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, occupy only a small area in the south-east, chiefly in the valley of the Teign, from Kingsteignton to Bovey Tracy; and there are a few isolated patches, as for example near Bideford and at Plymouth. These beds consist of clays, some of them of much value, with flints from the chalk, and gravels and beds of sand derived from the wearing away of older rocks. The most interesting feature of this formation is the lignite of Bovey Tracy, on the eastern edge of Dartmoor. Lignite, otherwise known as brown coal, consists of the imperfectly fossilised remains of tropical or sub-tropical vegetation, such as the palm, cinnamon, and laurel, amongst which are found lumps of resin. By far the most abundant remains are those of a very large tree allied to the sequoia of California. It is very remarkable that in the Pleistocene clay above the lignite are found stems and twigs of Arctic birch and willow, suggestive of a far colder climate than prevailed in Tertiary times, when the trees that went to form the lignite were growing. 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. 7. Natural History. It is generally believed by naturalists that the ancestors of most of our fauna and flora reached this country at a time when what we now call the British Isles formed part of the mainland of Europe, and when there was no intervening sea to bar the way.