therefore, to be reasonable grounds for supposing that the chalk under London is not more than from 600 to 650 feet thick.*

Assuming this estimate to be correct, then the approximate depth to the Lower Greensand at or near London can easily be calculated. For if we take the Tertiary strata at an average thickness of 200 feet, the following measurements will probably be near to the truth :—

It therefore appears probable that the Upper Greensand might be reached at a depth of from 800 to 900 feet, and the Lower Greensand at a depth not exceeding 1000 to 1100 feet beneath London.

* Or as I have given rather extreme numbers in the third column, the probable thickness may be estimated at between 500 and 600 feet. The greatest depth to which the chalk has been pierced at London is 300 feet.

§ 15. On the Height to which the Water from the Upper and Lower Greensands would probably rise at London.

127. THIS point depends upon the elevation of the line of outcrop of the water-bearing strata above the level of the ground at London. As the general surface of the country rises gradually around London both to the north and south, and also, but less rapidly, to the west, the outcrops of the Lower Tertiaries, Upper Greensand, and Lower Greensand, which take place in those directions, are on levels necessarily higher.

If the outcropping surfaces of any of these deposits always kept the same level, and the flow of water in their underground mass were perfectly unimpeded, we could calculate exactly the height to which the water would rise above the ground at London in a tube driven through the intervening and overlying strata into the water-bearing beds beneath, -for it would be nearly to that of the outcrop of these beds themselves. But this exposed surface, far from presenting an uniform level, exhibits great irregularities; it forms a zone maintaining on the whole a considerable elevation above the Thames at London, but in which occasional depressions, caused by transverse valleys, occur.

128. The following list shows the approximate height of the surface in the different geological tracts, keeping as far as possible to the lowest levels of the country, and within the boundary of the effective area of the several deposits. They are taken chiefly from Bradshaw's "Maps of Canals and Rivers," and from the different railway levels. They are all reduced to the same standard; viz., that of Trinity highwater mark at London Bridge (note p. 38). These heights

may not always be quite correct, as several of the original ones were not taken on the exact geological spot required, and in those cases they are merely estimated.

In the two Greensands, the places are in regular succession from east to west.

* Further westward, and beyond the boundary of the effective area, the valley levels of the Lower Tertiaries attain the following heights:-Old Basing, 254 feet; Newbury, 236 feet; Sonning near Reading, 120 feet.

The Lower Greensand, one mile west of Devizes, is at a height of only about 180 feet.

The relative average levels of the outcrop of the different water-bearing deposits are, therefore, as follows

129. The mean surface planes of the several formations, taken in the direction of their range and at their lowest levels in the several districts they traverse, form curves of considerable irregularity. This is owing, in great part, to their intersection at intervals by river courses, at which points the curves reach their lowest depression; while between them the country rises, and the curves attain their higher elevation. These breaks are frequent south of London, but on the north the curves are longer and less deeply indented. With regard to the height at which water stands at the exposed outcrop of the several water-bearing deposits;—if it moved freely, and without impediment, through the strata, it would naturally fall speedily to the level where the outcrop was lowest. But the resistance caused by friction and capillary attraction so greatly retards the horizontal passage of the upper layers of the water, that, although necessarily always seeking to find its lowest level, it tends, owing to these causes, to accumulate above that level in the interior of the hills and rising grounds (see fig. 18). This continues until the water reaches such a height above the adjacent valleys, that the increasing pressure it exercises counterbalances the resisting forces, and then any further addition displaces an equal quantity of the water already accumulated, forcing it out as springs over the lowest marginal edges of the outcrop. Consequently, as the resistance depends upon the distance the water will have to travel and the lithological character of the strata, so, in proportion to the height and distance of any part of their surface above and from the lowest levels

L

of the same districts, will the water-line be placed on a higher level. Mr. Hopkins, of Cambridge, has suggested to me, that the resistance caused by the texture of the sands is of that nature, that the flow of water through them may, in fact, be compared to the flow of a viscid fluid whose cohesion varies according to the lithological character of the strata. On this view of the subject, the difference of level in the water-line at the outcrop of the strata will be more readily understood.

130. In the valley of the river Medway, at Maidstone, the water in the Lower Greensand stands at nearly about the level of Trinity high-water mark at London: but in continuing along the range of the exposed outcrop to the westward of Maidstone, the line of water level in the strata is found to rise to a certain extent with the general rise of the surface, attaining probably an elevation of about 240 or 250 feet. The transverse valley of the Darent again lowers this line to a point nearly coincident with the level of the valley— there about 220 feet above the Thames at London. Proceeding further westward, and keeping to the valleys, the following are the approximate highest and lowest levels at which the water stands in the Lower Greensand:-Westerham 220 feet, Merstham 200 feet, Reigate 250 feet, Dorking 120 feet, Shiere 230 feet, Guildford 100 feet, Farnham 180 feet ; then crossing over to the zone of northern outcrop and proceeding eastward: Abingdon 130 feet, Leighton Buzzard 250 feet, Hincksworth, near Baldock, 120 feet, and at Cambridge the level of the water again descends to about that of Trinity high-water mark at London.

In the same way with the Upper Greensand the line of water level is about 250 feet high at Merstham, 200 feet at Betchworth, 100 feet at Dorking, 320 feet at Devizes, 120 feet at Wallingford, 330 feet at Tring, nearly 400 feet at Luton, and 120 feet to the north of Hitchin.*

* These numbers are only general.