* * * * *
GEOLOGY OF THE OASIS.
[Sidenote: Previous geological work.]
The earliest geological observations made in Baharia Oasis appear to
have been those of Cailliaud,[35] who visited the district in 1820,
and recorded the occurrence of volcanic rocks within the depression.
In 1874 the celebrated Rohlfs Expedition was organised, and in
that and the succeeding year traversed large portions of the Libyan
Desert. The work of this Expedition, forms indeed, the basis of our
knowledge of the geology of Egypt generally. The eminent geologist
and palæontologist of that Expedition, Karl A. Zittel, did not,
however, visit Baharia, and the only information with regard to
the geology of this oasis was derived from specimens collected by
Prof. Ascherson. After examining these, Zittel stated[36] that the
floor of the oasis consisted of sandstones, marls, etc., presenting a
considerable resemblance to the lower part of the Overwegi stage;[37]
but concluded, that as they were succeeded at a very short interval
by Nummulitic Limestone, precluding the possibility of the higher
Cretaceous beds, except in a very attenuated form, they must be of
Tertiary age.
In 1894, Capt. H. G. Lyons, R.E.,[38] visited Baharia, and 5 miles
N.N.E. of Zubbo discovered a bed containing small examples of
_Exogyra_. These were examined by Prof. Zittel and referred to as
undersized specimens of _Exogyra Overwegi_ Beyr.[39] This discovery
of Capt. Lyons’ thus showed the series to be undoubtedly of Upper
Cretaceous age.
In the latest edition of Prof. Zittel’s map, the depression was
shown as consisting of Cretaceous beds surrounded by an Eocene
plateau, which was also supposed to separate it from the oasis of
Farafra to the south.
[Sidenote: Geological Survey, Oct.-Dec. 1897.]
A glance at Plate I will be sufficient to show that the work of the
Geological Survey has resulted in large and important additions to our
knowledge of this oasis. The Eocene is seen to form the surrounding
plateau only on the north; while the Cretaceous, consisting of several
well-marked divisions, occupies not only the depression itself, but
forms part of the surrounding plateaux on the east and west sides,
and is continuous with that of Farafra to the south. Moreover, the
relations of the two great formations of Cretaceous and Eocene are
now shown to be those of unconformability and overlap. The actual
extent of volcanic rocks and post-Eocene lacustrine deposits, the
latter a newly-discovered and important series, is now shown for
the first time.[40] Important folding has been detected and numerous
organic remains have been discovered.
The deposits met with in the district admit of classification as
follows:—
RECENT.
1. Sand-dunes, salines, and superficial deposits.
POST EOCENE (OLIGOCENE?).
2. Basalt and Dolerite intrusions.
3. Ferruginous sandstones and quartzites, with
limonite and pisolitic iron-ore, probably
lacustrine in origin.
EOCENE (MIDDLE-LOWER.)
LOWER MOKATTAM- { 4. Limestones with _Nummulites_, _Operculina_,
UPPER LIBYAN. { Echinids, _Lucina_, etc.
UPPER CRETACEOUS.
DANIAN | 5. Thick-bedded White Chalk and grey
crystalline limestone.
CENOMANIAN }
(Some TURONIAN } 6. Limestones and variegated sandstones.
or SENONIAN). }
CENOMANIAN | 7. Sandstones, clays and marls.
These groups will now be dealt with in succession, commencing with
the oldest.
UPPER CRETACEOUS.
CENOMANIAN.
7.—_Sandstones, Clays and Marls_—These, the lowest and oldest
beds, form the floor and parts of the walls of the depression. They
are best developed and exposed in the north end, where their maximum
thickness is about 170 metres; the base of the series is not seen,
and thus their total thickness cannot be estimated. In the north
part of the oasis they are capped, with apparent conformability, by
Eocene limestones, although an intervening band of limestone-grit
may occasionally occur. In the south, the series is followed by
the higher Cretaceous divisions, while in the isolated hills within
the depression these beds are capped either by Eocene limestones,
by basalt or dolerite, or by Post-Eocene ferruginous sandstones
and quartzites.
They consist of friable false-bedded variegated sands and sandstones,
with harder dark-brown ferruginous bands, alternating with sandy
shales and clays, passing through every gradation. Some of the
sandstones are micaceous.
The clays are frequently saliferous, and bands of fibrous gypsum
are occasionally seen.
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., Metres.
Eocene).
Shaly clays with ferruginous bands, occasionally
containing _Exogyra_ shells; alternations of shales
and shaly sandstones. 7·4
Soft shaly and crumbly sandstones, with many hard
ferruginous bands and shaly clays. In the sandstone
occur solid vertical rods of ironstone 5
centimetres in diameter, with concentric structure 21·0
Gray marls with plant-stems and leaves 1·5
Yellow sandstone, well-marked thick compact bed,
false-bedded 11·5
Hard ferruginous band with fish-remains 0·2
Gray shales 1·5
Soft beds of crumbly sand, harder sand-rock, with
clayey layers and ferruginous bands 6·0
Shaly clays with dark-red ferruginous sandy layers,
with gasteropod and _Exogyra_ shells. Green shaly
sandstone on top, and gray sandy clays below 16·0
White and gray-bedded sand-rock 9·0
Obscured beds, one containing small _Exogyra_ and
gasteropods. 15·0
Shaly sandstone with hard reddish nodular
ferruginous bands containing gasteropods
and _Exogyra_. 3·5
Sandy beds, hidden by downwash 6·0
Ferruginous bands containing _Exogyra_ casts }
} 20·0
Sandy shaly clay, false-bedded, with white }
sand-rock. Plant-remains }
Bed of white sand-rock 1·0
Thinly laminated gray shale 2·0
Reddish-brown nodular ferruginous band 0·2
Sandy or clayey beds, obscured by downwash }
} 9·0
Sandy clay }
Sandy beds with dark-brown ferruginous bands 13·0
Sandstone with hard ferruginous bands,
full of _Exogyra_ 6·0
Sandy bed, crowded with well-preserved
_Exogyra_ shells 0·2
Yellow sandy clay with numerous _Exogyra_ casts 3·0
Dark carbonaceous shale with obscure plant-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.
_Top._ (Limestone, the basal bed of the intermediate series).
Limestone, the basal bed of the intermediate series.
Sandy beds with hard brown ferruginous concretionary }
bands, containing numerous casts of _Cyprimeria_ }
(_Artemis?_) _Arca_, _Cucullæa_, _Odostomopsis }
abeitrensis_, _Baculites_ aff. _syriacus_, and }
_Neolobites Vibrayeanus_ }
} 50 m.
Clays, often shaly }
}
Hard ferruginous sandstone with pockets of }
clean white sand }
Numerous other sections at different points were examined and
measured. Such a series is naturally very variable, and individual
beds do not maintain their characters over wide limits, so that
correlation of different sections is not possible, except in the
roughest manner.
