CHAPTER II.. * * * * *

* * * * *

SURVEYING METHODS AND GENERAL RESULTS.

Crossing the Libyan Desert from Maghagha and Minia respectively, the two parties of the Geological Survey met at a joint camp close to Zubbo, one of the chief villages of the oasis. It was desirable to fix this meeting-point as a primary station for the subsequent plotting of the maps. Cailliaud gave the latitude of Zubbo as 28° 21′ 47″, and its longitude as 26° 43′ 46″ E. of Paris, (equivalent to 29° 3′ 55″ E. of Greenwich). Jordan gave for Bawitti the position N. lat. 28° 21′ 12″, long. E. of Greenwich 28° 56′ 45″. Taking the difference of latitude and longitude between Bawitti and Zubbo as found by the Survey, viz., lat. + 55″, long. + 4′ 16″, we have as the equivalent of Jordan’s latitude for Zubbo 28° 22′ 7″, and for his longitude 29° 1′ 1″ E. of Greenwich. Thus, while there is a fair agreement between the two authors as regards latitude, there is a difference of 2′ 54″ in the values of the longitude.

The longitude is of course always the difficult matter in the geographical determination of places not easy of access. Cailliaud’s value rests on the method of distance and azimuths, both roughly determined only; Jordan, on the other hand, used the absolute though not very precise method of lunar distances measured with a sextant. The Survey parties depended entirely on direct measurement by means of measuring-wheels from known points in the Nile Valley; the values obtained in this way are tabulated below.

I.—Traverse from Maghagha to Zubbo.

Maghagha Railway Station, long. E. of Greenwich 30° 50′ 49″

Recorded west departure, Maghagha to Zubbo, 187·77 km. 1 56 46 ------------- Giving long. of Zubbo 28 54 3

II.—Traverse from Minia to Zubbo.

Minia Railway Station, long. E. of Greenwich 30° 45′ 39″

Recorded west departure, Minia to Zubbo, 176·32 km. 1 47 50 ------------- Giving long. of Zubbo 28 57 49

III.—Traverse from Zubbo to Minia, via the south end of the Oasis.

Recorded east departure, Zubbo-Minia, 174·87 km. 1° 46′ 45″

Minia Railway Station, long. E. of Greenwich 30 45 39 ------------- Giving long. of Zubbo 28 58 54

The arithmetic mean of these three determinations is 28° 56′ 55″; owing, however, to the breakdown of the measuring-wheel during the outward traverse from Maghagha, and the consequent necessity of estimating a part of the distance traversed by the time taken in marching, the different traverses are not equal in value, and the longitude finally adopted by the survey, as the best approximation after investigation of the various sources of error in the measurements, was 28° 58′ 34″. It would thus appear that Jordan’s position may be a little too far east, the difference amounting to 2′ 27″, or about 3½ kilometres, while Cailliaud’s value would place the position at a rather less distance west of that adopted.

The survey observations confirmed the accuracy of Jordan’s latitude (28° 22′ 7″), from which the value found by Cailliaud differs, as already remarked, only slightly.

The surveying operations within the oasis were based on a rapid plane-table triangulation from a measured base line within it, details being simultaneously sketched in, on a scale of ¹⁄₅₀₀₀₀. The site chosen for the base was a level stretch of ground extending between the camp at Zubbo and an isolated clump of date-palms to the north-east; the length of this line was found by repeated wheel-measurement to be 3·88 kilometres. The two parties ran off their triangulations from this base to the surrounding hills, and carried on the mapping southward by plane-table, taking stations chiefly on the hills and prominent points of the scarps, and not meeting again till Ain el Haiss, in the southern part of the oasis, was reached. Here a test was made as to agreement of the two sets of maps, only a small difference being found. The position of Ain el Haiss, as found by taking the mean of the two determinations, is latitude 28° 2′ 11″ N., longitude 28° 39′ 19″ E. of Greenwich; this places the spring about 4′ 18″ east of Jordan’s determination (lat. 28° 1′ 55″ N., long. 28° 13′ 47″ E. of Green.).

