distance of a few miles from London the waters from these two deposits appear to be more distinct, and have each a separate character sufficiently apparent even without the aid of analysis: but at London the few that have been analysed show a chemical composition very nearly alike. Professor Brande observes, however, that in most cases the water from the Lower Tertiary sands is not so pure as that from the chalk.

The Lower Tertiary sands are in the main siliceous, although carbonate of lime is frequently present in not inconsiderable quantities. They contain subordinate beds of clays sometimes carbonaceous and pyritical,-layers of fossil shells,—the green silicate of iron in dispersed grains,—and occasional crystals of sulphate of lime; consequently the water from this source, although generally good, cannot be always depended upon, it being occasionally slightly chalybeate,-sometimes not to such an extent as to be perceptible to the taste, or to be unfit for domestic use, at other times so much so as to render it unusable for any purposes. But the springs usually tend to improve by use.

*

The decomposition of the iron pyrites (sulphuret of iron) by the joint action of the vegetable matter, and the water in the same beds, sets free not only the salts of iron, but also gives rise apparently to the evolution of sulphuretted hydrogen.

In sinking wells this gas is sometimes met with, especially in wet and rainy weather. The quantity generally evolved is extremely small, and hardly perceptible when the flow of water is constant, but after being shut off for a few hours, the water, when first drawn, is sometimes perceptibly impregnated with it. This is more particularly apparent in some of the wells in the valley of the Lea. Exposure to the atmosphere for a few hours removes this impurity.

This is more particularly the case with the lesser springs in the bed of sand and conglomerate immediately at the base of the London clay.

At Waltham Abbey the water from these sands is used to a considerable extent, and is reported to be very good. The same at Water Lane, Edmonton.

At Merton this water is preferred to that of the Wandle. At Tooting and Garret it is generally liked.

Annexed (p. 224) are the particulars of a recent analysis, by Mr. R. Warington, of the waters from the Lower Tertiary sand and from the chalk in the same well at Kentish-town. They show a singular agreement, but one which would, I think, rather indicate a transfer of a portion of water from the sands to the chalk, than from the chalk to the sands.

In many wells, when first constructed, the water from these sands is very impure, but this must not be considered as a criterion of their quality, for it is found that after the well has been in use for a few months the water almost invariably improves. The defects arise from local causes, and are, I believe, more particularly to be attributed to the variety of mineral character introduced into the Lower Tertiary in the neighbourhood of London, by the setting in of the fluviatile beds so conspicuous at Woolwich and Lewisham.*

The Chalk.-The waters from this formation have been more frequently examined than any others. The quantity of

* Mr. R. Mylne informs me as the result of his experience that, “The water from the sands below the London Clay is generally both soft and wholesome, but instances sometimes occur to the contrary in new wells, particularly in the southern districts; the water when first drawn being occasionally charged with impurities, impregnated with iron, and unfit for domestic use. Under these circumstances either by continuous pumping, or after a lengthened use, a progressive improvement in the quality of the water invariably takes place.

“As the strata are known to differ considerably in their mineral structure, it is probable that the quantity of salts which they contain vary in different localities; and consequently any excess of these soluble products at particular points might give rise to these exceptional cases. By the constant action of purer waters flowing from more distant points the objectionable ingredients are thus rendered less apparent and gradually pass off in solution.

"In one district a few miles north of London the water obtained from the sands by any new boring usually contain a strong odour, with a peculiarly disagreeable taste which continues for two or more months, when the water at length attains its proper condition.

saline matter held in solution varies very considerably accordingly as the water is from the springs and streams on the surface, or from Artesian wells.

In the former the water generally contains about 18 to 22 grains per gallon of solid residue, of which from 15 to 20 consist of carbonate of lime, 1 to 2 of chloride of sodium (common salt), 1 to 2 of sulphate of lime, and traces of a few other substances. These proportions are tolerably constant. The change exhibited in the water from the chalk beneath London is remarkable. The solid residue has increased on the average to about 55 grains per gallon, in which the carbonate of lime is present to the extent only of 1 to 7 grains; whilst there appears 12 to 18 grains of carbonate of soda or potash, together with 5 to 20 grains of the sulphates of the same bases, and 10 to 20 grains of chloride of sodium.

