Hill' (where the gravel a 1 occurs), to be found on the south side of the river any equivalent of the gravels x 1, x2, and x 3 of the northern slope. Conversely, on the west of London, although there are gravels on the Richmond and Wimbledon Hills at a far higher level than the gravel of the lower ground, yet on the north side nothing occurs there to correspond with them, as the gravel on that side spreads over the north slope in a continuous sheet.

2

Mr. Whitaker speaks of partial terraces in the Thames gravel commencing and continuing for a short distance until they are again lost in the great sheet of the deposit, and has so shown them on the Geological Survey Map, such as that of Hyde Park and that of Clapham and Wandsworth Commons. The former of these, however, does not correspond to the quasi-terrace of gravel that covers so much of the Richmond and Wimbledon Hills, while the latter is on the same side of the valley. These partial terraces (which are in fact much less persistent than they are represented in the Map), although irreconcilable with any cutting down of the valley from higher to lower levels, are in no way at variance with the unequal action of an upthrow, while nothing approaching to a persistent terrace corresponding to another on the opposite slope can be shown to exist in any part of the valley between Richmond and the sea.

We will now take the formations that are common to both sides of the Thames river. It does not appear to have been suspected that the brickearth beds are divisible into three most distinct formations, but such is the case. The formations are the following:

1. The Lower Brickearth (x4).-The Brickearth of Ilford, both that of the Uphall and that of the London Road field, is a deposit underlying the Thames gravel, and unconformable to it. Like the much newer deposit of Grays, it contains Cyrena fluminalis and other purely freshwater shells, and has a thickness at Ilford of nearly twenty feet. It may be seen in the field on the London Road resting in one part direct on the London clay, while in another part it has a thin band of shingly gravel beneath it. In the Uphall field its position relatively to the Thames gravel (x 4") is best shown, the two deposits being unconformable. (See Section No. 2.) Northwards from Uphall field the brickearth disappears under the gravel, which, there denuded to a thickness of 4 or 5 feet, increases to

1 Shooters' Hill forms an isolated remnant of the original valley, in which the Thames gravel was deposited; the valley, as will be shown, having on the east of London, with that exception, had its southern slope destroyed by subsequent events. 2 The gravel occupying the higher ground above the Thames gravel to the west, beyond Uxbridge, is the gravel of the Middle-drift, a formation older than the Boulder-clay.

3 Mem. on sheet 7 of Geol. Survey. Mr. Whitaker also (p. 92) points out the discrepancy between the section given by Mr. Prestwich, in his "Ground beneath Us," of the position of the gravel of Clapham and Wandsworth relatively to that of the Wandle valley and the true position of those gravels, and I agree with Mr. Whitaker in so far as he says that the Wandsworth and Clapham gravels join in one part with the gravel of the Wandle valley, for I have traced the Wandsworth and Clapham gravels on their south-western side into inosculation with that of the Wandle valley, down Burntwood-lane, as well as on the north side into inosculation with the gravel nearer the Thames that sweeps round into the Wandle valley.

W.

SECTION 2.-Section exhibited by the Uphall Brick-field, Ilford.

d Warp. The asterisks to the west of section denote potholes formed by denudation anterior to the deposit of c.

The asterisk on the east of section indicates the

a and b form r4' of other sections.

place

whence

the remains of Elephas

a Clayey brickearth, principally derived from the London clay, containing freshwater shells. e Thames gravel (x4" of other sections).

primigenius

b Bright yellow sand, also containing freshwater shells.
(described at vol. i., p. 241, of this Magazine) were extracted.

its normal thickness of 12 to 15 feet at Ilford Station, a distance of only half a mile. Eastwards from the Station, and but half a mile further, the lower brickearth again comes up from under the gravel at the London Road field where the gravel is almost wholly denuded from it. This brickearth has a very limited development, but may everywhere be distinguished from the upper brickearth by its resting direct upon either the London clay or the lower tertiary sands, and by its position relatively to the Thames gravel. It occurs in the pits near Erith, where it has a thickness of about 30 feet,' resting on Lower London Tertiary sand, and is almost horizontal, but it passes northwards towards the river and beyond the pits under the gravel. It occurs also in Wickham Lane Brickfield (see Section No. 2), where a thickness of 15 feet of the deposit is exposed thrown into an inclined position, the dip being very marked and amounting to 18°. With the exception of small patches on the tops of the hills west of Wickham Lane, which the denudation accompanying the violent break-up in that direction has spared, these are the only exposures of the lower and oldest brickearth. The deposit seems to have been very limited, but it is that which has furnished the chief part of the Mammalian remains that enrich several private as well as the National collections.

