Large well-formed groups of barytes crystals are occasionally found in the sandstones of the oasis-floor, and hard thin vein-like masses of a sandstone, in which the cementing material is barytes, are frequently met with in the eastern part. The weathering of these veins, which are evidently produced by the action of infiltrated solutions in cracks, gives rise to the peculiar spheroidal belted nodules which often strew the ground, and of which the existence has been already mentioned by Ascherson and Zittel.
Although these strata are, as a rule, unfossiliferous, organic remains are common in certain beds, and especially in the dark-brown ferruginous concretionary bands of the sandstones. The commonest form is Exogyra, individuals of which are found crowded together in some bands; in most cases they are pseudomorphs in ironstone.
In the hills between Mandisha and El Bawitti this series consists of soft false-bedded variegated sands and sandstones with ferruginous bands, alternating with sandy shales, the whole locally showing a slight westerly dip of between 3° and 7°. Some of the sandstones contain flakes of colourless mica. The whole series is capped and protected by a hard bed of dolerite, which has been intruded along a bedding plane into the series, and now forms the summit of these hills.
The following section was measured about 4 kilometres west of Mandisha—
| Top. | Metres. | |
| More or less columnar dolerite, much broken up by weathering. | 10 | |
| Sandstones, sandy clays; unfossiliferous | 65 | |
| Ferruginous sandstone with casts of Exogyra; sandrock and sandstone, with ferruginous unfossiliferous bands | 10 |
The section is much obscured by downwash and talus of angular fragments of dolerite, sand, shale, etc. This downwash of shale and clay is often covered by a hard coating of salt and sand cemented together, sometimes having the appearance of regular beds.
The following is a detailed section measured at the well-marked isolated conical hill near the western escarpment, 10 kilometres north-east of El Qasr (See [Plate I.]) The series is here capped by 6 metres of nodular and hard compact limestone with Nummulites, Operculina, Lucina, etc., of Lower Eocene (Libyan series) age.
| Top. | (Hard compact limestone with Nummulites,etc., Eocene). | Metres. | |
| Shaly clays with ferruginous bands,occasionally containing Exogyra shells; alternations ofshales and shaly sandstones. | 7·4 | ||
| Soft shaly and crumbly sandstones, withmany hard ferruginous bands and shaly clays. In the sandstone occursolid vertical rods of ironstone 5 centimetres in diameter, withconcentric structure | 21·0 | ||
| Gray marls with plant-stems andleaves | 1·5 | ||
| Yellow sandstone, well-marked thickcompact bed, false-bedded | 11·5 | ||
| Hard ferruginous band withfish-remains | 0·2 | ||
| Gray shales | 1·5 | ||
| Soft beds of crumbly sand, hardersand-rock, with clayey layers and ferruginous bands | 6·0 | ||
| Shaly clays with dark-red ferruginoussandy layers, with gasteropod and Exogyra shells. Greenshaly sandstone on top, and gray sandy clays below | 16·0 | ||
| White and gray-bedded sand-rock | 9·0 | ||
| Obscured beds, one containing smallExogyra and gasteropods. | 15·0 | ||
| Shaly sandstone with hard reddish nodularferruginous bands containing gasteropods and Exogyra. | 3·5 | ||
| Sandy beds, hidden by downwash | 6·0 | ||
| Ferruginous bands containingExogyra casts | ⎱ ⎰ | 20·0 | |
| Sandy shaly clay, false-bedded, withwhite sand-rock. Plant-remains | |||
| Bed of white sand-rock | 1·0 | ||
| Thinly laminated gray shale | 2·0 | ||
| Reddish-brown nodular ferruginousband | 0·2 | ||
| Sandy or clayey beds, obscured bydownwash | ⎱ ⎰ | 9·0 | |
| Sandy clay | |||
| Sandy beds with dark-brown ferruginousbands | 13·0 | ||
| Sandstone with hard ferruginous bands,full of Exogyra | 6·0 | ||
| Sandy bed, crowded with well-preservedExogyra shells | 0·2 | ||
| Yellow sandy clay with numerousExogyra casts | 3·0 | ||
| Dark carbonaceous shale with obscureplant-remains | ⎱ ⎰ | 15·0 | |
| Sandy beds with bone-fragments | |||
| Floor of Oasis. | 168·0[41] | ||
At the extreme north end of the depression the cliffs and the lower part of the conspicuous dark hill, Jebel Horabi, are composed of shales, clays and sandstones belonging to this series. The cliffs are capped by Eocene limestone-grits and limestone as in the section just described. The upper part of Jebel Horabi, however, consists of a mass of ironstone, often coarsely pisolitic, with some limonite, and red and yellow ochre. This iron-ore appears to replace the sandy beds more or less irregularly at the base, but the great mass of mineral above is probably a later and quite distinct deposit. (Vide under [Post-Eocene]). Traces of shells occur in places.
In the isolated, limestone-capped hill ([Plate I]), 6½ kilometres north-west of Ain el Haiss (the northern spring) these beds are more fossiliferous than usual.