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Book II. Chap. II. Beds between the

breadth of this tract varies generally from one to two miles,but if the Cambridgeshire gait is properly referred to thesebeds, it is there considerably greater.

(e) Thickness. Near Folkstone the cretaceous varietiesoccupy about 200 feet, and the inferior argillaceous beds aboutthe same. From 300 to 400 feet may probably be assumed asa fair average thickness for these beds; which, like all otherstrata, are very variable in this respect when compared in dis-tant points.

(/) Inclination. The strata are alw'ays conformable to thesubjacent chalk, and therefore generally approach the horizon-tal position ; but in the disturbed ranges of the Isle of Wightand Purbeck, they become nearly vertical.

(g) Height. The cretaceous and siliceous varieties, espe-cially when they assume an harder texture, frequently forman under terrace beneath the escarpment of the chalk hills :this may be particularly seen in Berkshire. The highest hillsthus formed, are those on which the old Roman camp calledSinodunum hangs over the Thames opposite Dorchester; theyare detached and bold, and are probably about 500 feet abovethe sea. The argillaceous beds, having offered less resistanceto the causes which have modified the surface of our continents,form low grounds at the base of the escarpment of these ranges.

(/;) Agricultural characters. The cretaceous varieties are,like the chalk itself, favorable to the growth of the beech.Where this and the argillaceous varieties are blended, a warmcrumbling marly soil, very rich and valuable, is produced ; butwhere the argillaceous forms exclusively occur, a deep stiffclay which requires the labour of years to render it mellow.Flint gravel, derived from the overhanging chalk ridges, ishowever often spread over this tract, and materially modifiesits characters.

(?) Phasnomena of Springs. The waters which percolatethrough the rifty strata of the superjacent chalk, are usuallythrown out by some of the more tenaceous beds in the upperpart of this series ; but having passed the line of these springs,it is necessary to sink the wells to a considerable depth, often200 feet, in order to pierce the retentive argillaceous strata andreach the waters percolating through the subjacent sandy beds,and thrown up by their argillaceous partings.