Page:Philosophical Transactions - Volume 145.djvu/147

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MR. HORNER ON THE ALLUVIAL LAND OF EGYPT. 131 Besides the above samples, I. to XVII., IEKEKYANBEY distinguishes in his sections of the pits layers of soils which are mixtures of the above varieties; thus we have No. Natureof the soil. XVIII. A mixture of black mud with clay, as if they were kneaded together. XIX. A mixture of compact black mud, clay and river-sand in. nearly equal quantities. XX. Nearly equal proportions of compact black mud, river-sand and fine rubbish. XXI. River-sand mixed with a little black mud. It will be seen that in all the excavations the downward progress was interrupted by filtration water. This is mainly derived from the Nile, but occasionally from side torrents after rain. As the river rises, the level of the water absorbed by the soil on its banks does not keep pace with the rise, for the water takes time to spread late- rally, according as the soil is more or less pervious; and should its descent be im- peded by a compact layer, it will continue to spread until it is exhausted at a con- siderable distance from the river. When the Nile falls, that portion of the filtration water which has not penetrated the soil to a depth below the river's ebb level, returns into the channel; but the amount returned will also depend upon the more or less pervious nature of the soil; and when retained by a compact layer, it will remain for some time at a higher ievel than the falling surface of the Nile. EXCAVATION A. Ten men were set to work at this spot. A trench was commenced from a point opposite to and 40 feet distant from the north face of the obelisk, and carried southwards, descending by steps, so as to form an inclined plane downwards. In two days the upper surface of a red sandstone block was reached, being the pedestal upon which the granite obelisk immediately rests. The upper surface of this block was 5 feet 6 inches below the surface of the ground immediately round the obelisk. A trench was opened opposite to the southern face of the obelisk, 35 feet distant from it, and when carried forward laid bare the pedestal to a depth of 41 feet from, its upper surface. The filtration water having been reached, it was baled out, and it was discovered that the pedestal rests i'mmediatelyon two layers composed of several blocks of limestone, and under the lower of these white sand was found. The pedestal is 6 feet 10- inches in height; the first layer of limestone on which it rests is 1 foot 4 inches, the lower layer 1 foot 3 inches in thickness. The limestone is the numnmulitelimestone of the country; the sandstone of the pedestal is identical with that of the neighbouring hill, Gebel Achmar, that is, the upper sandstone in an indurated state. An iron bar 16 feet long and 1i inch in diameter, was worked perpendicularlyby T2