THICKNESS OF THE LONDON CLAY.

303

and north. The former is faced up by the nearly vertical beds of the plastic clay formation, which had been uplifted with it; and the latter is covered above with nearly horizontal layers of the freshwater, and upper marine formation.

The highest elevation of the London clay beds, is the summit of High Beech in Essex, 759 feet above the level of the sea. On the northern half of the Isle of Sheppey, the London clay is observed at 90 feet high, stretching 4 miles in length, and declining gradually towards the east and west. Whole acres of the cliffs are sometimes broken down at once into the sea. Since the days of Henry the Eighth, the sea has by this action, encroached about a mile on the north-west coast of the Isle of Thanet; the church of Reculver which has had its wall and half of its yard now washed away, having been in those times at the above distance. In thickness the London clay bed is very variable. At a spot, one mile east of London, it has been found only 77 feet thick; at Tottenham, 110; at White's Club-house, St. James's, 235; while in Portsmouth 266 feet were perforated without passing through the clay. It is said to have been dug into at lord Spencer's at Wimbledon in Surrey to the depth of 530 feet without getting through it. At High Beech in Essex, its thickness cannot be less than 700 feet. With the singular exception of the Isle of Wight, the beds of the London clay are nearly horizontal. Its mechanical disturbance is here demonstrable. Among the vertical strata of the plastic clay formation of this island, there is a thick layer of rolled chalk flints, standing also

upright, which must have been thrown into this position from the flat posture in which gravitation originally strewed them. The disturbing force seems to have operated with the greatest effect in a lateral direction. The central line or axis along which it has acted may be traced nearly east and west to the extent of no less than 60 miles, viz. from the eastern end of the Isle of Wight to Abbotsbury in Dorsetshire. The vertical chalk beds, which have also partaken in this great dislocation, terminate indeed at Whitenose, but a highly inclined saddle of the substrata may be traced in the prolongation of the same line.

The first section of this extraordinary stratum, in advancing westwards, is seen at Culver Cliffs, a magnificent range of precipices near the east end of the isle, forming a promontory which separates Whitecliff bay on the north-east, from Sandown bay on the the south-west. The superstrata of the plastic clay and sands may be observed at Whitecliff in an upright position, forming low cliffs. Immediately to the south, the chalky strata tower to a stupendous height, with an inclination to the horizon of about 70°. Southwards, round the cape, this angle falls to 50°. The direction of the dip is north-north-east. All the beds of chalk towards what had been their upper side, contain alternating strata of flint-nodules, the whole of which with the exception of a few detached ones in the body of the strata, are in a most extraordinary condition; broken in every direction into fragments varying from three inches diameter, to impalpable particles. Yet the flints so shivered, by some mysterious concussion,

LOWER FRESHWATER FORMATION,

305

retain their outward form and position in the bed. The chalky coating still invests them undisturbed. When this is removed, fine lines are perceived which correspond to the internal fractures, as we see in a piece of unannealed glass, cracked by a scratch. Like the glass, too, the flints fall asunder when moved. The fragments are all as sharp and irregular as possible, but devoid of crystalline arrangement.

The London clay is an extremely dense soil, rendered productive round the metropolis only by excessive working. It is almost impervious to water, and containing besides much saline matter, cannot be expected to give origin to good springs. But the alluvium above the clay, affords a very considerable supply of water. What rises from the sands of the plastic clay strata beneath, is limpid and soft, gushing up so suddenly after cutting through the London clay, that the welldigger is in danger of being drowned at the instant of its issue.

§ III. LOWER FRESHWATER FORMATION.

This is but a partial deposit occurring particularly at Headenhill, Isle of Wight, in a series of beds of sandy, calcareous, and argillaceous marls, mingled often with a little coaly matter. Some of these appear to consist almost wholly of the fragments of freshwater shells, many sufficiently entire to show their species.

Section of Binstead Quarrics, illustrative of this formation. On the summit lies,

1. Blue clay, containing many loose masses from the Upper freshwater formation of variable thickness.

Feet.

2. Limestone composed of coarse fragments of freshwater shells, 2

3. Do. in smaller fragments,

4

The organic remains of the above beds are lymneus, planorbis, and cyclostoma, perhaps the helix, and a bivalve, resembling the fresh water muscle. No marine exuvia have been detected in these beds. This formation at Headen hill is extremely irregular in its range, and cannot be continuously traced for more than a few hundred yards, so that it is to be seen only in apparently insulated masses, emerging from beneath the mouldering slope. The whole of the north end of the Isle of Wight has been referred to the fresh water formation, but the confusion of the strata is so great, that it is difficult to find any of them in their original seats. Hence fresh and salt water shells are intermixed on the west and north coasts, though now and then they occur in alternate layers. This formation may be traced to a considerable distance east of Ryde, perhaps as far as Nettlestone. Its greatest height is 90 feet; its greatest thickness 63. In the above section it is only 14.

§ IV. UPPER FRESH WATER FORMATION.

This lies above the preceding bed, separated from it by a 6-inch seam of sand. It is covered by alluvium, as on the summit of Headen hill. This

UPPER FRESHWATER FORMATION.

307

upper fresh water stratum is a yellowish white marl, containing a large proportion of calcareous matter. It is the best characterised bed of Headen hill, and contains crystalline veins of pure carbonate of lime, along with abundance of the same fresh water shells enumerated above, unmixed with any marine exuviæ. It extends to many parts north of the middle range of chalk hills. In the neighbourhood of Calbourne, and onward to Thorley, several quarries have been opened in it, which yield an excellent durable stone. This bed forms the upper part of Headen hill, 400 feet above the sea, and is 55 feet thick. Elsewhere it occupies low situations. The occurrence in England of a distinctly marked fresh water formation was first observed by Mr. Webster, Sec. Geo. Soc., and it may, generally speaking, be considered as limited to a portion of the Isle of Wight. We should be careful, says the Rev. W. D. Conybeare, not to confound these fresh water strata, which from their alternation with the regular marine formations, clearly belong to a period anterior to that at which our continents finally passed into their present condition, with the occasional occurrence of fresh water shells in alluvial tracts, belonging possibly to a very recent date, and certainly to one less ancient than the above. Near Kew, in Surrey, land and river shells were found in sand and gravel overlying the remains of elephants, &c., and therefore certainly posterior to the catastrophe which inhumed these animals. Though many of the shells found on Headen hill, be thin and brittle, they are quite entire; whence, it is inferred, that the shellfish must have lived and