As a rule, it is the magnesium salts which are the most harmful constituents, the sulphate giving a strong purgative character to the water. Such salts are naturally present in greatest quantity where the rocks are gypseous, as for instance along the coast to the south of Halaib, where the wells of Ti Kureitra, sunk in gypsum and lined with blocks of selenite, yield water of so purgative a character that all my Arabs who drank of it became violently ill. In districts where the rocks are of very basic igneous types, such as gabbros and serpentines, the salt-content may also be fairly high. Water from diorite country is somewhat better, that from granite better still, and that from sandstone best of all. There are practically no wells in clayey strata within the region here treated of, but Bir Qoleib, which is on the road from Daraw at about two days’ journey east of the Nile, is sunk at the foot of a clayey scarp, and the water, when I partook of it in 1907, was clouded with clayey matter so finely divided as to be unfilterable through a Berkefeld filter even under strong pressure, while the physiological action of the water was to produce very marked constipation. Wells near the sea coast are liable to be very salty from infiltration of sea water. As a rule, the very salt wells, such as Bir Murra, Bir Muelih, Bir Shalatein, and Ti Kureitra, are used only by camels and sheep. When the Arabs have to drink purgative water for lack of other supplies, they often mix milk with it, and I have found it wise to imitate them in this respect, with liberal addition of brandy in some cases. Wells are generally named after the wadi in which they occur, _e.g._, Bir Abu Hashim is the well in Wadi Abu Hashim. A bitter well is often called Bir Murra.[118] A deep well is called _Sararat_ by the Bisharin, _e.g._, Bir Sararat Seyet is the deep well in Wadi Seyet.