In the southern part of the depression fossils are more frequent,
otherwise the beds are very similar.
The series under consideration is by no means poor in organic remains,
although the latter are more or less restricted to certain bands
and localities. No doubt most of the fossil shells have been removed
by solution, etc., as those now existing are generally preserved as
pseudomorphs in limonite.
The following is a list of fossils which have been obtained from
this series:—[42]
_Cucullæa_ sp. | _Odostomopsis abeitrensis_.
|
_Arca_ sp. | _Natica_ sp.
|
_Exogyra flabellata_. | _Baculites_ (_syriacus?_)
|
_E. mermeti._ | _Neolobites vibrayeanus_.
|
_Venus_ (_Artemis?_) sp. | Fish-teeth and bones.
Silicified wood and plant-remains (including leaves of Dicotyledons).
As regards the age of these beds, it has already been mentioned
that the discovery of _Exogyra_ by Capt. Lyons some years ago, in
the basal bands of the series, showed them at least to be Cretaceous,
although their correlation with the Overwegi stage, on the supposition
that the specimens collected were referable to _E. Overwegi_, is now
shown to be inadmissible. The fairly extensive assemblage of organic
remains obtained from this and the overlying series enables the age
of the beds to be determined with accuracy, and Dr. Blanckenhorn,
who had the first opportunity of making a careful examination of
the collection, has pronounced them to be Cenomanian.[43]
The general lithological character of the beds and the prevalence
of false-bedded sandstones containing in places silicified wood
and plant-remains, including leaves of large trees, point to the
conclusion that the beds of this series were deposited in fairly
shallow water, perhaps in an estuary, at no great distance from
the land.
6.—_Limestones and Variegated Sandstones._—In the walls of
the south end of the oasis, the series just described is always
overlain by a bed of hard, and usually more or less crystalline,
limestone. This limestone forms the lowest member of the series
now under discussion. At the extreme southern end of the oasis,
on the western side, this basal limestone forms a narrow platform,
of only some 200 metres width, separating the respective escarpments
of the two series 6 and 7. On the same side further north the upper
escarpment is usually further back, and in some localities the beds
do not form a single well-marked escarpment at all.
The series also occurs forming a line of hills within the depression,
these hills owing their existence to a remarkable syncline, by which
the beds in question have been folded down to the level of the floor
of the depression.
The beds of the series form, as a rule, a well-marked group,
individual beds often being traceable for great distances and
correlable in different parts of the oasis.
The group as a whole consists of alternations of brown limestone[44]
(with grey crystalline varieties) and variegated sandstones and
clayey beds. A typical section, as exposed on the west scarp in
N. lat. 27° 53′, is as follows:—
_Top._ (Buff-coloured limestone and white chalk of Metres.
Division 5.)
1. Soft brown sandstones and sandy beds,
much obscured 7·6
2. Sandy beds, passing up into a very hard grey
concretionary sandstone, showing ripple-marks 4·6
3. Thin-bedded dark sandstones, mostly very hard 1·5
4. Hard compact thick-bedded grey sandstone,
very concretionary, false-bedded 2·1
5. Shaly sandy beds with ferruginous concretions 1·5
6. Brown impure limestone 0·6
7. Soft variegated sandy beds and sandstone 9·0
8. Rather friable buff-coloured sandstone, more
or less false-bedded, with peculiar
iron-staining 6·0
9. Gray rather concretionary sandstone, with
dark-brown ferruginous bands 3·0
10. Brown crystalline limestone with calcite,
passing into harder grayish crystalline
limestone, flinty at top 6·1
11. Gray marls and shaly clays 3·0
12. Brown crystalline limestone 0·6
----
45·6
_Base._ (Sandstones and clays of Division 7.)
In Jebel Hefhuf, the long narrow ridge-shaped hill a few kilometres
south-east of El Bawitti (Plate VII), the beds of this series are
implicated in the fold, and are found tilted at angles of 30 and 40
degrees. They here consist of a considerable thickness of crystalline
limestone, calcareous grits, shaly clays and sandstones; the softer
beds always form a gully between the harder.
The following were noticed at a point about 7 kilometres from the
western end of the ridge:—
_Top._ Metres.
Calcareous grit, usually siliceous, with bones }
(often a true bone-bed). }
}
Sandy clays, etc., with silicified wood[45] }
}
Thick-bedded brown sandstone, siliceous and } 21
ferruginous in part. }
}
Green and gray iron-stained sandy clays }
}
Thick-bedded brown crystalline limestone }
Brown crystalline limestone with calcite cavities
near base 24
--
_Base._ 45
Below come the clays and sandstones of the lowest series, No. 7.
In the section above, the limestones are usually too crystalline to
show any traces of organic remains, although numerous casts of shells
occur at certain points. The calcareous grit, a very well-marked
bed, has been observed at points widely distant within the oasis;
in places it is a true bone-bed,[46] often highly siliceous and
passing into a hard quartzite. Its occurrence here, at the top of
the Cenomanian series and below the White Chalk (to be described
later) is of great interest, as its position shows it to be probably
homotaxial with the bone-beds of Dakhla Oasis[47] and the Eastern
Desert,[48] where they are of Campanian age. The bone-beds on this
horizon must thus have been deposited over an enormous area.
If followed round from the south end of the oasis along the east
scarp, the beds of this division are found to gradually become
more and more calcareous, consisting often almost entirely of
limestone,[49] and to dip more and more to the south-east, till in the
neighbourhood of the large sandstone-hill near the scarp in latitude
28° 5′, where the tilting reaches its maximum, they dip at 45°
and partly disappear under the sandy covering of the ground between
the hill and the scarp. Further north the dip grows less and the
beds re-appear, forming the top of the main scarp at the oasis-edge.
The organic remains in Division 6 include ammonites, echinids,
with _Ostrea_, _Exogyra_ and masses of _Serpulæ_; fossil wood
and fragments of bone also occur. These fossils are in places
abundant, some bands being almost entirely formed of the shells of
_Exogyra_. The ammonites, although apparently specifically identical
with those of the lower series, attain a much greater size. The
assemblage of forms and the prevalence of bands of limestone point
to deeper water conditions than the foregoing underlying series.
The following is a list of forms obtained as far as determined:—
_Rhabdocidaris_, probably nov. sp.
_Diplopodia marticensis_.
_Heterodiadema libycum_.
_Toxaster radula_.
_Hemiaster lusitanicus_ (_roachensis?_)
_Serpula_ (_Galeolaria_) _filiformis_.
_Pinna_, probably nov. sp. | _Corbula_ sp.
|
**_Inoceramus cripsi_. | **_Natica_ sp.
|
_Ostrea_ sp. | _Tylostoma syriaca_.
|
_Exogyra flabellata_. | _Turritella_ nov. sp. aff. _nodosa_.
|
_E. olisiponensis_. | **_Turritella_ sp.
|
_E. mermeti_. | _Cerithium_ sp.
|
_Plicatula Reynesi_. | _Pterodonta_ aff. _inflata_.
|
_Spondylus_ (?) | _Murex_.
|
_Modiola_ sp. | **_Fusus_ sp.
|
_Cardium_ sp. | **_Drillia pleurotomoides_.
|
_Isocardia_ sp. |
_Nautilus Munieri_.