The two parties made a third connection at the extreme south end of the oasis-depression. Our observations for this point give its latitude as 27° 48′ 13″ N., and its longitude as 28° 32′ 19″ E. of Greenwich, placing it very near the position shown on Jordan’s map.

The plane-table method making use of the magnetic meridian, it was imperative to determine the amount of declination of the compass. This was done at one point only, viz., at the south end of the oasis, the value found (by observation of the transit of Polaris) being 4° 50′ W. The declination is fairly constant over the entire area, except near the eruptive dolerite masses, the magnetite in which causes a very sensible deflection of the needle; in the neighbourhood of these, however, the surveying was carried on independently of the compass. With regard to the yearly change of declination, we have as data the previous observations of Cailliaud, who found the declination at Zubbo in January, 1820, to be 12° 13′ W.; of Jordan, who obtained the value 6° 56′ W. in March, 1874; and of Capt. Lyons, whose observations with a Bamberg declinatorium at Mandisha in April, 1894, gave the value 5° 8·9′ W. Tabulating these:—

Observer. Date. Observed Yearly change. Declination W.

Cailliaud January 1820 12° 13′

Jordan March 1874 6° 33′·6 6′·3

Lyons April 1894 5° 8′·9 4′·2

Geological Survey December 1897 4° 50′ 5′·3

In view of the magnitude of the diurnal variation, which may range up to 10′ of arc, and our present lack of knowledge of the distribution of this diurnal variation during the twenty-four hours, a comparison after so short an interval as that between the last two observations is not to be trusted. It would seem proved from the three foregoing observations that the yearly variation is at present decreasing, the mean from 1820 to 1874 being 6′·3 as against 4′·2 for the period 1874-1894. This decrease is also noticeable in comparisons of the declinations observed at different times in other parts of Egypt.

The altitudes above sea-level of the principal points, more especially in the eastern half of the oasis, were determined with a Watkin aneroid barometer, which had been compared with the Cairo standard mercurial barometer. A fairly long stay was made at the Zubbo camp, and the altitude of this point may be regarded as fairly accurately fixed by the observations tabulated below:—

-----------------------+-------------+--------------+--------------- | Zubbo. | Cairo. | Date and Time. | Bar. | Bar. | Difference. |(corrected). | (corrected). | -----------------------+-------------+--------------+--------------- | mm. | mm. | mm. | | | October 12, 6 p.m. | 753·54 | 761·10 | 7·56 | | | „ 14, 8 a.m. | 754·94 | 763·04 | 8·10 | | | „ 6 p.m. | 753·79 | 761·53 | 7·74 | | | „ 15, 7.30 a.m. | 754·55 | 762·60 | 8·05 | | | „ 9 p.m. | 754·30 | 762·64 | 8·34 | | | „ 17, 7.15 a.m. | 754·18 | 761·84 | 7·66 | | | „ 18, 7 a.m. | 753·79 | 761·64 | 7·85 | | | noon | 752·52 | 761·34 | 8·82 | | | „ 19, 8.30 a.m. | 752·65 | 762·31 | 9·66 | | | Nov., 26, 3 p.m. | 754·81 | 763·29 | 8·48 | | | „ 27, 8 a.m. | 754·81 | 764·12 | 9·31 -----------------------+-------------+--------------+--------------- Mean | 8·32 --------------------------------------------------------------------

Since 1 mm. of mercury corresponds at the mean temperature of observation (20° C.) to 11·4 metres of height, we have height of Zubbo camp above Cairo observatory = 8·32 × 11·4 = 94·7, or say 95 metres. Since the observatory is 33 metres above sea-level, the camp at Zubbo is 128 metres above sea-level. Jordan’s altitude for Bawitti, which probably lies at about the same level as Zubbo, is 113 metres, and when it is remembered that the point of the Survey’s observation lay not in Zubbo itself, but at the camp on elevated ground some 10 or 12 metres above it, the results show a very good agreement.

At Ain el Haiss three barometric observations were taken on different days; the comparison of these with the Cairo records would place this point 156 metres above sea-level. Jordan’s value is 122 metres, the number of observations on which this figure is based not being stated, it is difficult to say which of the two altitudes is the more probable.

