I ntroduction T here are few series of rocks in the southern half of Great Britain which have been so much neglected by geologists as the Denbighshire Grits and Flags; and the reason is not far to seek. As a rule, these rocks form rounded hills which are often completely covered with heather, and there are many square miles of country without a single exposure. The beds are frequently much folded and faulted, and it is often a work of great difficulty to make out their relations to each other. Fossils are by no means common, except in a few localities; and the most important are graptolites, which the earlier geologists considered to be of little value for stratigraphical purposes. Under such circumstances it is not surprising that these rocks have tempted few geologists to enter the field, and that the observations which have been made on them have in general been somewhat cursory. The series, nevertheless, deserves more than a passing notice. It belongs to one of the most widely-spread facies of the Silurian in Britain ; and now that the stratigraphical value of graptolites is so generally recognized, it has become a matter of some interest to see how far these graptolite-bearing deposits correspond with those of other regions. The area over which my observations have extended includes the whole of what may be called the Llangollen basin ; but the present paper is occupied chiefly with that part in which the beds are best exposed and in which