**_Pachydiscus peramplus_.
_Neolobites vibrayeanus_.
[Footnote **: Indicative of a somewhat later age than Cenomanian.]
The above fauna taken as a whole has a decided Cenomanian aspect,
although some of the forms (marked **) are indicative of a somewhat
later age, i.e., Turonian and Senonian. As the next bed above is
the Danian White Chalk, it is probable that some of the upper beds
of this series form a transitional stage between the Cenomanian and
Danian. The presence of the bone-bed, as already mentioned probably
of the same age as the Dakhla and Eastern Desert bone-beds, i.e.,
Campanian, supports this view. In any case, however, the great
thickness of beds, of Senonian-Danian age, of the southern oases of
Dakhla and Kharga, including the bone-beds, the _Exogyra Overwegi_
series and the ash-gray shales, is here apparently only represented
by a few metres of deposits.
DANIAN.
5. _Thick-bedded White Chalk and grey crystalline limestone._—This
series conformably overlies the upper member of the last. On the
west side the chalk forms a well-marked, snow-white, tortuous
escarpment, lying at a considerable distance (5 to 10 kilometres)
from the oasis-wall. Towards the south, however, it approaches the
depression, its escarpment following immediately above and behind that
of the underlying series. South of the extreme end of the depression
proper, the chalk escarpment forms a long narrow bay, the furthest
point of which is 20 kilometres distant. Thence it runs northwards,
so that to the south-east of the depression the beds of the series are
seen at some little distance from the oasis-edge, but further north
they appear to come to the edge and to overlie the brown limestone
as part of the main scarp in the neighbourhood of the large hill
already mentioned in latitude 28° 5′ N., where the strata dip so
steeply into the plateau. There is some difficulty in correlating the
beds of this group on the two sides of the oasis, for while on the
west they are chalky and in places fossiliferous, being only in part
altered to a crystalline limestone, on the east, doubtless owing to
the folding which produced the strong dipping already referred to,
they are almost entirely composed of hard crystalline and horny
greyish-white limestone, apparently devoid of fossils. (_Vide_ p. 60).
Within the depression it is probable that the White Chalk is
represented in part by some of the hard gray crystalline limestone
in the synclinal fold of Jebel Hefhuf.
On the west and south-west sides of the depression the White Chalk
covers a considerable area, which is characterized by its rough
surface and by numerous depressions, some of considerable size,
eroded by wind-borne sand. To the south it extends to Farafra,
being continuous with the chalk of that oasis.
Lithologically, on the west side of the oasis, the series is
represented by a thick-bedded, snow-white, pure foraminiferal chalk,
30 to 45 metres thick, partly altered in the upper part into hard
gray crystalline limestone. It was probably deposited in water of
considerable depth.
On the west side of the depression the White Chalk yielded a fair
assemblage of fossil-remains, the chief among which were great numbers
of corals. In the upper part Nautili are occasionally met with,
while _Spirorbis_, _Pecten_, _Gryphæa_, fragments of _Inoceramus_
and other shells are fairly common. In addition echinids occur
sparingly and sharks’ teeth are seen here and there. One of the
best fossil localities is 24 kilometres north of Ain el Haiss.
The following is a list of those obtained:—
_Cælosmilia_. | _Exogyra overwegi_.
|
_Linthia_ nov. sp. | _Pecten farafrensis_.
|
_Spirorbis_. | _Spondylus_ nov. sp. aff. _S. Dutempleanus_.
|
_Inoceramus_. | _Nautilus_ sp.
|
_Gryphæa vesicularis_ | _Corax pristodontus_.
As regards the age of this White Chalk the fossil facies has a very
young Cretaceous aspect. Zittel[50] has already shown that the White
Chalk of the Libyan Desert, studied by him in Farafra, Dakhla and
Kharga, is Danian, and the identity of the chalk of Baharia with
that of those oases was satisfactorily proved on the traverse from
the latter oasis to Farafra.
The White Chalk brings our description of the Cretaceous series of
Baharia to a close. As will be shown, it is overlain unconformably by
Eocene deposits. Throughout the deposition of the Cretaceous in this
area it is clear from the character of the beds that the sea-floor
had been continually sinking. After the deposition of the white chalk
subsidence probably ceased and the area became one of elevation,
the Cretaceous beds rising to form land. During this elevation much
folding must have taken place, and subsequently gradual subsidence
set in until in Lower Eocene times the area again became marine and
the deposition of the rocks now to be described began to take place.
* * * * *
EOCENE.
UPPER LIBYAN—LOWER MOKATTAM.[51]
4. _Limestones with Nummulites and Operculina_.—In the north part
of the oasis, both in the walls and in the isolated hills within
the depression, the lower Cenomanian beds, the “Sandstones, clays
and marls,” are directly overlain by buff-coloured or yellowish
limestone of Eocene age, containing _Nummulites_, _Operculina_,
_Ostrea_, etc. The beds of the two series being horizontal, the
junction is one of apparent conformability, but in many localities the
base of the Eocene is marked by a bed of limestone-grit, indicating
the break in continuity of deposition which is known to have occurred
from the respective ages of the beds in question and from a study
of the same beds further to the south.
In a traverse across the plateau to the west of Bawitti,
Operculina-limestones with nummulites were found to form the surface
of the desert. Again on the traverse from Maghagha to the oasis,
already described, no Cretaceous beds were crossed, so that the
plateau N.N.E. and N.W. of the northern end of the oasis is entirely
formed of Eocene rocks. On the east side, however, no certain evidence
of Eocene age in the beds capping the scarp has been found south
of the fault which cuts the scarp as the prolongation of the sharp
syncline of Jebel Hefhuf.
Further south, on the west side, a very dissimilar succession was
found to that on the north. Here at a distance of 20 kilometres from
the oasis wall, at a point 30 kilometres north-west of Ain el Haiss,
the uppermost member of the Cretaceous, the White Chalk itself,
is overlain by a hard gray crystalline limestone containing an
abundance of _Nummulites_ and _Operculina_, with _Lucina_, _Fusus_
and _Natica_. The actual junction of the two series is here difficult
to detect, as the White Chalk is itself altered in its upper part
into a hard gray crystalline limestone, simulating the Eocene beds
themselves, and there is little difference of dip between the two
series. The following foraminifera have been determined from this
locality by Mr. F. Chapman:—[52]
_Textularia? gramen_ | _N. discorbina_
|
_Globigerina bulloides_ | _N. sub-discorbina_
|
_Operculina complanata_, | _Orbitoides_ (_Discocycloides_)
var. _discoidea_ | _dispansa_ (_O. dilabida_, Schwager).
|
_Nummulites gizehensis_, |
var. _Pachoi_ |
|
_N. curvispira_ |
Within the synclinal fold at one point (Plate VII), 17 kilometres
north-east of Ain el Haiss, Eocene echinids and nummulites were
found, but the beds from which they had weathered had been almost
completely removed by denudation. The echinids have been recognized
by Blanckenhorn as the following:—
_Porocidaris Schmedeli_.
