  Three main points characterise the geological features of Devonshire; the simplicity of the system in the west, north-centre and south-west of the county; the comparative complexity and variety of the strata in the east and south; and, most remarkable of all, the extraordinary number of outcrops of igneous rock, from the great mass of Dartmoor granite, which has no parallel in England, to the hundreds of small dykes or elvans that are scattered chiefly over the southern region, although some occur to the north and east of Dartmoor. The oldest rocks in Devonshire are probably not, as was once thought, the granites, but the highly altered or metamorphic formations in the extreme south; that is to say, the mica and quartz schists and the hornblende epidote schists which extend from near Start Point to Bolt Tail, a district which, owing in great measure to distortion by volcanic upheaval, includes some of the most picturesque scenery in Devon. Next in order of age is the series called Devonian, after the name of the county, in which they were first distinguished from the Old Red Sandstone. They are, however, by no means confined to Devonshire, but are very widely distributed, covering a large part of Cornwall, and occurring on the continent of Europe, especially in Russia, and in Asia and North and South America. The Devonian beds—which are found both in the north and south, occupying two distinct areas separated by widespread deposits of culm or carboniferous measures—were, it is thought, formed in open water, and probably at the same time that the Old Red Sandstone of the adjoining county of Somerset and elsewhere, which is not found in this county at all, was being deposited in estuaries and land-locked seas.
DIAGRAM SECTION FROM SNOWDON TO HARWICH, ABOUT 200 MILES. This cross section shows what would be seen in a deep cutting nearly E. and W. across England and Wales. It shows also how, in consequence of the folding of the strata and the cutting off of the uplifted parts, old rocks which should be tens of thousands of feet down are found in borings in East Anglia only 1000 feet or so below the surface.] 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 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 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. A smoothly-weathered granite Tor, Dartmoor By far the most important and striking of these volcanic formations is the great granite mass of Dartmoor, one of the most prominent features of the county, measuring 225 square miles in extent, and constituting the largest granitic area in England. Granite is a volcanic rock, formed, it has been suggested, by fusion at a great depth and under great pressure, and consisting in the main of three minerals, quartz, felspar, and mica. That of Dartmoor is, on the whole, grey and coarse-grained, but it varies a good deal in colour, fineness, and composition. Its real origin is obscure. It has been assigned by various experts to various periods, and it has been called "the sphinx of Devon geology." There can, however, be no doubt about the great disturbance which has been caused in the county by upheaval and by the intrusion of melted rock, which has bent, broken, and A long interval of time appears to have followed the laying down of the Culm measures, during which so vast an amount of shattered rock was worn away that when the beds that come next in order—the New Red Sandstones—were formed, they were, in places, deposited directly upon the Devonian, the superincumbent carboniferous or Culm strata having entirely disappeared. The New Red Sandstones occur chiefly in the east of the county, where their lower beds fill up old creeks and valleys in the carboniferous system; and they extend northwards from the coast past Exeter as far as Holcombe Regis, forming broad bands on either side of the Exe, characterised by the high fertility of the overlying soil, and with one long spur traversing the heart of the county, past Crediton and Exbourne, with isolated patches round Hatherleigh, and with another and less extended prolongation a few miles west of Tiverton. The Lower New Red consists of clays, conglomerates, red breccias and sands, in which occur many outcrops of trap, the evidence, not only of numerous eruptions, but of eruptions extending over a long period of time. These beds contain no fossils, except in fragments of older rocks. The Middle New Red, in the form of thick beds of red marl and red and white limestones, well seen on the south Footprints of Cheirotherium, New Red Sandstone The Rhaetic beds are not well seen in Devonshire. They occur on the coast between Lyme Regis and the mouth of the Axe, and in the estuary of that river, but are much hidden by landslips of cretaceous formations from above. One layer, consisting of black shale, with bivalve shells such as Cardium and Pecten, contains also a bone-bed, with remains of fish, such as Acrodus and The Lower Lias is exposed in a narrow strip of coast from the Devonshire border to the mouth of the Axe, and to a greater extent in the valley of the river above Axminster. It has been divided on the coast into four distinct zones, each characterised by its own particular species of ammonite. The cretaceous formations occupy a much wider area, but they also are confined to the southern part of the county. The Greensands of the Blackdown and Haldon Hills have been divided by geologists into fifteen layers, varying in thickness from a few inches to as much as thirty-five feet, some with few fossils, and some very rich in animal remains. Trigonia and Inoceramus are found in almost all the zones: other forms less widely distributed are Murex and Turritella. Chalk occurs on the south coast from the Dorset border to Sidmouth; and in isolated patches it extends inland as far as the Blackdown Hills, and also further west, in the Haldons. The Lower Chalk, well seen on the coast and to the west of Hinton, is made up of calcareous sandstones, with ammonites and pectens. The Middle beds, composed of white chalk with flints, the zone of Terebratulina gracilis, is exposed at Beer. The lower and harder layer is characterised by Rhynconella. The Upper Chalk also holds many flints, with echini; Holaster in the lower, and Micraster in the upper strata. Last of all come the tertiary deposits, which, however, To the Pleistocene period also belong the gravels and alluvial deposits of some of the river valleys (those of the Exe and the Teign, for example), the blown sands of Braunton Burrows and elsewhere, the raised sea-beaches, the submerged forests, and the cave-deposits which are alluded to in other chapters. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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