At the remaining camps within the oasis, and at the camps en route between the oasis and Minia, corresponding observations were taken, the number of comparisons with Cairo varying from two to seven at an individual station. These observations being reduced and corrected by comparison among themselves gave the levels of the different camps with some degree of approximation to accuracy, and the altitudes of intermediate points were found by interpolation based on barometric readings. The resulting altitudes will be found on the map (Plate I); where no altitudes have been taken by the Survey, the values given by Ascherson on his map have been inserted.

The statistics relating to the oasis, and the methods of cleaning out wells, are based on information supplied by the Government officials at Bawitti, and may be taken as fairly reliable. The particulars regarding water-supply are of course based mainly on direct observation during the survey of the villages.

The botany of the oasis having been fully studied by Ascherson no attempt was made by the Survey to collect or describe the plants met with. The abundant growths of the beautiful maiden-hair fern (Adiantum Capillus Veneris) will not, however, fail to strike even the casual visitor to the old Roman aqueducts, which still serve as the principal water-channels of the oasis. Nor were the animals of the oasis made the subject of any detailed observations, although the existence of several of the species of lizards and snakes common to the Nile Valley was recorded, and specimens collected when easily obtainable.

Baharia is not rich in archæological remains, and, with few exceptions, even those existing were not examined by the Survey, though the positions of all ruins met with during the work were mapped. In a later chapter will be found a connected though brief account of all the antiquities noted, the publications of previous authors being referred to wherever the descriptions cannot be given from personal observations.

The principal point attended to in the topographical mapping by the survey was the accurate delineation of the bounding scarps of the oasis and of the large number of hills within it. These features, of which an accurate map was essential for any proper consideration of the geology, had been only rapidly sketched by previous travellers, and the precise shape of the oasis was still unknown. In the cultivated spots, on the other hand, much had been done by Cailliaud, Jordan and Ascherson towards mapping the detail. Hence, beyond re-determining the precise positions of the main points and the general limits of the cultivated areas, no attempt at detailed mapping in these areas was undertaken by the Survey parties, it being felt that it would be preferable in the limited time available to concentrate attention on the almost totally unknown features, so essential to any geological consideration, rather than to devote considerable time to the details of the inhabited spots.

Thus, while the field maps resulting from the survey represented the oasis for the first time in its true shape, and the hills within it in their true relative magnitudes and positions, they fell somewhat short of the maps of Ascherson and Cailliaud in the number of springs, ruins, etc., shown. The more important ruins overlooked by the survey have, however, been inserted approximately from existing maps, and the whole result (Plates I, III and VII), is an advance on the existing representations.

The geology of the oasis was very carefully investigated, this being a field in which comparatively little had been done, and a number of very interesting results were obtained. The principal points in this connection worthy of note resulting from the detailed examination of the scarps and hills are—

(i) The existence of a marked unconformity between the Upper Cretaceous and Eocene strata, thus confirming the unconformity between these two great systems which had been noted earlier at Abu Roash; this unconformity has now been remarked in many parts of Egypt. The palæontological proofs of this unconformity were obtained from the western scarp, the beds of the eastern side, though they show the same thickening, being much poorer in fossils;

(ii) The occurrence of an extensive series of Upper Cretaceous beds of Cenomanian to Danian age within the depression and forming a large part of the desert to the west;

(iii) The precise extent and relations of the dolerite capping some of the sandstone hills in the north of the depression;

(iv) The existence of well-marked folding having an important bearing on the origin of the oasis;

(v) The presence of ferruginous sandstone deposits of later origin than the primary formation of the oasis-hollow, though long anterior to the date when the work of excavation, which gave the oasis its present form, took place.

These points will be found discussed at some length in the chapter on the geology of the oasis.

* * * * *

The return traverse via Farafra to Assiut is left out of consideration owing to its great length and consequent low value in the determination of the longitude of Zubbo. It agrees however, very closely with the others, owing to compensation of errors.]