_Echinocyamus luciana_.
_Sismondia Sæmanni_.
The following two species of nummulites have been determined by
Chapman:—
_Nummulites Beaumonti_.
_N. sub-Beaumonti_.
[Sidenote: Beds of east side of depression.]
In the scarp due north of Harra, the Cenomanian sandstones and clays
with _Exogyra mermeti_ and _E. africana_ are conformably overlain
by a hard yellow crystalline limestone containing small nummulites;
so that here the Eocene clearly overlies the Cenomanian beds. If this
upper limestone bed be followed southwards along the scarp (see the
sections on Plate VIII) it appears to be continuous as far as the
road leading to Ain Haswi, where the ground forming the bottom of the
oasis-depression rises to the level of the limestone, and the clays
are completely hidden by sand and limestone-debris. There commences
here, however, a considerable thickening of the limestones, and hard
brown limestones, with calcite-filled cavities, and some soft earthy
beds come in; so that here we have the beginning of the series (No 6)
which has been described as recurring on the opposite scarp in the
same latitude, and which forms the hills north-west of Harra. The
topmost bed of the scarp is a hard crystalline limestone, which
appears continuous with that containing nummulites further north,
and the top of the plateau is therefore probably here also Eocene. A
little further south, the plateau-surface is covered with countless
small hills of white chalky limestones with siliceous beds; these
beds were searched without result for fossils, but as they overlie the
hard crystalline rock, above mentioned as probably Eocene, they must
be of Tertiary age, and therefore do not correspond with the Danian
white chalk of the western plateau. As the large hill near the plateau
east-north-east of Ain el Haiss is approached, the thickening of the
lower crystalline brown and grayish-white limestones is very marked,
and here they probably comprise both Cenomanian and Danian rocks; but
the dip south-eastward is so strong that the Eocene must be close to
the edge of the plateau, while the folding doubtless accounts for the
crystalline nature of the rocks and their poverty in fossils. South of
the large hill, a second plateau is met with at a distance of about a
kilometre eastward from the main scarp; this second plateau consists
of soft chalky beds similar to those above-mentioned, underlain by
clays and sandstones, while the main scarp still exhibits brown
and grayish crystalline limestones, though both the dip and the
thickness of these beds become gradually smaller. As the south point
of the oasis is approached the limestones thin out, till at the end
only a few beds of hard yellow-brown limestone occur in the clays;
these beds are continuous with those containing Cenomanian fossils
on the west plateau. Thus at the south-east portion of the scarp the
Danian is either absent, or represented by very thin beds at a little
distance from the oasis. It might be suggested that the soft chalky
limestones which form the second or upper plateau correspond with
the Danian white chalk of the west side; but several considerations
tend to negative this view. In the first place, it has been mentioned
above that similar soft chalky limestones occur at about the same
horizon further north, where they overlie beds which are probably
Eocene. Again, on the line followed from the south end of the oasis
of Minia, nummulites were found in abundance only a few kilometres
away from the oasis. And lastly, these chalky limestones never show
folding comparable with that of the lower beds, a fact which seems
nearly conclusive in view of the unconformity known to exist at
other points between the Cretaceous and the Eocene.
If the Eocene age assigned above to the chalky limestones of the
eastern plateau be correct, it is possible that the clays and
sandstones which underlie and separate them from the Cretaceous
limestones may represent the Esna Shales of the more southern oases,
though sandstones are elsewhere absent from this series.
_Unconformity and overlap._—From the above description of the
Eocene rocks of Baharia we see that their relation to the underlying
Cretaceous system is one of unconformable overlap.[53] In the north
part of the oasis we have the Eocene (Up. Lib.-Low. Mok.) overlying
directly the lower beds of the Cenomanian; further south, on the
west, the same beds overlie the White Chalk of Danian age, and no
doubt at an intermediate point the Eocene overlies the intermediate
beds. On the east side, though the actual demarcation of the Eocene
and Cretaceous beds is a matter of some difficulty, the difference
of dip between the two systems in some localities is well marked.
It seems certain then that after the deposition of the Cretaceous
beds in this region, elevation took place with a considerable amount
of folding (which will be noticed fully further on). The land thus
formed underwent a marked denudation before subsidence took it below
the surface of the sea and allowed of the deposition of the Eocene
deposits just described.
POST-EOCENE (OLIGOCENE?).
3. _Ferruginous grits and quartzites, with limonite and pisolitic
iron-ore._ As already mentioned, one of the most striking
topographical features of the oasis is the number of isolated
black, and for the most part perfectly conical, hills within
the depression. These hills are composed of the lower Cenomanian
sandstones and clays of the Cretaceous, and owe their existence
and dark colour to protecting caps of very hard, dark, ferruginous,
silicified grits and quartzites, often associated with limonite. The
first impression obtained on examining these rocks, is that they are
merely silicified and ferruginous bands of the sandstone series below,
which forms the general oasis-floor and part of the walls and hills.
A careful investigation of the deposits over a large area, however,
shows this view to be untenable; for while in most of the isolated
conical hills within the depression, the beds in question cap the
lower sandstones and clays (No. 7) of the Cenomanian, on a portion
of the edge of the western plateau they cap the bed of limestone
(basal member of No. 6) which itself caps the lower sandstones and
clays. (Plate I). There must therefore be an unconformable overlap
below the two series.
This is further borne out by the facts, first, that the ferruginous
silicified grit has never been observed to pass under the limestone
in the walls of the oasis, although found capping hills in close
proximity; and secondly, that the limestone never occurs below
the ferruginous silicified grit in the isolated conical hills. The
former would happen if the deposit in question represented the top
of the sandstones and clays, the latter if it represented the next
bed above the limestone.
Furthermore, these beds are found capping hills close to the
oasis-wall, and occurring on exactly the same level as the limestone
capping the latter, suggesting at first sight some sort of connection
between the two; the beds are, however, so entirely different that
it is not possible to imagine the one to be an altered condition
of the other; moreover, if such were so, the gradual passage of
limestone to the ferruginous beds should be visible, but such has
never been observed.
The only possible view of the origin of these beds, consistent
with the above facts, is that they represent a far younger deposit
than the strata on which they lie, a deposit formed in fact in a
slight depression in the Eocene and Cretaceous rocks, long anterior
to the time when erosion was carving out the area to its present
form. The pisolitic character of the iron-ore of Jebel Horabi and
the usually large amount of ferruginous material, as well as the
general character of the beds, indicate shallow-water lacustrine
deposition and precipitation. No organic remains have as yet been
observed in these deposits.
