Anyone who studies the relation of the rocks of the Lizard Peninsula to those immediately to the north must realize that Porthallow, the little village in a sheltered cove near St. Keverne, is a critical locality in the problem, and one indeed that may hold the key to the solution of the relative ages of the rocks on either side of the sinuous line called the Lizard Boundary. Porthallow is at the eastern extremity of that line. The western extremity is on Polurrian Beach, near Mullion, where a very clear fault with a breccia about 22 feet thick separates two totally distinct formations, sandy and argillaceous killas on the north from hornblendegneiss on the south. At Porthallow, however, although there are cliffs on either side of the cove washed by the sea, the geology is far from having the textbook clearness of Polurrian Beach and there is room for much difference of opinion. Text-fig. 1 is a sketch-map of the locality based on the Ordnance Survey 25 in. to the mile map, and first the main features of the geology will be described. For the neighbouring country the reader is referred to the Survey 1 in. map (Sheet 359) and to the 6 in., Sheet lxxxi N.E.