Page:Philosophical Transactions - Volume 145.djvu/153

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137
MR. HORNER ON THE ALLUVIAL LAND OF EGYPT.

MR. HORNER ON THE ALLUVIAL LAND OF EGYPT. 137 Synopsis of the Soils sunk through in the nine excavationsat Heliopolis. By an examination of the preceding tables and the diagram, Plate IV., it will be seen that the soils consist of two principalkinds I. EARTHS(1, 2, 3), more or less sandy and calcareous, varying in colour forom a dark blackish brown to a light grey, but evidently so nearly allied, passing by such insensible shades into each other, and having, with slight variations, so great a resem- blance to the modern Nile sediment, that they may all be classed as belonging to what is commonly called Nile mud, the earthy matter deposited by the river during the inundations; and II. SANDS(4, 5, 6, 7), partly mixed with indurated portions of Nile mud, but chiefly a pure quartzose sand, similar to that of the adjoining desert. I have distinguished the chief varieties of each kind by different shadings in the accompanying Plate;, whereby it will be more readily seen, that in the same hori- zontal plane, even in this limited space of half a square mile, there is a very con- siderable differencein the nature of the soil. Although it might, a priori, have been expected that fine earthy particles gradually subsiding from tranquil water, year after year, would form a series of thin layers, in none of the excavations was there an instance of the lamination of the sediment. To this remarkablefact, observed in all the excavations, both here and on the site of Memphis, I shall have occasion to refer in a subsequent part of this memoir. When we consider the small amount of sediment left annually by the inundation in any one place, it is very difficult to conceive how there should be in any one spot so great a thickness of one kind of sediment without any lamination or other sign of successive deposition. For example, in the Excavation E. there is scarcely any per- ceptible differencein the nature of the soil to a depth of 122.feet, a thickness which, if accumulated by annual deposits, would be the work of a vastly long period*. But this great amount of thickness of one kind of soil becomes still more remarkable when we find other varieties at the same level in the immediate vicinity, as may be seen in the sections of these excavations. It is evident that other causes than the tranquil operation of annual inundations must have been at work in the formation of this portion of the alluvial land. The crystalline quartzose sand, it will be seen, was found to the greatest amount in the pits nearest to the desert; and as it is not at all probable that matter so coarse would be suspended in the inundation water, especially in this locality, the layers of sand were most likely blown across the valley from the desert. But further general remarks, and all inferences as to the secular increase of the alluvial deposits, the main object of this inquiry, I must defer, until I shall have had an opportunity of laying before the Society an account of the far more extensive researches that were carried on in the year 1852 in the district of Memphis, and

  • Were we to adopt the estimate of secular increase given by M. GIRARD, viz. 5 inches in a century, it

would amount to 3000 years. MDCCCLv. UT