102	ARABIA  INFELIX
main wadis. These channels have been traced with great skill and a marvellous eye for level: occasionally they present the striking portent of water running uphill. This occurs as follows: a flimsy barrage of thorny bush and stones is laid across the dry bed of the wadi and, just above this point, a broad irrigation channel will be cut in a gentle slope up the bank, with its intake pointing up stream and its outlet onto the adjacent fields.
When the spate comes down, it is checked by the slight obstacle in the wadi and, during that check, the first shallow rush of the stream swirls up the slight ascent of the intake and down into the nearest field, to be let through gaps in the surrounding bunds (or earthen banks) onto successive fields, as far as the supply will hold out. A big spate will of course gather force and sweep over the flimsy obstacle in its bed but, having no great grip on such a barrier, will not carry it down stream. The barrage is left for a future occasion, probably stronger than before, as the flood-wrack will have helped to bind it together. The gaps in these bunds are closed at all other times, and a rainstorm may be relied upon to flood each field within its scope ; thanks to the bunds, the water stands until it sinks into the ground, so each field gets the full benefit attainable.
Agriculture in the mountains has to meet very different conditions. To begin with, the terrain is rocky, and water-channels cannot be cut, as in the loamy soil of the lowland valleys. Then, too, the rainfall is greater, and the tremendous surface drainage, on such hard ground, cannot be handled by the primitive means available to local husbandmen.
Moreover, the soil has been formed under other conditions. In the lowlands, all the larger wadis have