X. The fossil flora of the culm measures of North-west Devon, and the palœobotanical evidence with regard to the age of the beds

Abstract

The deposits of Upper Palæozoic age in the south-west of England differ remarkably in their general characters from those developed on the north side of the Bristol Channel. This conclusion applies not only to the Devonian rocks, but also to the succeeding Carboniferous series. In South Wales and Monmouth, the Devonian beds are of the Old Red Sandstone type, whereas, the Devonian succession of North Devon exhibits a marked, though not an entire change, in both petrological characters and palæontological facies; a change which becomes even more marked in the series of limestones, volcanic, and detrital deposits developed in the southern portion of that county. The South Wales coalfield, the largest and most important productive measures in this country, consists of a sequence of coal-bearing strata, resting upon beds of Lower Carboniferous age, for the most part similar in character to those occurring in our other English coalfields. In Devonshire, and in portions of the neighbouring counties of Somerset and Cornwall, a Carboniferous basin of considerable size is developed, occupying more than 1,200 square miles. In many important respects these rocks again differ somewhat markedly from their equivalents in South Wales. They form a succession of deposits of a somewhat abnormal type; being composed of sediments of extremely varied nature and origin, both detrital and organic. They are especially characterised by a general absence of carbonaceous material of any economic importance. These Carboniferous rocks are spoken of as the Culm Measures, a name first applied to them by Sedgwick and Murchison in 1837. These authors in their classic memoir, published in 1840, gave the first accurate description of the physical structure of the beds, and proved conclusively their Carboniferous age. It may be pointed out, however, that De la Beche, in 1834, was the first to indicate the Upper Carboniferous age of that portion of the Culm Measures which forms the subject of this memoir; his conclusion being based on plant remains identified by Professor Lindley. De la Beche also added considerably to our knowledge of the Culm Measures in his ‘Report on the Geology of Cornwall and Devon,’ published in 1839. Since then, John Phillips, Holl, T. M. Hall, and others, and, in more recent times, Messrs. Hinde and Fox, and Mr. Ussher, have all contributed important information on this subject.