I. Introduction. The present paper deals more particularly with the petrography of the younger Tertiary deposits of Western Cornwall which, by reason of their mode of occurrence and lithological similarity to the well-known fossiliferous beds of St. Erth, have been generally assigned to the Pliocene age. These deposits occur as isolated patches of detrital material resting in eroded hollows in Palæozoic or older rocks in the south-western part of the county, and are located at St. Agnes, 4 miles due south-west of Perranporth; at St. Erth and Canons Town in the Hayle-River valley; at Crousa Common, a mile and a half south-west of St. Keverne (Lizard district); and at Polcrebo, near Crowan, 4 miles south of Camborne, where an outcrop of gravels of alleged Pliocene age is found. With the exception of the St. Erth Beds, no fossils have hitherto been discovered in these deposits, and their stratigraphical position has been inferred entirely from general geological and topographical considerations. The object of this paper is not only to establish the relative ages of these deposits by petrographic methods, but to demonstrate the importance of this mode of attack in problems of stratigraphical correlation where palæontological evidence is in some cases lacking. In addition, the mineralogical factors concerned are used as a guide to the determination of the origin of these sediments and, in conjunction with topographic data, to restore the geography of this part of Britain in early Pliocene times, in so far as the evidence at our disposal allows.
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Henry Milner (1922) studied this question.