The North Devonian beds, which extend from the coast as far south as the latitude of Barnstaple, consist of slates, grits, and sandstones which, it is believed, judging from the organic remains in them, were formed in shallow water and near shore. Their lower strata, the Foreland grits, Lynton beds, and Hangman grits, contain some fossils and various kinds of coral. But the Middle beds, the Ilfracombe and Morte slates, are much richer in animal remains; of which perhaps the most remarkable are primitive palaeozoic fish, such as the very curious armoured pteraspis; while corals and bivalve shells are abundant and characteristic. The Upper Devonian is less fossiliferous, but contains some large trilobites, various marine shells, and some land-plants.
The South Devonian, which covers nearly all South Devon and a large part of Cornwall, is somewhat different in character, consisting chiefly of slates, with coralline limestones, varied by volcanic outcrops or elvans—a word said to be of Cornish origin, and meaning "white rock." To judge from its fossils, it was deposited in deeper water than the contemporary beds in the north of the county. The Lower and Middle beds are also far richer in animal remains; and the Middle Devonian of the south, which is the most typical of the series and includes the limestones of Plymouth and Torbay, is crowded with shells, trilobites, and corals. Among the shells, bivalves—such as Stringocephalus, which occurs only in the Devonian formations—spiral univalves, and corals are very abundant. There are also many crinoids, distinct from those of the carboniferous limestone, while perhaps the most characteristic form is the rare and curious Caleola sandalina, differing from all other corals in having an operculum. There are not many varieties of trilobite, but the large Brontes flabellifer is not uncommon.
Logan Stone, Dartmoor
The Lower beds of this series contain fewer organic remains, although a good many fossils are found, including fragmentary remains of various fishes which have not yet been identified. The Upper Devonian is, on the whole, very poor in fossils.
Between the two Devonian areas, and occupying a large part of the centre of the county, are the carboniferous or coal-bearing measures, containing, however, not true coal but anthracite, which has more carbon in it than is found in ordinary coal; and these beds are perhaps more often known as Culm, from the Welsh cwlwm, a knot, in allusion to the fragmentary condition in which the mineral is frequently found. Anthracite, which elsewhere and especially in South Wales is a most valuable fuel, is here clayey and impure, and in thin seams. It is worked to a small extent, to be ground and made into a paint called Bideford Black. The Culm measures consist of grits, shales, and sandstones, with beds of chert and limestone containing fossil plants and other forms of marine life. Fish are few, only two species having been identified. The anthracite occurs in the middle Culm, and there are other remains of plants in both the middle and upper beds. The upper Culm is well seen on the coast near Clovelly and by the river Torridge, where it has been bent by volcanic upheaval into curious and beautiful curves. These measures, in general, are characterised by many outcrops of volcanic rock, some of which were probably contemporary, that is to say, they were poured out while the culm was in process of formation; while others are intrusive, or were forced up through the strata after these had been solidified into rock. These igneous rocks are found in great variety.
