In this part of its course the rocky sides of the valley, which sometimes closely approach the river, are composed of marls and gypsum, with occasional selenite, overlaid with sandstone, with a topping of breccia or conglomerate, and rise at places to a height of 200 ft.
The mineral was probably included with selenite under Pliny's term lapis specularis.
Thus the sulphate constitutes the minerals anhydrite, alabaster, gypsum, and selenite; the carbonate occurs dissolved in most natural waters and as the minerals chalk, marble, calcite, aragonite; also in the double carbonates such as dolomite, bromlite, barytocalcite; the fluoride as fluorspar; the fluophosphate constitutes the mineral apatite; while all the more important mineral silicates contain a proportion of this element.
It consists mainly of grey dreary flats covered with selenite; and a little below the surface, gypsum.
Soc., 1898, 20, p. 595), from the analysis of silver selenite and the reduction of the double selenium ammonium bromide, give values from 79 2 77 to 79.3 6 7; whilst J.
Meyer (Ber., 1902, 35, p. 1 59 1) by the electrolysis of silver selenite in the presence of potassium cyanide obtained the value 79.22.