I. Introduction. The region herein discussed is that portion of Wales which lies, roughly, between the River Dovey and Pembrokeshire; it is bounded on the west by the coast of Cardigan Bay, and on the east by an irregular line coinciding in part with the base of the Wenlock rocks and in part with the margin of the Upper Palæozoic rocks. It comprises, therefore, the whole of Cardiganshire and a great part of Carmarthenshire and Pembrokeshire, together with portions of Montgomeryshire, Breconshire, and Radnorshire—an area exceeding 1800 square miles in extent. This tract is represented upon geological maps by ‘Lower Silurian’ colour, only diversified by small areas indicating igneous rocks, or by tracts of yellow dots indicating grits and sandstones. Ever since the days when Sedgwick and Murchison laboured to introduce order among the old slaty and volcanic rocks of Wales the region defined above has always remained something of a geological puzzle: the apparent uniformity and unfossiliferous character of the strata over wide areas, and the compression which they have suffered by folding, faulting, and cleavage, combined to defy for a long time all attempts at unravelling its structure. In more recent years the discovery of numerous fossiliferous beds among these strata has made possible the application of the zonal method, based in this case upon the range in time of various genera and species of graptolites. By this method, combined with close attention to the lithological characters of the rocks, attacks have been made upon this ‘geological wilderness,’