Possibly too, in extending the work (as I hope to), it may be well to mark areas where beds below the chalk may contribute to its water supply, or where rather the water system of the chalk may join that of some underlying bed. This, of course, would take place where any mass of permeable upper greensand is not divided by clayey chalk marl from the chalk above, in which case there will be but one water system. There is one remarkable part of the chalk tract surrounding the London tertiary basin, where, from the absence not only of the clayey chalk marl and the underlying (but much more local) upper greensand, but also from the thinning out of the usually persistent gault, a markedly impervious bed of clay, it results that the hard ferruginous sand (or carstone) of the lower greensand at once underlies the hard massive chalk. In the north-western corner of Norfolk, where this takes place, from from Hunstanton southwards to near Sandringham, the water systems of the chalk and of the upper part of the lower greensand join; though the lower part of the latter formation is divided off by clayey beds. The supply at Hunstanton Waterworks, close to the base of the chalk, may, therefore, be derived from this composite source, whereas the supply for the Sandringham estate is from the chalk, the clayey chalk marl coming in at that part, and causing an outflow from the more permeable beds above.

The maps which form the text on which this paper is founded, though based on the later geological maps (with drift), follow the latter absolutely in only one particular, that is in the area coloured as bare chalk. As, however, the object in constructing the maps in question was contrast and conspicuousness, their colours are quite different from those of the geological maps; and, whereas, on the latter the chalk is shown by a very pale green tint, on the former it is emphasised by light carmin. It is only fair to the maps to say that they were not made as diagrams, to be seen at a distance; but to be examined more closely, and that they have been coloured at various times, as occasion enabled me to take them up. When the work

has been further extended, and a large set of maps can be coloured at once by a better colourist, a more harmonious effect may result.

We may now consider the evolution of these "Chalk Area Maps" from the Geological Survey Drift Maps. It will be at once seen that the geological maps are the far more complicated of the two, having a great number of colours and tints as against the five of the other set. My geological friends may, therefore, be inclined to regard the evolution as a case of degeneration; but, perhaps, engineers may not object to the comparative simplicity of my maps, and to their disregard of theoretical considerations and of geological classification. It should be clearly understood that these maps are meant to show one thing only, that is the accessibility of the chalk to surface-water: they do not always show of necessity the nature of the surface; but their colours are, in some parts, quite independent of what occurs at the surface, which may be neutralised, for our purpose, by something else that occurs beneath.

1. BARE CHALK.

As in this case the maps follow the geological ones, there is no need to say more than that bare chalk is taken to include those parts where there is merely a thin soil over the chalk, for that soil is practically unimportant as regards water being pervious.

2. CHALK COVERED ONLY BY BEDS OF A PERMEABLE KIND.

This includes tracts marked by many colours on various geological maps, permeability being by no means connected with geological age. In colouring these tracts, it was essential to consider not merely the beds shown as at the surface on the geological maps, but to make certain that nothing came between any bed of a permeable character and the chalk, as it is clear, for instance, that ten feet of clay in or beneath 100 feet of sand would stop the downward passage of water through the sand.

The geological beds that fall into this class are as follows: but it should be understood that this is only where one of them rests directly on the chalk, without any bed of an impermeable or mixed kind coming between, or where two or more of them succeed each other without any intervening bed of another kind :

Thanet Beds, where these are of sand, as in Surrey and West Kent.

Oldhaven and Blackheath Beds, in the few places where they have cut through the under-lying Woolwich and Reading beds, and rest on the Thanet sand or as outliers on the chalk.

Red Crag and Norwich Crag, where they rest on the chalk. Parts of the tracts coloured with the latter on the geological maps are not included, however, on account of the occurrence therein of the clayey Chillesford beds, which must stop the downward passage of water.

Sands and Gravels of the Glacial Drift.
Sands and Gravels of the River Drifts.

3.-CHALK PROTECTED BY BEDS OF MIXED OR
VARYING CHARACTER.

In this case it is not essential either that the chalk should be directly covered by beds of the kind indicated, or that such beds should be at the surface; it is only needed that they should occur somewhere between the surface and the chalk, and that no absolutely impermeable beds should come in; permeable beds are of course made of no account by the presence of these mixed beds. Many colours of the geological map find themselves massed in this set, as may be seen from the following list ::

Thanet Beds, where to a great extent clayey, as in East Kent.

Woolwich and Reading Beds, from their varying character, here clay, there sand. There are places, however, where it may be well to include this series with the impermeable beds. The overlying Oldhaven and Blackheath beds, in themselves permeable, are almost wholly carried with this

underlying series, except where, as above noted, they have cut through the latter.

Loams of the Glacial Drift. Some of the brick-earth of this age is so clayey, however, that it has been classed as impermeable.

Loams of the River Drifts.

Brick-earth and Clay-with-flints of the high chalk tract. These are beds which commonly occur over the chalk hills, and which, though of a more or less clayey nature, seem not to be wholly impermeable, for they do not give rise to drainage-streams; moreover, they seem, in part at least, to be the result of the dissolution of the chalk by the infiltration of acidulated water. Some outlying patches of sand in East Kent have been classed with these deposits, as they often have a clayey base.

Alluvium.-The silt and peat of our marshes might be thought to be impermeable; but though this may often be the case, these deposits are sometimes sandy, and sometimes very thin, the permeable gravel beneath coming up near the surface.

4-CHALK PROTECTED BY IMPERMEABLE Beds.

Here, again, as with the last division, the beds in question need neither rest at once on the chalk nor form the surface; their occurrence anywhere between the surface and the chalk is decisive, and relegates the tract where they occur to what, from our point of view, may be called the useless division, save for those parts of it where the surface waters flow towards the chalk. The large tract marked by the grey colour on the maps, owes its classification almost wholly to the two great clay deposits, the London clay and the boulder clay; but some other beds contribute slightly, as shown by the following lists.

Woolwich and Reading Beds.-In North Suffolk, and in Norfolk, where these beds are quite masked by drift and crag, never cropping out to the surface, they seem to consist chiefly of clay, as proved by well-sections, and it is best to class them with impermeable beds. This may

have to be done in parts of the Hampshire basin also, when the re-survey of that tract for drift enables us to construct chalk-area maps of it.

London Clay-As it is not only where this thick mass of clay forms the surface that it stops the downward passage of water, but also wherever it underlies other beds, it follows that all the overlying permeable deposits have now to be classed with it. This includes the whole of the Bagshot series, so largely composed of sand, which forms such large areas in both the London and Hampshire basins, and also the whole of the very local Coralline crag, which occurs only over a few square miles of Suffolk. Great part of the red crag and of the various drift sands and gravels are also included. All such tracts of permeable beds over London clay, are, of course, of some importance as regards water supply, on a comparatively small scale, and over part of Norfolk and Suffolk no other source of supply from wells is used.

I need hardly say that, in the case of the London clay, my maps add nothing to our knowledge, the position and extent of that deposit being well known, except in East Norfolk and the adjoining part of Suffolk: the above remarks have been made merely to explain the grouping of other beds with it. There are places where the loamy beds at the base of this clay may be sandy enough to allow of the downward passage of water, but such areas must be insignificant.

Brick-earth of the Glacial Drift.-In some parts this deposit is not only of fair thickness, but also of an essentially clayey nature, so that it has then to be classed as impermeable. It often, too, extends underneath deposits of a more permeable kind, and so cuts off their water from the chalk; the underground extension is, however, sometimes hard to trace.

Boulder Clay-This not only covers the chalk, or permeable beds overlying the chalk, over large areas, but also extends under some of the drift gravels and sands, so that it has a great effect in protecting the chalk from the access of