2. The Thames Gravel (x 4'').-Next in succession comes the great sheet of gravel which, wherever not denuded, consists of a persistent well-marked deposit, reaching occasionally to a thickness of 20 feet, but usually from 12 to 15; spreading over and far beyond the lower brickearth, and up the north side of the valley to a height of more than 100 feet above it. There seems no reason for regarding this gravel as other than marine; but at a few places near its edge, as at Brentford and West Hackney, freshwater shells have been found in a band of sandy clay, either intercalated in the gravel or underlying it. Mammalian remains, too, have been found in a few places in it, a feature common also to the marine sands and gravels of the Middle-drift.

2

3. The Upper Brickearth (x 4'').—This is the brickearth which has so large a spread in the

A thin band of shingly gravel occurs in it towards the upper part. 2 Morris, Quart. Journ. Geol. Soc., vol. vi., p. 201. Prestwich, îb., vol. xi., p. 107.

Thames Valley, and rests on the gravel. Consisting of a tawny loam, it is much inferior in thickness to the lower brickearth, being usually from 5 to 8 feet in thickness, and it has not furnished that rich series of fossils which has been obtained from the lower. It appears, however, to be everywhere conformable to the gravel, but does not generally reach so far up the valley as the more outlying portion of the gravel. Its chief development is in the east near Wanstead and Tottenham, and in the west around West Drayton; but it has a great extent in many places between both extremities of the valley. The next Section, No. 3, exhibits the relation of the lower brickearth to the gravel and the upper brickearth, and the dislocation by which the lower has been brought into its inclined position at Wickham.

4. The Grays Brickearth (x 5').- The Grays deposit is distinct from all those described. It consists of 15 feet of blue clay, containing Cyrena fluminalis and other pure freshwater shells, overlain by 20 feet of sand which in places is false-bedded. The clay is underlain by a few feet of flint-gravel coarser and more angular than the Thames gravel (x 4"), and that again by redeposited Chalk, termed by the workmen "Bullhead." Section 4 shows the deposit, as exposed by the three pits, and its relation to the Thames gravel and upper brickearth. Here we see that the upthrow and denudation which succeeded the deposit of the upper brickearth, and preceded the Grays deposit, has along the line of section. removed all but a fragment of the upper brickearth, and has in one place removed the gravel (x 4") and the subjacent Thanet sand (b), exposing the Chalk; while at one extremity of the section a newer, and subsidiary, valley in which the Grays deposit (x 5') rests, has been formed by the cutting down of the Thames gravel and Thanet sand to the Chalk, as well as of the Chalk itself. In all respects the position of this subsidiary valley corresponds to that of the valley of the Cray cut through Dart

SECTION 3.-From Wanstead Flats Brick-field to the Wickham Lane Brick-field. (Length, 7 miles.)

R. Roding Uphall
Fault Brick-field

Barking
Level

Plumstead
Marsh

Bostol

Brick-field

ford Heath, while the gravel at the base of the Grays deposit corresponds with the gravels of the Cray valley.

SECTION 4.-From Stifford Bridge to Grays Thurrock Brick-fields. (Length, 24 miles.)

3. Blue laminated clay,

x4" Upper

a Chalk. containing bands of sand abounding in freshwater shells, 15 feet.

Thames gravel.

Thanet sand.

brickearth.

5. The Forest and Peat (y).-For a few miles on the east side of London, we find a bed of peat, containing the stems of trees,' and resting on the gravel (x 4") or else upon the upper brickearth (x 4'). When this formation was cut through in forming the sewer through the Plumstead Marshes, stools of trees were exposed at the base of the peat, rooted into the gravel at a depth of some 20 feet below the surface of the marsh (see y in section 3). This peat is overlain by the marsh-clay (z) formed by the river-mud before the stream was embanked.

