LYDIAN-STONE. 15S Geognostic Situation. It occurs very frequently along with common flinty- slate in beds in primitive clay-slate ; but it has not been found in any of the older primitive rocks. It occurs in Masses of various sizes, imbedded in grey-wacke, and in beds that alternate with strata of that rock. A rock Ver y nearly allied to it occurs in beds in the oldest coal formation, viz. that associated with the old red sandstone, ar >d in some newer coal formations. Geographic Situation. It is found near Prague and Carlsbad in Bohemia ; at Hainchen near Frey berg in Saxony; in the Hartz; and Jl * the Moorfoot and Pentland Hills, near Edinburgh. I Use. This mineral is sometimes used as a touchstone, for *scertaining the purity of gold and silver. When we ' Vls h to determine the relative purity of different kinds of gold and silver alloys, we draw the alloy across the sur face of the stone, and compare the colour of its trace ' Vl th that of the pure metals, or of known compounds of Ihese metals, and we thus obtain by simple ocular inspec tion a pretty correct knowledge of the purity of the al loy. •A. good touchstone should be harder than the metals 0r metallic compounds to be examined: if softer, the powder of the stone mixes with the trace of the metal, an d obscures it. It must also possess a certain degree of r °ughness on its surface, in order that the metal may leave * sufficiently distinct trace or streak ; it must not, how- Tol. I. N mr,