I. Introduction. The area dealt with in the following pages forms the western half of the 1-inch Ordnance Survey Map, New Series, Sheet 121, with small contiguous strips in Sheets 120 & 137. It was mapped by us in the course of official work for the Geological Survey, in 1912 & 1913. The memoir on Sheet 121 is in preparation; but both it and the map which it accompanies have been greatly delayed by the War. This applies even more emphatically to Sheet 137. In the projected memoir on Sheet 121, special attention will be devoted to the Wrexham-Ruabon Coalfield. In preparing the present paper, we have been influenced by a desire to publish in a concise form1 an account of the older rocks which are more of theoretical than economic importance. We have attempted to describe the district as a geological unit, whereas the Survey Memoir must be primarily an explanation of the map and of the evidence on which it is based. We are, therefore, greatly indebted to the Director of H.M. Geological Survey for permission to publish the following account, which is divisible into two parts. In the first, we aim at stating new facts and important modifications of the hitherto accepted ideas of the stratigraphical sequence, without traversing in detail the work of previous investigators where their conclusions have been confirmed by us. In the second part, we give a fuller description of the tectonics than the space available in the official Memoir would allow. Our