In the formations before described we have the following succession of events subsequent to the complete formation of the original valley by denudation through the Boulder-clay or Upper-drift-First, the deposit of the lower brickearth (x 4'); Secondly, the deposit of the Thames gravel (x 4"); and Thirdly, the deposit of the upper brickearth (x 4"). These are the deposits of the original valley. Then comes the upheaval of portions of the original valley, the dislocation of its deposits, and the extensive denudation of the uppermost of them (x 4''); with the more partial denudation of the intermediate deposit, the gravel (x 4"); while of the lower (x 4') a portion was detached that eventually has become isolated by a fault behind the Chalk cliff, capped with Lower Tertiary sand and pebble, which forms the southwest side of Bostol Hill, in Kent, as shown in section 3. By the same upheaval and denudation we have the southern slope of the original valley east of London destroyed, with the exception of a remnant forming Shooters Hill, and in its place the formation of the subsidiary valley of the Cray, and of that in which we find the Grays deposit resting; both being cut through the Thames gravel (x 4") and subjacent Thanet sand down to and into the Chalk itself. Then we have the accumulation in one of these of a fluviatile deposit (x 5') some 40 feet thick, underlain by another gravel (x5) which we also find resting on the Chalk in several places in the Cray valley.

Further we find the gravel (x 4") and the upper brickearth (x 4") where not denuded, forming after this upheaval a land surface, and supporting a growth of forest; that surface then

1 Mostly of yew, but oak and pine occasionally occur.

cast down so as to form an intercepted drainage, giving rise to a swamp which engendered a peat growth; by the agency of which the portion of the forest so thrown down has been preserved from the destruction that by atmospheric or else human agency has overtaken the rest of it. Lastly, we have the introduction of the river over the previous land-surface and peat-swamp, bringing sediment which by its deposit has formed the marsh-clay of its former shallow bed, before that bed was reduced to its present narrow limits, and deepened by embankment.

In all this, save to the extent indicated in the original valley by the gravels (x 1, x 2, and x 3), we find nothing analogous to terraceformation, or to the modification of an estuary by the successive elevation of the land and cutting down of its bed, until the estuary has become a river, as is described to us to be the case with the valleys of the Somme and the Seine.

(To be continued in our next.)

IV.—The SeA AGAINST RAIN AND FROST; OR THE ORIgin of ESCARPMENTS.

By D. MACKINTOSH, F.G.S.

S every part of the crust of the earth has at one time been the surface, it follows that all questions connected with the origin of the present "form of the ground" must be very important, and that on their issue the progress of Geology must in a great measure depend. But on this subject a very wide difference of opinion at present exists. According to one party, consisting of Professor Ramsay, Mr. Jukes, Mr. Geikie, Colonel Greenwood, Dr. Foster, and others, the more abrupt inequalities of the earth's surface have been produced by subaerial or atmospheric causes. According to the other school, embracing Sir Charles Lyell, Sir Roderick Murchison, Professors Sedgwick and Phillips, Mr. Edward Hull, etc., the sea has been the principal denuding or excavating agent.

The subaërialists start with the assumption that the sea tends to plane down the land, and that its newly-elevated bed presents a surface divested of abrupt heights or hollows. But this assumption is at variance with the generally-received principle of physical geography, that the bottom of the sea, at any given time, is as uneven as the dry land; and it is equally opposed to the established geological fact that mountains have been islands and islands moun

1 There is a very extensive series of table-shaped mountains under the North Atlantic Ocean. Their comparatively level surface may be partly owing to their being covered with sand. Along their western and north-western edges the depth suddenly descends to 600 feet, and along the southern edge to 900 feet. The descent is nearly perpendicular. Besides these inequalities in the Atlantic there are deep furrows which run north and south (see article "Sea" in Penny Cyclopædia, and "Section of the Bed of the Atlantic" in Maury's Physical Geography of the Sea). The bed of the German Ocean, though very favourably situated for becoming a level surface, would likewise appear to be far from uniform in depth (see Lyell's Principles of Geology).