It should be mentioned here that it is frequently impossible to
draw any sharp of line of demarcation between these deposits and the
undoubted Cenomanian sandstones below, when they rest on the latter
in the hills within the depression. The sandstones themselves are
frequently ferruginous and limonitic in their upper layers. This,
however, is easily explained on the supposition that there would have
been considerable infiltration into these porous sandstones forming
the lake-floor, with consequent deposition of ferruginous material.
Lithologically the beds in question present a considerable similarity
to the quartzites and hard ferruginous sandstones of Jebel el Ghudda
and Gar el Hamra on the road from Feshn to the oasis (ante, p. 18,
20), and also to the beds of Jebel Ahmar, near Cairo, and in parts
of the Fayum. In the absence of evidence of their precise age,
they may be provisionally classed as Oligocene.
[Sidenote: Jebel Horabi.]
A short description of these beds in a few special localities will
now be given.
This well-marked hill, situated at the extreme northerly end
of the depression, consists of a mass of ferruginous material,
including limonite, pisolitic iron-ore, red and yellow ochre, etc.,
lying on a series of sandy shales, clays and sandstones belonging
to the Cenomanian (Series No. 7). Thin bands of limonite, etc.,
occur in the clays and sandstones, but the great mass of mineral
appears to form a distinct deposit capping, and in part replacing,
the former. The iron-ore occurs in every stage of purity.
Samples from Jebel Horabi, analysed by Mr. A. Lucas, gave the
following results:—
Pisolitic ironstone Ferric oxide 58·68% ≡ 41·07% iron.
Limonite „ 81·06% ≡ 58·84% „
[Sidenote: Isolated hills in centre of Oasis.]
These hills all show the same characters, with the exception of one
or two capped by limestone or basalt.
They consist of sandstones, with occasional shales and clayey bands,
of the Cenomanian series (No. 7) capped by a hard brown or black
silicified ferruginous grit, which frequently passes into a typical
quartzite. This cap may be of any thickness up to some seven or eight
metres. Its junction with the sandstones below is generally obscured
by the mass of talus lying on the slopes, but where visible it is
difficult to draw any line of demarcation between the two, there
being in some cases a more or less gradual increase of hardness and
ferruginous material from the upper part of the sandstones upwards. In
one hill an ordinary yellow sandstone was observed, when followed
up, to contain an increasing number of ferruginous concretions,
at first in isolated lumps, or strings, but higher in such quantity
as to present the appearance of a breccia, which latter gradually
passed up into a hard dense mass of ferruginous quartzite. This
at first sight suggested a similar age for the whole rock-section,
but would be quite explicable on the supposition of infiltration,
as suggested above.
2.—_Basalt and Dolerite._—Three large hills in the north-west
of the oasis, notably Jebel Mayesra, Jebel Mandisha, and the
northern half of Jebel Hefhuf, are capped with a basic volcanic
rock, the existence of which was noted by Cailliaud[54] so long
ago as 1820. Ascherson[55] further studied the distribution of
this rock in 1876, and collected specimens which were carefully
examined later by Prof. Zirkel.[56] The latter diagnosed the rock
as a typical plagioclase-basalt, strongly resembling that of the
Giant’s Causeway; it is finely holocrystalline, containing augite,
plagioclase and olivine, with some magnetite and ilmenite, and very
sparing flakes of biotite; the resemblance of the rock to basaltic
intrusions in Tripoli and at Abu Zabel (between Cairo and Bilbeis)
is remarked on by Zittel and Ascherson, and the opinion that it was
intruded into the sandstones in later Tertiary times is put forward.
The basaltic intrusions occur at four separate points in the oasis,
representing two more or less inclusive areas; the total area covered
by the rock is about 14 square kilometres. The intrusion appears
to have been in the form of laccolites; in Jebel Hefhuf the basalt
is intrusively interbedded in the Cenomanian, between the lower
sandstones and the overlying limestone, both of which rocks show
distinct signs of contact-metamorphism. The sandstone shows generally
very little alteration, though in some places much hardened; while the
limestone is highly crystalline and of a beautiful red colour near the
igneous rock. The crystalline nature is, however, more due to folding
than to contact-metamorphism, as it occurs in all the limestones of
the neighbourhood. In several places the vertical pipes, up which
the molten mass was thrust, with disturbance of the sandstone,
can be traced; and dykes are occasionally seen near the edge of the
deposits. The section seen in Jebel Mandisha is as follows:—
_Top._ Metres.
Columnar basalt and dolerite, much broken up by
weathering 9·2
Sandstones, clays and sandy shales 64·0
Ferruginous sandstones with casts of _Exogyra_ }
(thin bed) } 9·2
}
Sand-rock and sandstone with ferruginous bands }
----
82·4
In a small ⅂-shaped hill in lat. 28° 46½′ N., long. 28°
48′ E. of G. a mass of basalt occurs which appears to be mainly
the remains of a large pipe or “neck.” The igneous rock here
shows flow-structure at the sides, being hard and crystalline
in the centre. It has altered the sandstones at the contact
very considerably, converting them into a greenish rock full of
chalcedony. Here, as in the other deposits, the basalt shows distinct
columnar jointing and the resulting blocks weather into spheroidal
masses where exposed.
The igneous rocks of Baharia are therefore Post-Cretaceous in age
and it seems reasonable to assume, as suggested by Mayer-Eymar,[57]
that they are of Lower Oligocene age, contemporaneous with the
basalt-sheets of the Fayum, of Abu Roash and the desert to the
west, and of Abu Zabel. The andesite neck passed on the road between
Maghagha and the oasis (p. 22) and the larger masses of Bahnessa and
other places in the Western Desert were likewise probably erupted
at the same time.
RECENT.
1.—SAND-DUNES, SALINES AND SUPERFICIAL DEPOSITS.
_Blown Sand_, carried along by the prevailing north and north-west
winds, is heaped up against all obstacles in its path, such as hills
and vegetation; sometimes it is carried over these obstacles, only
to fall and accumulate in the wind-shadow on the other side. The
cultivated lands of the villages are for the most part surrounded
by sand-accumulations, and some low dunes occur in various parts of
the oasis; the positions of the latter will be better gathered from
the maps than from any mere description. Baharia itself is more free
from sand than any of the oases to the south of it, but even here
the deposits of this material are very considerable, and as already
mentioned extensive sand-dunes occur on the desert to the east,
at a short distance from the depression.
_Salt-swamps_ occur in the north of the oasis, having probably been
produced by the overflowing of springs forming lagoons, which have
taken the salt from the surrounding clays, the water from the springs
only containing a small percentage of salt. The salt-swamps round
Mandisha are now covered with a thin rough dry white crust of salt
mixed with clay and sand; when this is broken through a salty sludge
is seen to exist below, sometimes in a quite liquid state; the depth
of sludge is probably not very great, but there is enough to immerse
a camel up to its neck, as was found by experience during the survey.
