They penetrate these to the depth of from ten to twenty feet, in order to reach a lower deposit distinguished by the name of _Nellan_, in which the objects of their search are found.

This is of so early a formation that it underlies the present beds of rivers, and is generally separated from them or from the superincumbent gravel by a hard crust (called _Kadua_), a few inches in thickness, and so consolidated as to have somewhat the appearance of laterite, or of sun-burnt brick.

The nellan is for the most part horizontal, but occasionally it is raised into an incline as it approaches the base of the hills.

It appears to have been deposited previous to the eruption of the basalt, on which in some places it reclines, and to have undergone some alteration from the contact.

It consists of water-worn pebbles firmly imbedded in clay, and occasionally there occur large lumps of granite and gneiss, in the hollows under which, as well as in "pockets" in the clay (which from their shape the natives denominate "elephants' footsteps") gems are frequently found in groups as if washed in by the current.
The persons who devote themselves to this uncertain pursuit are chiefly Singhalese, and the season selected by them for "gemming" is between December and March, when the waters are low.[1] The poorer and least enterprising adventurers betake themselves to the beds of streams, but the most certain though the most costly course is to sink pits in the adjacent plains, which are consequently indented with such traces of recent explorers.