Notwithstanding, however, the increase in the total amount of the salts, the presence of alkaline carbonates, and the decrease in the quantity of the salts of lime, render the water soft and generally good for many domestic purposes.

Various reasons have been assigned to account for the large quantity of common salt contained in the chalk water beneath London. It has been suggested that, owing to the exhaustion which has taken place in the wells, there is now an influx of the brackish waters of the Thames through the fissures of the chalk at its outcrop between Woolwich and Gravesend. It, however, need not necessarily arise from this cause. The quantity of chloride of sodium in the water also by no means increases in proportion as we ap

"The fact of water being temporarily surcharged with the products of the strata through which they pass is often exemplified in the springs of the extensive gravel deposits resting on the London clay. A disturbance of the ground from the construction of sewers and other similar works occasionally intercepts some of the subterranean channels communicating to the wells, and the water, consequently drawn from a new direction, is liable to become largely charged with sulphates of lime and other mineral and earthy salts, rendering it for a considerable period bitter to the taste, and wholly unfit for general purposes."

proach that outcrop of the chalk.* This salt is commonly, or rather almost constantly, present in all sedimentary rocks. Some recent experiments of M. Maumené have shown that it exists in as large a proportion in the wells of Rheims, (a city which is situated in the midst of an extensive chalk plain, 300 feet above the level of the sea, and 140 miles distant from it), as in the wells at London. He gives several analyses in which the quantity varies from 15 to 22 grains per gallon.

It is possible that this salt may have been left in the chalk by the ancient ocean in which that great deposit was formed. We know it by its organic remains to be of marine origin. The fine texture of the mass and its strong capillary attraction, would render the drainage from it—after elevation into dry land—of the water with which it must have originally been saturated, a work of a very great length of time. But during the ages since its actual surface has been exposed to the action of rain water, the upper beds have been so washed that comparatively little of the chloride of sodium remains in them. But where the beds are very deep seated, and more especially when covered and protected by Tertiary strata, then although they may have been in a state of constant saturation, there has not been a sufficient flow and change in the water to effect the removal of the salt originally left in them.

Another objection against any general infiltration of the Thames water, is the great variation which exists in the proportion of the saline ingredients in all the deep well waters that have been analysed. No two are alike. The quantity varies from 33 to 70 grains per gallon. This no doubt arises from the different levels to which almost all the wells are sunk, for the chalk varying slightly in its composition from

* In two Artesian wells near the river at Greenwich, the waters of which were analysed by Prof. Graham, the chloride of sodium was present only to the extent of 0.37 and 3.12 grains per gallon.

the lower to the upper beds, and the water passing chiefly along the planes of stratification, the mineral matter which it takes up is likely to vary according to the different portions of the formation with which it comes into contact. If the waters were derived from the Thames, it is probable that their saline ingredients would show less variation, although the same lithological cause of differences would still of course continue to operate, but it would cause an addition to a constant quantity that would scarcely allow of the extent of variation which now exists. The difference in general chemical composition of these waters would also be difficult to reconcile.

In support of the infiltration of the river water, it has been shown that some of the deep wells in and around London are influenced by the tide to the extent of as much as from 2 to 4 feet, whence it has been inferred that there are communicating channels between the river and the subterranean strata, through which the water finds a passage.

But this is a phenomenon common in water-bearing strata, particularly in those which are compact and in which water is transmitted through fissures, both near tidal rivers and near the sea. It is of frequent occurrence on our own coasts ; and is exhibited sometimes in the chalk plains of Picardy to a distance of several miles from the sea. Many of the wells round Abbeville are affected by the tide. exercised by the sea at high and low tide has been found to

The varying pressure

* A curious case occurs at Noyelle-sur-mer (Somme).' Some meadows required a supply of fresh water for the cattle. An Artesian well was sunk to a depth of 55 feet in the chalk. A supply of water was obtained rising at high tide just to the surface of the ground, but lowering 6 feet as the tide fell. A large pond was therefore excavated at the top of the well, and now the water, which is perfectly fresh and good, is forced in by the rise of the tide ; the orifice of the bore is then closed by a valve, and the fall of the water back into the subterranean reservoirs, as the tide retreats, is thereby prevented. A large and fresh supply of water is thus forced or pumped up at each successive tide by these natural and most effective means.

Baillet," Bulletin Soc. d'Encouragement pour l'Industrie Nationale, 1822.