_Clays._—Sandy clays of recent origin occur at isolated spots on
the floor of the oasis, notably near Ain el Haiss, where they are now
being rapidly removed by the action of the wind. Many of the springs
are also depositing a certain amount of clay on the low ground.
THE TECTONICS OF THE OASIS.
The rocks of the district have undergone a considerable amount of
disturbance along certain lines.
The most important fold is a well-marked syncline beginning at the
west scarp in lat. 28° 7′ N. and running across the depression in
a direction 30° north of east until it strikes the eastern escarpment
in lat. 28° 23′. This synclinal fold is remarkably sharp, the width
being generally only a few hundred metres, but the dips so strong
as to bring the limestone beds nearly vertical in places. Along the
line of fold we find the whole of the series 6 and 7 brought down
to the level of the floor of the oasis, but on either side the beds
so quickly regain their horizontal position that no trace of the
folding is to be seen (Plate VII). The White Chalk in these folds
is always altered by pressure into a hard crystalline limestone,
as are also the limestones of the Cenomanian below. The fold is not
actually continuous across the area, but is in reality made up of a
number of long narrow ellipsoidal depressions, or cup-shaped hills,
in which the end-dips are always less strong than those of the
sides. At its most southerly point, where it runs into the scarp
14 kilometres north-west of Ain el Haiss, the two series 6 and 7
are found dipping northwards at angles up to 40° (and at one spot
78°). On the other side of the fold the same beds dip southwards
at only 5°, and the appearance of the fold at this point suggests
the existence of an accompanying fault (fig. 1).
[Illustration: Fig. 1.—Section across Syncline, 12½ kilometres
N.W. of Ain Haiss.
_a._ Shales and sandstones with _Exogyra_, _Arca_, _Cucullaea_,
etc., _Neolobites vibrayeanus_, etc. _b._ Hard crystalline limestone
with flints. _c._ Sandstone passing into hard quartzite. _d._
Variegated sandstones with bands of sandy limestone. _e._ Hard
dark-brown siliceous crystalline limestone. _f._ Calcareous grit
(≡ bone-bed) passing in part into sandstone, and capped by hard
crystalline limestone with shells.]
Between this point and Jebel Hefhuf the fold is marked by a line
of low isolated limestone-capped, cup-shaped hills, the largest and
best marked of which is 16 kilometres north of Ain el Haiss. Here the
maximum dip amounts to 55° on the western side of the basin-shaped
hill.
Jebel Hefhuf is, perhaps, the most striking part of the fold. The
southern portion owes its existence entirely to this syncline,
consisting as it does of a long, and remarkably narrow, ridge-shaped
hill of folded rocks. Dips of 50° and 60° are common in places,
and the limestones implicated in the fold are usually rendered hard
and highly crystalline. The highest crystalline limestones in Jebel
Hefhuf probably represent the White Chalk, but the beds are too
crystalline to show any traces of fossils.
It is noteworthy that all the basaltic rocks of the oasis occur on the
north-west side, and at no great distance from this syncline. To the
north-east of Jebel Hefhuf the line of fold passes apparently between
the range of elongated hills west of Harra and the larger somewhat
triangular hill to the north of it, and although the limestones and
clays of these hills show a considerable amount of folding they do
not exhibit it in so striking a manner as Jebel Hefhuf.
Continuing the line till it cuts the last scarp, we arrive at a
prominent sandstone headland, in the gullies on either side of which
the folding can be very well seen. To the west of the headland the
beds are thrown sharply up in the valley so as to be nearly vertical;
this up-throw has resulted in an actual fault, the denuded edges of
the vertical sandstone and clay-beds being overlain by a horizontal
bed of limonite. The nose of the promontory is of sandstone, this
being thrown up somewhat on the south-east side of the fault;
the upper sandstone and clay beds dip south-east again somewhat
sharply on the face of the headland and come down to the level of the
oasis floor. It would seem therefore that at this point we have the
syncline between two sharp anticlines, the north western of which
has culminated in the fault.
The southern end of the oasis is cut out in a well-marked gentle
anticline.
On the west side from Ain el Haiss southwards the whole succession
of Cretaceous rocks dip steadily to the north-west at from four to
five degrees, while those on the east scarp show an equally constant
dip to the south-east, which increases further north, till in the
neighbourhood of lat. 28° 5′ its magnitude reaches 45°; it becomes
gradually less further northwards. Owing to this strong tilting of the
beds, the large hill near the scarp consists entirely of sandstones
and clays with a cap of quartzite and limonite, although nearly
or quite as high as the scarp; the beds here recover their nearly
horizontal position at a very short distance north-west of the line of
folding, as do also those of the plateau on the other side the line.
Besides the above-described disturbances of the beds, there are some
smaller local disturbances which are extremely well-marked. Thus
for instance the large flat-topped hill in N. lat 28° 7′ and
E. long. 28° 48′, like several smaller hills E.S.E. of it, bears
a cap of limestones, although its height is less than that of the
sandstone hills around it. The section of this hill at its western
side is as follows:—
_Top._ Metres.
Silicified limestone 0·6
Hard impure chalk 2·4
Silicified limestone 1·2
Pale green soft sandy and clayey limestone 6·1
Sandy ferruginous conglomerate 0·3
Clays with sandy and ferruginous layers,
extending to oasis floor. approximately 22·0
----
32·6
The beds are horizontal, and the neighbouring smaller hills which
possess a similar limestone covering are precisely of the same
height. It would seem impossible to account for these limestone
caps in the midst of higher sandstone hills except by assuming that
they mark the positions of islands, subsequently denuded, in the
post-Eocene lake which deposited the limonite caps on the other hills;
at the same time they evidently represent part of a synclinal fold,
though the horizontal position and non-crystalline character of the
beds show that this syncline must have been wide and shallow. It is
scarcely possible to think there can have been a locally calcareous
deposit from the lake which generally deposited the sandstone and
limonite; the sudden transition from the black sandstone and limonite
of the surrounding hills to the snow-white limestones of those under
consideration is so strongly marked in the field as to forbid such
a supposition. At the same time, no fossils were found in the beds,
so they cannot be satisfactorily correlated with the limestones of
the oasis-scarps, and they are of somewhat variable character at
different points of the hill.
It is evident then that in the south part of the depression the
Cretaceous beds as a whole form a large flat anticline, which is
continuous with that of Farafra to the south.[58] This bears out
the statement of Capt. H. G. Lyons,[59] some years ago, that he
believed Baharia and Farafra Oases to lie on an anticlinal at right
angles to that of Dakhla and Kharga. The anticlinal and synclinal
folds of Baharia are parallel and were evidently produced by the
same earth-movements.
[Sidenote: Age of the folding.]
