The

water; although that will depend materially upon whether the formation has, as a whole, been elevated, subsequently to its consolidation, into such a position above the sea level as to admit of its drainage and of the subterranean passage through its mass of the water from its outcropping surface. I do not now take into consideration the occurrences of rock salt, gypsum, or any local deposit of mineral matter. These are particular occurrences arising from some specific cause. present observations refer solely to general conditions. They are made to show a general principle tending to counteract the solvent power of water, and to maintain the great mass of subterranean waters in a state of comparative purity,and are not intended as a sufficient explanation of the occasional and interesting local causes which give rise to unusual saline states of the waters, and to mineral springs.*

Synchronously with the accumulation of some of the older sedimentary strata, we find traces of the commencement of organic life. This introduces an important element in the chemical changes which took place in these rocks; for as animal and vegetable matter became widely spread, it could not fail frequently to affect and modify the chemical nature of the deposits; particularly if, as there appears every reason to believe, the temperature of the surface was then considerably higher than it is now. The presence of animal and vegetable matter in a state of decay, either dispersed in accumulating strata, or else diffused in water, tends to deoxidize various minerals and salts. Under these conditions sulphuretted hydrogen and carbonic acid gases would be generated by the decomposition of the sulphates, and the latent combustion of carbonaceous substances: the peroxide

*For information on this subject I would particularly refer to the valuable Papers of Dr. Daubeny, in the Edinburgh New Philosophical Journal for 1831, and in the Transactions of the British Association for the Advancement of Science, for 1836: to Dr. Gairdner's treatise On Mineral and Thermal Springs," Edinburgh, 1832: and to Prof. James Forbes's Paper in the Philosophical Transactions for 1836.

66

of iron would also be converted, first into the protoxide and then into the carbonate. The presence of vegetable matter would further affect the alkaline silicates, by supplying carbonic acid to their bases, whereby the silica would be precipitated, and alkaline carbonates disengaged. The effects of subterranean heat on rocks so composed, would likewise have to be considered.

This subject is one of too great an extent to allow me to do more than indicate thus generally its importance and interest, with reference to the influence of former geological changes upon the present condition of the waters in some of the permeable sedimentary deposits.

APPENDIX E.

On the Cost of some Artesian Wells in England and on the Continent, with reference to the probable expense of sinking Wells of this description into the Upper and Lower Greensands beneath London.

INTO this question, as matter of exact estimate, I am not competent to enter. But as some particular facts serving as a rough criterion of the probable cost of such works may be thought desirable, I have collected a sufficient number of cases to illustrate the actual expenditure that has been commonly incurred, and the great difference of cost between shafted and bored wells; and as data for some general calculations.

Owing to the water-level in the Artesian wells of London being at a depth of 40 to 60 feet below Trinity high-water

mark, it is, in almost all cases, found advisable, or necessary, to sink shafts to a short depth below that level, and then to bore either into the Tertiary sands, or into the chalk when the supply of water from the former is, as now frequently happens, deficient. This not only causes a considerable increase in the first cost, but it also necessitates a large outlay for engines; whilst the pumping up of the water becomes a not unimportant annual charge.

But, on the contrary, when the water rises above the surface, and boring alone suffices, the expense is comparatively light; it increases, however, very rapidly in proportion to the depth.*

The first part of the following list (1 to 3) serves to show generally the cost of the large deep wells in London; and it will not be found surprising that the expense has been considered a serious objection to their extension. Where a smaller supply of water is required, and the works consequently are not so heavy, or where the difficulties of the work have been less, a great reduction in the cost is apparent. (4).

The latter part (6 to 8) of the list exhibits the much more moderate cost of true Artesian wells, i. e., wells in which the water overflows at the surface, and which therefore only require tubular bores. At Cambridge, although short shafts (12 to 15 feet deep) are necessary, the wells are so numerous that the expense of construction seems to be unusually small.

Now, as the waters from the Upper and Lower Greensands would probably rise everywhere above the surface at London, tubular bored wells would alone be required, without pumps or engines to bring up the water to that level. But the depth to which it would be necessary to bore much exceeds

10 feet can be bored for 5s.; 40 feet 27. 10s.; 60 feet 5l. 5s.; 100 feet 137. 15s., and so on in proportion. The charges, however, vary, and are often less than this. But this shows the proportion generally, and points out the rapid ratio of increase.

that of any of the wells in or near London. We therefore have no evidence from actual experience in this country respecting the cost of such works,* and we must refer to those which have been constructed in France. Some of the data there furnished are peculiarly applicable, as we find wells traversing strata, considerable portions of which are of the same lithological character as those occurring beneath London.

1 A shaft sunk through the Tertiary strata, and a bore into the chalk.

2 A shaft sunk the whole depth.

3 No particulars given.

4 Shafts sunk to base of the London clay, then bored the remaining depth.

5 Bored the whole depth, just reaches the chalk.

6 Bored the whole depth,-ending at the top of the lower sands overlying the chalk.

7 Bored the whole depth through the London clay, Lower Tertiary sands, and into the chalk.

8 Bores through the Gault to the top of the Lower Greensand.

* The Southampton and Chichester wells, though deep enough, do not meet the case. + Davison, Proc. Inst. Civ. Eng. for 1842, p. 194. The additional cost for engine and pumps amounted to 1,3517.

Braithwaite. "Includes the hire and repair of temporary pump, and the cost of two new sets of permanent pumps." Proc. Inst. Civ. Eng. for 1843, p. 165. § Braithwaite, Report of the General Board of Health on the Water Supply, 1850. Evidence, Appendix No. II. p. 98.

Zool. Soc. Reports, 1836; but since carried seven feet deeper. Includes cost of engine house and reservoir. ¶ Communicated by Dr. G. O. Rees.

***

Report of Committee of Visitors, January, 1850. Includes cost of pumps. ++ Manuscripts of the late Dr. Mitchell. Mr. Nightingale. §§ Mr. Deck.

c Bored the whole depth through the New Red Sandstone.

M. Degousée has recently informed me of his having contracted to bore an Artesian well at Rouen to the depth of 1080 feet (through the Lower Cretaceous and Oolitic series) for 1,600.,-expenses of every description to be defrayed by him. This he states he is enabled to do in consequence of using some new machinery, and by the application of steam power.

M. Degousée has also constructed three Artesian wells in different parts of France to a depth of about 820 to 830 feet each, at an expense, including tubes and all expenses, of from 600l. to 1,000l. The Calais well offers a very near counterpart of the deposits which occur beneath London, but the difficulties of the first 240 feet much exceeded those which would be met with here, and the Chalk is probably 100 to 200 feet thicker. There and at Paris, the first 1,000 feet cost less than 3,000%., and at Donchery apparently not much more than 20007.

Communicated by M. Mulot, the engineer of the work, who states however that

a similar work could now be executed for 10,000l.

This includes two sets of tubes and the constructions over the well. There were a number of extra expenses attending this work, arising from the novelty of the undertaking. The contract for the first 1312 feet was 4,0007.

Report of M. Legros-Devot, mayor of Calais. This well was first carried down to a depth of 1047 feet for 31007., or rather for that sum less 10,000 francs, to which M. Mulot would have been entitled, had the work been successful. The tubes were only temporary. The Tertiary strata presented considerable difficulties.

§ Degousée," Guide du Sondeur." M. Degousée also mentions that the sixteen Artesian wells which he constructed at and near Tours cost together 5,872., the average depth being nearly 500 feet. They traverse the chalk and part of the Greensand series. Only part (less than half) of these wells were tubed. Guide, p. 450.

|| Dr. Granville, letter to the Times, Aug. 23rd, 1850.