I. Introduction. (1) The Ordovieian and Silurian Complex of Central Wales. If an examination be made of any general geological map of Wales, in which the outcrops of the various formations are marked with the colours habitually used by the Geological Survey, the attention is certain to be arrested by the broad spread of pale pink or purple colour which occupies the central portions of the map. It will be seen that this tint spreads over an area representing some 1800 square miles, or nearly one-fourth of the area of the Principality: stretching from Machynlleth to Caermarthen, and from the shores of Cardigan Bay nearly to the town of Builth. Round this unbroken field of pale tint upon the map sweep mere strongly–coloured bands which mark the outcrops of the bordering formations; but within the central field itself little or nothing is given which would enable one to gather an idea of the sequence or distribution of its rocks. Even upon the latest Index Maps ot the Geological Survey, we find merely a few scattered reference–letters,— b2, b3, b4, b5,—which indicate that within this great region rocks are known to occur belonging to the Llandeilo, Caradoc, and Llandoverv formations; but no attempt is made to mark out the boundaries of the various groups. The reasons are not very far to seek: we need only refer to any published description of the rocks themselves. In the words of the late Walter Keeping, 'Central and West Central Wales is made up almost