Within the oasis-area too, some other traces of folding are visible;
in lat. 28° 11′ N., long. 29° 1′ E. are two small hills of clays
and sandstones (the latter locally green in colour), the beds of which
dip about 30° E. and W. respectively, showing a local anticline
whose axis runs nearly north-and-south. In this neighbourhood, as
in the oasis generally, it is not possible to trace the folding in
the beds of the low ground, owing to its thick sandy covering.
Another point of disturbance is on the western plateau in N. lat. 28°
26′, E. long. 28° 49′, some 11½ kilometres north-west of El
Qasr. Here (Fig. 2) along a N.E. and S.W. line, crossing an otherwise
level plateau, the Eocene beds, and probably also the Cretaceous
below, are thrown up in a sharp anticlinal fold, which however, is
not continuous but consists of a number of isolated bulges, with dips
up to 50°. The plateau at this point consists of Eocene Operculina
and Nummulitic limestone, and the effect of the anticlinal fold is
to expose the beds of sandstone below, which, as already mentioned,
are probably Cretaceous.
[Illustration: Fig. 2.—Sketch showing probable relations of Eocene
and Cretaceous in anticline on Western Desert plateau, 11½ kilometres
N.W. of El Qasr.
EOCENE.—1. Grey crystalline limestone. 2. Hard white gritty
chalky limestone. 3. Grey calcareous sandstone. 4. Limestone with
_Nummulites_ and _Operculina_.
UPPER CRETACEOUS.—5. Calcareous-grit. 6. White
sandstone. 7. Crystalline limestone. 8. Cenomanian clays
and sandstones with _Exogyra flabellata_, _E. Mermeti_ and
_E. olisiponensis_, etc.]
With regard to the age of the tectonic movements which produced the
synclinal and anticlinal folds of Baharia, it must be admitted that
it is not easy to speak with absolute certainty, owing to want of
evidence at one or two points. It appears certain, however, that
they belong to two great movements, separated by a vast interval in
time. The first disturbance of the beds would appear to have taken
place at the end of the Cretaceous period, when it seems probable
that the area in question underwent upheaval into dry land. It
seems tenable that it was during this movement that the large flat
anticline, so well seen in the south part of Baharia and in the north
of Farafra, was produced, as the Eocene beds forming the plateau
immediately to the east of the Cretaceous beds do not show any sign of
disturbance.[60] The subsiding Cretaceous land, on which these Eocene
deposits were laid down, must therefore have had the form of a long,
flat, irregular ridge of anticlinal structure, probably extending
from Dakhla through the oases of Farafra and Baharia to Abu Roash.
The other folds of Baharia, being parallel to the anticline just
dealt with, would at first sight appear to date from the same
movement. With regard to the syncline, however, two observations
appear which force us to consider its age as very much later; these
are, first, the occurrence of nummulites and other Eocene fossils
within the fold at a point 16 kilometres north of Ain el Haiss,
and secondly, the fact that at the northern extremity of the fold,
where it meets the eastern scarp, the Eocene beds are affected. With
regard to the first of these, the beds themselves, from which the
fossils had been derived, were not observed to be actually affected
by the fold, and therefore the evidence here, although suggestive,
is not conclusive. The evidence at the north end of the fold,
however, if substantiated, proves that the age of the fold must be
post-Eocene. With regard to the anticlinal fold on the north-western
plateau there is no question, as this clearly affects the Eocene
beds; it also is parallel to the other folds. These folds affecting
Eocene beds, lead us to the conclusion that there was another period
of possibly ever more important earth-movements, of the exact date of
which we cannot be certain, although it is not impossible that it was
closely connected with those great earth-movements of Pliocene times,
which gave rise to the chief topographic fixtures of N.E. Africa
and S.W. Asia.
[Sidenote: Summary.]
_Summary of the Geological History of the Oasis._—The oldest[61]
sedimentary deposit in Egypt is the Nubian Sandstone of the
Cretaceous. From the general absence of marine shells in this rock
we may premise that the deposit was laid down in an inland sea or
lake, which must have covered an enormous extent of country. In the
northern part of Egypt the upper part of the Nubian Sandstone becomes
fossiliferous, the fauna having an undoubted Cenomanian aspect; this
is the case in Wadi Araba, at Abu Roash and in Baharia. Further to
the south, however, as in Dakhla, the Nubian Sandstone first becomes
fossiliferous in Senonian times. We may explain this difference
in the two localities on the supposition that a gradual subsidence
was taking place with a corresponding gradual encroachment of the
sea from the north, which covered the northern part of the country
(including Wadi Araba, Abu Roash and Baharia) in Cenomanian times
but did not reach the latitude of Dakhla till considerably later,
i.e., in Senonian times.
From the Cenomanian onwards throughout the Cretaceous, the
Baharia area was one of continued subsidence, the lithological and
palæontological characters of the beds showing evidence of a gradual
increase in conditions of depth, until the maximum was reached in
the deposition of the White Chalk in Danian times.
Between the deposition of the uppermost Cretaceous rocks and the
Eocene it is probable that a considerable interval elapsed, during
which the Cretaceous was elevated into land, with much folding
and fracturing of the rocks and subsequent denudation.[62] It was
probably during this upheaval that the Cretaceous of Baharia assumed
its anticlinal structure.
Subsequently, subsidence taking place, the Eocene sea submerged the
area, and deposits were laid down on the uneven Cretaceous land in an
unconformable and overlapping manner. In Baharia the lowest member
of the Eocene of Egypt, the Esna Shales, is not present, although
further to the south towards Farafra it has been observed. On the
east side, some sandstones and clays met with below the white chalk
beds of the outer plateau may belong to this division.
The first undoubted Eocene deposits in the Baharia area are the
limestones with _Operculina_ and _Nummulites_ which unconformably
overlie different members of the Cretaceous in the north and west
sides, and eastward of the south end. The whole of the Eocene deposits
are here, however, only a few metres thick, which contrasts strangely
with the enormous thickness of the deposits of the same age in the
Nile Valley. This is intelligible, however, on the supposition that
near the subsiding Cretaceous land the conditions for continued
accumulation of deposits were not so favourable as further to the
east, where deeper water conditions obtained.
Subsequently, in Post-Eocene times, the whole underwent upheaval, and
it is probable that during this elevation the main synclinal fold[63]
was produced, together with the minor anticline. The evidence for
placing the date of the formation of the syncline anterior to the
deposition of the ferruginous grits, limonite, etc. (Series No. 3)
stands on the following basis: the absence of proof of the folding in
question having affected the beds of Series 3, and the presence of a
horizontal deposit of limonite on the upturned edges of the strata,
at the point where the fold meets the eastern scarp (page 66). About
the same time, probably, basalt and dolerite was intruded into the
Cenomanian rocks below.
[Sidenote: Formation of depression.]
