The usual classification adopted for the British Coal-Measures is that of Upper, Middle, and Lower Coal-Measures; between the Upper and Middle divisions there occur certain transition or passage beds which are known as the Transition series. Continental writers, and more recently Mr Kidston of Stirling, have attempted with considerable success to correlate the Coal-producing strata by means of fossil plants[57]. [Illustration: 10 in. Massive clay-shale with a few coal films in the lower part. 10½ in. Shale full of thin streaks of coal. 14 in. Massive shale with a few streaks of coal and iron pyrites. 5½ in. Bastard coal; more coal than shale. 6½ in. Good coal, with masses of iron pyrites. 1½ in. Coal and seat-rock mixed. 5 in. Seat-rock. Fig. 4. Vertical section of the Bassey or Salts Coal seam, Rushton Colliery, Blackburn (Lower Coal-Measures). From a specimen 4 feet 4 inches in height, presented by Mr P. W. Pickup to the Manchester Museum, Owens College.] Finally, some reference must be made to the occurrence of Carboniferous rocks underneath more recent strata. In a geological map, or bird’s-eye view of a country, we see such rocks as appear at the surface; by means of deep borings, however, we are occasionally enabled to follow the course of older beds a considerable distance below the usually accessible part of the Earth’s crust. In the neighbourhood of London, Dover, and other places we have Tertiary and Mesozoic strata forming the surface of the country, but below these comparatively recent formations, the sinking of deep wells and other borings have proved the existence of a ridge of Palaeozoic rocks stretching from the South Wales Coal-field through the South-east of England to northern France, Belgium and Westphalia. It is from rocks forming part of this old ridge that characteristic Coal-Measure plants have been obtained from the Dover boring. In Fig. 5 is shown an almost complete pinnule of _Neuropteris Scheuchzeri_ Hoffm., a well-known fern, marking a definite horizon of Upper Carboniferous rocks[58]. The small hairs on the pinnules, shown in the figure as fine lines lying more or less parallel to the midrib and across the lateral veins, are a characteristic feature of this species. [Illustration: FIG. 5. Imperfect pinnule of _Neuropteris Scheuchzeri_ Hoffm., showing the characteristic hairs as fine lines traversing the lateral veins. From a specimen obtained from the Dover boring and now in the British Museum. Nat. size.] VII. =Permian.= Reference has already been made to the earth-foldings which marked the close of Carboniferous times; “the open Mediterranean sea of the Carboniferous period in Europe was converted into a large inland sea, like the Caspian of the present day, surrounded by a rocky and hilly continent, on which grew trees and plants of various kinds[59].” In parts of