
3.	Dark-brown clayey sands with gypsum and grey sandy clays with obscure plant-remains. Occasional Ostrea		14
4.	Hard, white, sandy limestone with numerous Ostrea at top; soft clays with gypsum		1
5.	Greenish and brownish sands and sandy clays with band of sandy limestone near top	⎱ ⎰	14
6.	Greyish-brown, impure, sandy limestone weathering into large globular concretions. Shell impressions
7.	Sandy clays and marls alternating with impure limestones; much gypsum. Occasional fish-remains and small oysters	⎱ ⎰	22
8.	Greenish sandy limestone with traces of shells
9.	Finely laminated grey-brown clays with black carbonaceous matter and fish-remains; saliferous		3
10.	White sandy limestone		1
11.	Soft yellow sandstones, etc.	⎱ ⎰	7
12.	White marls with fish-scales, etc.; base not seen. (This bed, and possibly also 9, 10, 11, should be reckoned as belonging to the Ravine beds)
	Total thickness		70

In the north of the Fayûm the series is characterized by the presence of one or more very constant well-marked beds of hard calcareous sandstone, which almost invariably weather into huge globular masses. These masses should be regarded as huge weathered-out concretions, rather than as water-rounded blocks, although no doubt in many cases their roundness has been increased by the action of the waters of Lake Moeris as the level of the latter gradually fell, and possibly still earlier during the invasion of the Pliocene sea; from the latter time also may date the millions of parallel vertical borings with which these and other exposed rocks are often perforated. In the various places where one of these beds forms the present surface of the desert the concretions may be seen in different stages of exposure, from the initial, where only just the tops are laid bare, to the final stage where the globes are left completely weathered out, as seen in the illustration (Plate VII). The appearance of the desert when covered for many square kilometres with thousands of these blocks is more easily imagined than described.

The lower beds of the Birket el Qurûn series form the island Geziret el Qorn, and consist of clays and sandstones containing a considerable number of organic remains. These beds were collected from and examined by Schweinfurth[44] in 1879, the mollusca being subsequently described by Mayer-Eymar,[45] while the vertebrate remains, which included cetacean bones and numerous fish-teeth, were submitted to Dames.

The following species were determined by Mayer-Eymar, who indicated that the fauna as a whole had a Bartonian aspect[46]:—

Upper Bed.

• Ostrea plicata, Defr.
• Arca Edwardsi, Desh.
• Lucina pomum, Duj.[47]
• Lucina cfr. tabulata, Desh.
• Cardium Schweinfurthi, May.-Eym.
• Cytherea Newboldi, May.-Eym.
• Tellina pellucida, Desh.
• Mactra compressa, Desh.
• Corbula pyxidicula, Desh.
• Calyptræa trochiformis, Lam.
• Turritella angulata, Sow.
• Ficula tricarinata, Lam.

Lower Bed.

• Astrohelia similis, May.-Eym.
• Goniastræa cocchii, d’Achiardi.
• Heliastræa acervularia, May.-Eym.
• Heliastræa Ellisi, Defr. (Astræa).
• Heliastræa flattersi, May.-Eym.
• Ostrea digitalina, Dubois.
• Ostrea gigantea, Sol.
• Ostrea longirostris, Lam.
• Ostrea producta, Delb. et Raul.
• Isocardia cyprinoides, Braun.
• Turritella carinifera, Desh.
• Turritella transitoria, May.-Eym.
• Turritella turris, Bast.
• Turbo Parkinsoni, Defr.
• Pleurotoma, sp.

The cetacean remains, belonging to the genus Zeuglodon, were described by W. Dames,[48] who compared them with the American species Z. macrospondylus and Z. brachyspondylus, but did not then consider them to represent a new species; in a later publication,[49] however, the same author described similar but more complete remains, also collected by Schweinfurth (from beds belonging to our Qasr el Sagha series), as a new species, Z. Osiris. A considerable number of fish-remains from Geziret el Qorn are also described in the earlier publication. Although the difference in size of the bones of separate individuals was considered by Dames to be sexual, it seems probable that there are two distinct species of Zeuglodon, as the smaller type appears to have a much greater upward range than the larger[50]; both species, Z. Osiris, and Z. Isis occur in the Birket el Qurûn series, and a very fine mandible of the larger was obtained from these beds in the cliffs near the west end of the lake.[51] More recently a third species has been discovered by Stromer and described under the name of Z. Zitteli.[52]