stone, and seams of cherts, are common in some of the sands. Subordinate to the whole are a few beds of green sands and clays, and at Nutfield a bed of fullers' earth. In some places the strata pass into soft porous sandstones, and in others they again become argillaceous;-a few are calcareous (Bargate stone). As a whole the Lower Greensand in these districts is very absorbent and permeable, and often forms extensive dry sandy heaths.

In Bedfordshire and the adjoining parts of Buckinghamshire and Cambridgeshire, this deposit forms a tract of a less bold character than in Kent and Surrey, but still sufficiently prominent and distinct; and here also it is more purely arenaceous and uniform than southward of London,-consisting of a mass of light yellow and ferruginous sands, with thin seams of iron-sandstone. The upper part of it frequently passes, as around Silsoe especially, into a thick bedded rock of soft coarse ferruginous sandstone. The sands of Woburn and Ampthill are well known.

76. The range of its outcrop is parallel with that of the Upper Greensand, the contour of which it follows, separated merely by the lower tract of the gault.* Its breadth is con

siderable; in Kent and Surrey varying generally from two to five miles, and in Buckinghamshire and Bedfordshire from three to four miles.

As this deposit is so much thicker and more variable and less defined in its structure than the Upper Greensand or the Tertiaries, its details are not so well known; as the description must consequently be more general, it may be convenient to consider the different divisions of the Map together.

The surfaces of the Lower Greensand in these several divisions exhibit the following important dimensions :

* Except in part of Berkshire and Wiltshire, where its outcrop is disturbed by faults, and is altogether wanting for a distance of many miles.

77. As with the Upper Greensand, the outcrop of this formation is usually bare and denuded, a thin layer of earth alone protecting the abraded edges of the arenaceous strata. In Kent and Surrey its surface is, with very few exceptions, without any covering of drift: the case is the same, but to a lesser extent, in Wiltshire and Oxfordshire. In Buckinghamshire and Bedfordshire it is often covered by thick beds of gravel, which is, however, for the most part sandy and permeable; but in the latter county the gravel is again overlaid in many places by the impermeable "boulder clay" drift. In Cambridgeshire and Norfolk the covering of drift is more general and impermeable.

The thickness of the lower greensand in the district under review has only been determined accurately at Folkstone.* At the other places the thickness is merely a rough estimate founded upon general observation.

* In the Isle of Wight, the fine coast section of the Lower Greensand has been carefully examined, both by Dr. Fitton and Captain Ibbetson; the former by line found the thickness of this deposit to be 805 feet, and the latter, by trigonometrical survey, 833 feet (Journal Geol. Soc. Vol. I. p. 190, and Vol. III. p. 330.)

This is a tolerably exact general measurement recently made with Mr. Austen. Mr. Middleton, who appears to have been a careful observer, differs somewhat from

78. At Folkstone about 150 feet out of the 406 consist of clay, and the remainder of sands and ragstone; at this point, therefore, we may take ths of the deposit to consist of permeable strata. But in proceeding westward the clays gradually diminish, and are replaced by sands, so that in the thickness of 680 feet at Chilworth we found a central group of mixed clays and sands, measuring only about 130 feet, or less than 1th of the mass. In Wiltshire the deposit consists of sands only, and apparently so in Oxfordshire. From Leighton Buzzard to Woburn and Biggleswade, a tract in which the Lower Greensand is largely developed, there are very few beds of clay interstratified with its sands. Taking, therefore, all these districts together, if we exclude rather less than a third of the mass as consisting of clays, it will probably more than meet the necessities of the case; and we may consider this group as consisting of 117 feet of impermeable clays, and of 250 feet of permeable sands. The mineral character of this series is so variable, that it is possible that even these middle clays, although so important in Kent, may not be spread over the whole underground area, and that there may be at some central point a communication between the different main masses of sand of the upper and lower divisions.*

me in his estimate of the thickness of this and the other deposits in Surrey. He assigns to them the following dimensions :-(" Manning and Bray's History of Surrey," Vol. III.)

