The Delta of the Nile, though not quite one-half as large as the Delta of the Ganges, presents nevertheless some features of peculiar interest. In many places where a vertical section is exposed to view, the phenomenon of stratification may be distinctly recognized. The upper part of the deposit belonging to each year is composed of earth of a lighter color than the lower part; and the whole forms a distinct layer of hardened clay, which may be easily separated from those above and below. This formation, therefore, corresponds exactly with those strata of shale which we so often meet with in the Crust of the Earth. Again, many of the old channels through which the Nile made its way to the sea in ancient times, have been since filled up and converted into solid land. The two extreme arms of the river, which formerly enclosed the Delta, were two hundred miles apart where they entered the Mediterranean. But these channels are now Alluvial Plains, and the base of the Delta is but ninety miles in length. Hence, though the quantity of land which has been formed by the sediment of the Nile is much greater now than it formerly was, the size of the Delta properly so called has not been increased but diminished. If we turn to the great continent of America, we are met by results not less striking and important. The Delta of the Mississippi is two hundred miles in length, and one hundred and forty in breadth. This vast stratum of mud is between five and six hundred feet thick, and covers an area twelve thousand square miles in extent. Each year it receives from the great _Father of Rivers_ a new accession of sediment which is computed at 3,700,000,000 of cubic feet. And besides this annual deposit of inorganic matter, we must not omit from our estimate the countless trees of various species and of gigantic size, which are torn up by the floods, carried along by the impetuous stream, and buried at last with the bones of animals, and works of human art, and other spoils of the land, in the mud of the Delta at the river’s mouth.[45] [Illustration] [Illustration] _CHAPTER VII._ STRATIFIED ROCKS OF CHEMICAL ORIGIN. _Chemical agency employed in the formation of mechanical rock--But some rocks produced almost exclusively by the action of chemical laws--Difference between a mixture and a solution--A saturated solution--Stalactites and Stalagmites--Fantastic columns in limestone caverns--The grotto of Antiparos in the Grecian Archipelago--Wyer’s cave in the Blue Mountains of America--Travertine rock in Italy--Growth of limestone in the Solfatara Lake near Tivoli--Incrustations of the Anio--Formation of travertine at the baths of San Filippo and San Vignone._