As a result of the sharp folds the upper limestones were cracked, and
their denudation by natural agencies followed, forming a slight hollow
similar in shape to that which the oasis now exhibits; the agent of
denudation cannot be stated with certainty, but whatever force came
into operation it would find easy work in the cracked-up rocks, and
still easier would be its task in partly removing the soft Cenomanian
sandstones and clays after the harder limestones had disappeared. The
primary excavation of the hollow was followed by the formation of a
great lake, in which were laid down deposits of sandstone, quartzite,
and iron-ore; this lake doubtless surrounded islands, represented
to-day by those hills which still preserve their limestone-caps;
it extended, or similar lakes existed, beyond the oasis-limits,
forming the quartzites and ferruginous sandstones passed on the
way from Maghagha to Baharia, and was perhaps continuous with the
Oligocene and post-Oligocene sea which covered a large part of the
country to the north.
In later times the area finally became continental and denudation
gradually sculptured the oasis to its present form; this sculpturing
would no doubt proceed rapidly in the moist climate which is known
to have existed in Egypt in Pliocene and early Pleistocene times,
and is being continued to-day by the powerful agency of the desert
wind-borne sand and changes of temperature.
The water-supply of the oasis is probably derived from the tropical
rains of the mountainous regions of Central Africa, the water from
which penetrates the ground and flows northwards along permeable beds
of sandstone, etc., in which it is confined by other impermeable
strata, until tapped naturally or artificially in the great oases
or depressions of the Libyan Desert.
* * * * *
[Footnote 35: _Voyage à Méroé_, etc., _op. cit._]
[Footnote 36: _Geologie, u. Palæontologie der Libyschen
Wüste_. Cassel, 1883.]
[Footnote 37: Zittel had divided the Cretaceous of the Western Oases
into the following main divisions:—
_Top._
1. Well-bedded limestone and earthy chalk.
2. Greenish and ash-grey shaly clays.
3. Beds with _Exogyra Overwegi_.
]
[Footnote 38: _On the Stratigraphy and Physiography of the Libyan
Desert of Egypt_, Quart. Jour. Geol. Soc., Nov. 1894, Vol. 50,
p. 535.]
[Footnote 39: Letter from Prof. Zittel to Capt. Lyons, dated May
19th, 1894. It was afterwards suggested (BEADNELL, _op. cit._
Int. Geol. Congress, p. 10) that these specimens were possibly not
_E. Overwegi_ at all, and Mr. Bullen Newton, of the British Museum,
who has undertaken the determination of the Baharia collection,
confirms this, stating that the so-called specimens of _Exogyra
Overwegi_, referred to in Capt. Lyons’ paper, are in reality
examples of _E. mermeti_.]
[Footnote 40: A short account of the geological structure of Baharia
has already been published:— BEADNELL, op. cit. (_Découvertes
Géologiques Récentes_, etc.) Int. Geol. Cong. 1900; Paris, 1900.]
[Footnote 41: This, with the 6 metres of Eocene limestone above,
gives the height of the hill above the floor as 174 metres. Ascherson
gives 162, but the difference is probably due to our readings being
commenced on the lowest ground in the neighbourhood, where the oldest
beds were exposed.]
[Footnote 42: The specific names given in this report are based on
determinations made by Dr. Blanckenhorn in Cairo and by Mr. Bullen
Newton at the British Museum.]
[Footnote 43: In connection with the discovery of Cenomanian deposits
in Baharia Oasis, _vide_, _Geological Magazine_, Nos. 127, 430, 432,
1900 (H. J. L. B.).]
[Footnote 44: A very striking feature of the brown limestones is
the abundance in them of calcite-lined cavities; these are often
arranged in planes parallel to the bedding, as is well seen for
instance in the hills north-west of Harra.]
[Footnote 45: Mr. Gorringe first drew attention to the presence of
fossil wood in these beds.]
[Footnote 46: An analysis of this bone-bed is given in _Report on the
Phosphates of Egypt_, Geol. Surv. Egypt, publication, Cairo. 1900.]
[Footnote 47: BEADNELL, _Dakhla Oasis, its Topography and
Geology_. Geol. Surv. Egypt. Report, 1899, Pt. IV. Cairo. 1901,
pp. 96-98.]
[Footnote 48: BARRON and HUME, _Topography and Geology of the Eastern
Desert of Egypt_, Geol. Surv. Report, Cairo, 1903.]
[Footnote 49: It may perhaps be that the lowest limestone bed
disappears, having thinned out, in which case it is difficult to
distinguish the sandstones and clays of this division from those of
the one below.]
[Footnote 50: Zittel, _op. cit._]
[Footnote 51: As it seems probable that the Lower Mokattam is a
somewhat local development of the upper part of the Libyan Series,
and it being difficult or impossible in many areas to separate the
two, we shall in our description of the Eocene of this oasis make
no attempt at a division.]
[Footnote 52: CHAPMAN, _Geol. Mag._ Dec. IV, Vol. IX, Feb. and March,
1902, pp. 62-67, 106-114.]
[Footnote 53: It should be noted here that the presence of an overlap
was suspected by Capt. H. G. Lyons as long ago as 1894, as in his
paper (_op. cit._ 535) he says:—
“At the north-east and east of the Baharia Oasis the Upper Mokattam
beds, characterized by _Ostrea Fraasi_ and _O. Cloti_ (as kindly
determined by Dr. Zittel), occur 30 miles north-east and 20 miles
east of Upper Cretaceous beds containing _Exogyra Overwegi_ in the
oasis, and with a difference in altitude of less than 200 feet. As
there is no marked dip of the beds, we have evidently an overlap
of the remainder of the Cretaceous beds and the Libyan and Lower
Mokattam beds of the Eocene, and this inference is borne out by the
short distance between the Cretaceous and Miocene outcrops to the
west on Dr. Zittel’s route to Siwa.”]
[Footnote 54: _Voyage à Méroé_, I., p. 189. The same observer
also noted the presence of nummulites in the northern scarp, and the
ferruginous nature of the sandstones composing the hills of the oasis
(p. 190).]
[Footnote 55: _Zeitsch. d. Ges. f. Erdkunde zu Berlin_, 1885, p. 134.]
[Footnote 56: _Geol. d. Liby. Wüste_, 1883, pp. 122, 123.]
[Footnote 57: _Le Ligurien et le Tongrien en Égypte_,
Bull. Soc. Géol. France, sec. III, vol. XXI (1893).]
[Footnote 58: BEADNELL, _op. cit. (Farafra Oasis)_ p. 28.]
[Footnote 59: LYONS, _op. cit._ pp. 537-540.]
[Footnote 60: _Op. cit._ (_Découvertes Géologiques Récentes_,
etc.) pp. 855-856.]
[Footnote 61: We except, of course, the very local areas where a
Carboniferous fauna has been detected.]
[Footnote 62: Compare, _op. cit. The Cretaceous Region of Abu
Roash_, etc.]
[Footnote 63: Or, as suggested above, these folds may have
been produced in later times, i.e., Pliocene, when important
earth-movements were known to have taken place in N.E. Africa and
S.W. Asia.]