* The Upper Cretaceous deposits have been penetrated, and the Lower Greensand reached, at the following places,—

A well near Wrotham in Kent, at the base of the North Downs, mentioned by Dr. Mitchell, the section of which gave

79. The height of the outcropping surface of the Lower Greensand above the level of London is, on the whole, very considerable. Commencing in the valley of Dart at Riverhill and Westerham, we there find the bottom of the valleys to be 210 to 260 feet, at Godstone about 450 feet, at Reigate from 250 to 280 feet, and thence to Farnham on an average 200 feet, above Trinity high-water mark,-excepting at Dorking and Guildford, where the Mole and the Wey cut through this formation and reduce the height of the surface to about 100 feet. North of London the surface of the Lower Greensand is 270 feet high at Leighton Buzzard, rises to about 300 to 350 feet in the neighbourhood of Woburn, thence gradually descends to 120 or 130 feet at Biggleswade, and to a still less height in Cambridgeshire, above the level of the Thames at London. These however are but the lowest points. In Kent and Surrey the Lower Greensand hills rise 200 to 600 feet above these levels, and in Buckinghamshire and Bedfordshire, 200 and 300 feet.

The surface of the Lower Greensand is so placed that it cannot, except in a few rare cases, receive any additional supply of water by drainage from adjacent districts; but from its permeability, its extent, and elevation above the

Below this came sand; and water rose to within 130 feet of the surface.

At Snodland on the Medway the water in a well (like that near Wrotham) rose over the surface.

At Gatton Park a well was sunk through 380 feet of firestone and gault; then 20 to 30 of red clay and pebbles. The water rose to within about 290 feet from the surface (the ground here is probably 400 to 450 feet high).

Dr. Fitton mentions a well at the Feathers Inn, Merstham, sunk 150 feet through gault to the underlying Lower Greensand, whence a plentiful supply of water was obtained. (Trans. Geol. Soc. 2nd Ser. Vol. IV. p. 140.)

The Rev. J. C. Clutterbuck informs me of a well a few miles north of Baldock where the Lower Greensand was reached under a depth of 170 feet of gault. The water rose (1834) to within 3 feet of the surface, and has maintained that level ever since.

At Cambridge, Artesian wells are very numerous. They traverse the gault (130 to 140 feet thick ?); and immediately on reaching the Lower Greensand, water is found, which rises to within a few feet of the surface of the ground.

level of the Thames, it is peculiarly well adapted to receive and transmit the rain-water falling within its own

area.

80. With regard to the passage of water from these beds at their outcrop to any point beneath the chalk, there is some uncertainty how far it may be interfered with by faults and disturbances; for in Surrey and part of Kent,* on a line parallel with the North Downs, and at a short distance from them, some very considerable faults, or rather flexures, run through the Lower Greensand, bringing at some places in their course, the lower beds of this formation, and occasionally the Wealden, to the surface of the ground in the midst of the Greensand area. If this could be attributed to one continuous line of disturbance ranging in equal force and uninterruptedly, it would certainly exclude much of the broad zone of exposed surface of the lower greensand from contributing to the supply of water in its deeper-seated beds below the chalk; but this I do not apprehend to be the case. Lines of disturbances are of very variable intensity, and, although interrupting occasionally the continuity of the strata, their effects, at some intermediate places, may be much less important; so that although the water may be kept back by faults in some parts of the greensand area, yet the communication is probably kept up in other parts where the disturbance has not been so great.

The tilting up of the chalk in the singular ridge of the Hog's-back, and the rise of the Wealden one mile to the south of Guildford, are no doubt owing to one and the same disturbance, which, if prolonged eastward, most probably passes to the south both of Dorking and Reigate, and is possibly

For a description of some of these disturbances see Dr. Fitton's paper, before quoted, pages 132-6, and Dr. Mantell's " Geology of the South-east of England.”

↑ Since the pages above were written Mr. A. K. Barclay has shown me that the tract of the Lower Greensand to the south-west of Dorking is distinctly separated into two areas by a broad tract of clay, which by its organic remains proves to belong to the Wealden formation. This is particularly apparent in the valley immediately