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water. A good example of this is given by one of the maps exhibited (sheet 50, S.W.), in which, on an ordinary geological map (without drift) nearly the whole of the 205 square miles represented, would be coloured as chalk—a most delusive thing, as, in fact, the area of bare chalk therein is very small, and the areas of chalk covered by permeable or mixed beds not large, whilst the protecting cover of impermeable beds spreads over by far the greater part.
The map to the north (50, N.W.) is much the same, though we have here rather more bare chalk; but in this sheet the whole area would be coloured as chalk, on an ordinary geological map, except for the alluvium of the streams.
A larger map, not yet coloured after the plan described (on account of the drift edition not being published) is also a case in point, for, in sheet 47, whilst about half the area (of 820 square miles) is shown on an ordinary map as chalk, the impermeable boulder clay occurs over the greater part thereof, and quite alters the character of the district.
Though for the most part a fairly stiff clay, there is one tract where the boulder clay is so largely composed of chalk, almost to the exclusion of anything else, that one can there hardly take it as absolutely impermeable. I allude to the north-western part of Norfolk, where the thin boulder clay of the higher ground is so like the merely weathered surface of the chalk that the officers of the Geological Survey often had some difficulty in dividing the two, especially in places without clear sections. In other parts of North Norfolk, too (near Wells), the glacial brickearth seems to be represented by a bed which is little else than reconstructed chalk, the specimen having yielded, on analysis, no less than 91 per cent. of calcic carbonate, the other constituents being, moreover, of a sandy rather than of a clayey character, and the whole having some likeness in composition to parts of the chalk.
From what has been said, it is clear, I think, that the "maps of chalk areas" exhibited are not merely another version, or condensation, of the Geological Survey maps, although depending so largely on the possession of these. They show, indeed, some things not shown on the geological maps, and for which approximate lines had to be drawn, and their grouping of the facts mapped is different from that of the geological maps.
It may be of interest, perhaps, to contrast the areas of chalk, &c., as shown by some of my maps, with those represented on geological maps without drift.
Thus, in sheet 7, whilst in the latter map nearly half the area, or roughly about 400 square miles, is coloured as chalk, half (or a little more) being of tertiary beds, and a small area (in the north-western corner) of beds below the chalk, a measurement of the four different kinds of area, described in this paper, gives the following approximate figures, which seem, however, to err a trifle on the side of
about 125 square miles. Chalk covered by permeable beds
(almost wholly London clay, there being
Again, in sheet 46, S.E., almost all coloured as chalk on the ordinary plan, we get the following figures, as before erring slightly in excess : Bare chalk.
about 76 square miles. Chalk covered by permeable beds Chalk protected by mixed beds .
67 Chalk protected by permeable beds.
27 In 51, S.E., where neither tertiary beds above nor older cretaceous beds below occur, but chalk alone, the figures are about 40, 8, I, and 155 respectively, various beds of drift (especially boulder clay) generally covering the chalk.
Whilst the general result of the work is to curtail the somewhat excessive estimates that may have been made in bygone years of the amount of cha!k area available for the absorption of rain, yet the chalk remains our chief waterbearing bed in the south-east of England ; for, though not always coming up to some of the sand-beds in permeability or porosity, it is pre-eminent over all other geological formations in thickness and extent of outcrop.
Although beyond the immediate object of this paper, there is one circumstance connected with the question of water supply from the chalk which I can hardly help alluding to from its great importance. The present state of the law, as regards deep-seated sources of supply, is the reverse of encouraging to those who advocate the use of pure well water ; for, by a recent legal decision, in the case of Ballard v. Tomlinson, it has been declared to be lawful for any one to pour any filth or noxious matter down his own disused well without any regard to the fact that he may thereby pollute the source from which his neighbours draw their supply! It would be a work of no great difficulty therefore to utterly spoil most well-waters, and this might not cnly be done by inadvertence, as in the case alluded to, but apparently of malice aforethought. If this is really the law of the land, or rather of the water, there seems to be an opening for law reformers. It is to be hoped, however, that the decision may be reversed in a higher Court than that from which it came. I need hardly say that these remarks apply to all water-bearing beds and not to the chalk only, and, to conclude, I may say the same of the line of observation that this paper brings forward.
WATER SUPPLY IN ITS INFLUENCE
ON THE DISTRIBUTION OF THE
By W. TOPLEY, F.G.S., Assoc. Inst. C.E. Geological Survey of England. Lecturer on Geology at the Institute of
Agriculture, South Kensington. ONE of the most essential conditions for the comfort and well-being of a population is water, and a little consideration
will show that the early settlements of a people have been where, and only where, water occurs.
In a broad and general sense, this fact is patent to allthe banks of rivers and streams are usually well populated -the wide areas of waterless districts are unpeopled ; but the fact is equally true in a very limited and restricted sense, not at first so obvious.
SECTIONS ILLUSTRATING THE ORIGIN OF SPRINGS.
x x Indicate the mere common sites of early settlements ; dots indicate porous beds ; lines indicate impervious beds.
The source of all water is the rain which falls on the land ; this acts in two different ways, according to the nature of the soil on which it falls. If the soil is porous, or pervious to water, a certain portion of the rain sinks in ; if the soil is impervious, the whole of the water either drains off the land into brooks, or passes back by evaporation into the air. The water which soaks into a porous soil or rock, accumulates there until it flows out again as springs, or is artificially tapped, and drawn away by means of wells.
The conditions under which springs most commonly occur may be illustrated by the following "sections," which represent the rocks cut through from the surface downwards. The positions of springs are marked x x x, and in all cases these also mark the positions in which villages and towns occur.
Springs occur near where a pervious bed overlies or underlies an impervious bed (Fig. 1), or where a valley reaches down to the level at which the rock is saturated with water (Fig. 2). In the case of valleys cutting deeply into the rock, the valleys themselves determine the level of saturation.
A soil which allows water to sink into it is a dry soil, and is therefore suited for habitation and for agriculture. Hence the main conditions which favour the settlement of a district are found in the same soil, or along the outcrop of the same bed. We thus see that geological structure controls the distribution of the population ; not only in such great features of the earth's surface as mountain-chains, plains, and valleys, but also in the minor divisions of the district.
The outcrop of a narrow band of porous rocks, between wide beds of clay, is strongly marked by the occurrence of a long line of villages, each of which obtains its water from shallow wells or springs. The cornbrash, between the Oxford clay and the great oolite clays, is an excellent example of this. So, too, is the marlstone rock-bed, between the upper and lower lias. Even a thin and comparatively unimportant bed of sand, ironstone, or limestone, if it only affords a small space fit for arable culture, will be marked by a line of villages (Fig. 3). A thin bed of ironstone in the lower lias of Lincolnshire is a good example of this.
When rocks rise from beneath a covering of clay there are often springs at the junction. Fig. 4 shows a very common arrangement in East Northumberland, where sandstone rises from beneath the boulder clay.
The base of the chalk escarpment, with the line of outcrop