The mineral composition of rocks, often an important test in comparatively unfossiliferous formations, such as the Trias, becomes of supreme value amongst the Archæan groups. It is very frequently the only kind of evidence available, and, used with due caution, may lead to decisive results. In Anglesey, as illustrated in Fig. 1, and already described, it is nearly as conclusive as the testimony of organic remains; for the great similarity of the dark schist in two areas so near together renders their correlation in the very highest degree probable. The gneissic rocks of the Malvern Hills have been classed as Laurentian on the ground of the lithological resemblance between the two groups, though 3000 miles apart; and the determination has been generally accepted. But in such a case as this, high probability is all we can expect. Other gneissic and schistose systems have been described in America, though none of them so closely resemble the Malvern series as the Laurentian.