and contains some very massive beds, as may be seen in some of the road sections near Basset Down and Clyffe Pypard.

The Middle Chalk.

About three miles from Chiseldon Station the cutting enters the middle or second division of the chalk; the dividing line between this and the lower chalk is marked by a line of hard nodular yellowish chalk known as the Melbourne Rock, but sections where this rock occurs are rare in this district. This nodular bed, however, may be seen on the surface of the ground near the junction of the lower and middle chalk. Unfortunately the cutting does not pass through this Melbourne Rock, but it may be seen in a pit close to the line.

The middle chalk is whiter and purer than the lower chalk and less argillaceous. Flints also begin to make their appearance, but rather sparingly.

This division of the chalk was laid down in a deeper sea than the lower chalk, and at a considerable distance from land.

It forms a distinct feature in the escarpment, as most of the sheep down-land is on the middle chalk, which is thus of importance agriculturally.

The middle chalk in our district is thinner than the lower, being only about 120ft. thick. Being thinner and the dip being the same its outcrop is much narrower than that of the lower chalk, and thus it covers a much less area on the north and west. This bed is marked 4b in the diagram.

The Chalk Rock.

The divisional bed between the middle and the upper chalk is known as the chalk rock. No good example of this bed is to be seen in the cuttings, though a smooth hard cream-coloured bed rather like part of it may be seen in the cutting south of the bridge by Ogbourne Maizey.

The chalk rock consists of five or six distinct beds of hard creamcoloured limestone, each of which has a layer of hard green-coated nodules at the top; each layer forming a marked plane of separation:

glauconite grains occur in it, and the thickness is about 8ft. Where fossils occur in the chalk rock they are very interesting, for the assemblage recalls that of the chalk marl, some of the species being very similar, and gasteropods common; hence it is supposed the sea became shallower at this epoch.

That this bed must have been cut through in making the line near Ogbourne Maizey is evident, because the line for some little distance south of the cutting by Ogbourne Maizey Bridge is ballasted with it.

The chalk rock may be seen in many pits on the high ground and the marked features of the country such as those on which Liddington and Barbury Castle stand, are due to this very hard bed, which has withstood denudation better than the softer chalk.

The Upper Chalk.

The upper chalk, next above the chalk rock, comes on in the first deep cutting after leaving Ogbourne Station.

We find that flints are very numerous in the upper chalk, and the layers often occur close together. These flints are so plentiful that they cover the ground, thus making a different soil to that of the lower and middle chalk.

West of Marlborough the upper chalk is very much cut up by valleys, and does not cover a very great area, but on the east it occupies more of the ground. On the north it only begins to make an appearance, and so it thickens gradually to the south, being about 150ft. in thickness at Marlborough Station well, where, however, the higher beds are absent.

The Marlborough Water Works well, according to information supplied to me by Mr. Fairbank, the engineer, is 143ft. 6in. deep, all in the upper chalk, which is not passed through.

The upper chalk is a most important division of the chalk for many reasons.

First, it is our great source for water-supply. The lower chalk is compact and clayey, holding up rather than containing water, which percolates very slowly through it. But when it contains the hard gritty beds seen near Chiseldon these would favour percolation,

and so when these beds are found the lower chalk might yield a good deal of water.

But neither this division nor that of the middle chalk can be compared with the upper chalk for purposes of water supply. The many layers of flint and the fissures so plentiful in this formation assist percolation, and the water sinks through the chalk till the level of saturation is reached. This level rises and falls with the amount of rainfall.

Agriculturally the upper chalk is very important, forming a good arable soil, especially when covered by the red clayey flinty soil known as the clay-with-flints, and only found on the upper chalk in this district. The carbonated rain water, the carbonic acid being derived from decaying vegetable matter, percolating through the flinty chalk, removes the carbonate of lime and leaves the clayey part of the chalk as a red clay stained by iron.

Thus, from an agricultural point of view, the lower chalk forms the heavy arable land, the middle chalk the down land, and the upper chalk, when bare, the light arable land.

The clay-with-flints and the Tertiary debris often fill pipes many feet deep. Mr. Codrington, in his paper on the Geology of the Berks and Hants Extension and Marlborough Railways, describes some pipes 30ft. in depth in the cutting between Savernake and Marlborough, near Wernham Farm.

Mixed with the clay-with-flints is to be found a good deal of mottled clay, the remains of the Tertiary beds.

In the brickyard on the top of Salisbury Hill, near the Marlborough Water Works reservoir, are some very good sections of both these clays. The pipes are lined with a thin coating of claywith flints and filled with a considerable thickness of Tertiary clays, the debris of the Tertiary beds.

It is this clay-with-flints and the existing outliers of the Tertiary beds, forming a soil so favourable to the growth of trees, to which Savernake Forest owes its existence. The forest area roughly marks out the clay area, and though to the west of the forest there is a considerable clay area bare of trees, this is because they have been cut down. The forest must once have extended as far

as the West Woods, this being the westerly limit of the clay. From the upper chalk also came the flints whence our forefathers obtained the weapons so well seen in Mr. Brooke's splendid collection in Marlborough.

Marlborough, thus favoured by its geology, was just the place for an important prehistoric settlement, and of this we have many evidences.

Here the forest and the down met, there were flints for their tools, and the river for their water-supply and for fish. Game they could find in the forest, and the down afforded pasture for their flocks, and arable land as well.

As I have gone at length into this matter in a paper published in the Marlborough College Natural History Society's Report, 1890, on "The Influence of Geology in forming the Settlement round Marlborough," I can only thus briefly touch on it here.

We must now proceed on our journey along the line, and go on from Marlborough towards Savernake.

After passing Hat Gate the strata which as far as Marlborough have a steady southerly dip, thus bringing on higher beds in the chalk, begin to dip very sharply the reverse way, to the north. Mr. Codrington, in his paper, makes the dip as high as 45°. The result of this is to reverse the outcrop of the strata, and the chalkrock, which we left at Ogbourne passing away under the upper chalk to the south, is now brought up to the surface and well shown in the deep cutting beyond Hat Gate, with the high north dip before referred to.

As this reverse north dip continues we still get lower and lower beds in the chalk, and very soon reach the lower chalk with the same hard gritty beds we had observed a little south of Chiseldon Station, and last of all we come once more on the upper greensand, which disappeared at the northern end of Chiseldon cutting.

Going on past Savernake, along the Andover line, we cross the northern end of the Pewsey Vale, and pass over the "anticlinal,” or uprise caused by some disturbance which has brought up to the surface, strata that would never have come up but for this.

That we are passing over this arch or anticlinal is clear, for the

upper greensand which outside Savernake Station can be seen dipping north, is seen in the cutting just before Grafton is reached to be dipping steadily and gently south, so that the same series of strata is repeated from Grafton to Collingbourne, where the upper chalk again makes its appearance, as was seen between Chiseldon and Marlborough.

It is interesting to compare the effect on the width of the outcrop of the strata of the sharp northern dip to the north of Savernake, with that of the gentle south dip from Grafton to Collingbourne.

It will be seen that the upper, middle, and lower chalk (4a, 4b, and 4c in the diagram) occupy a very small space north of Savernake as compared with the wider space occupied by the same beds between Grafton and Collingbourne. The sharp dip, too, affects the line of the outcrop which west of Burbage is seen to be almost straight, while to the east of that place it winds about and follows very much the natural contour of the country.

I have now taken you all the way from Chiseldon to Collingbourne on our geological railway journey, and necessarily at express speed, as I have only sketched the bare outline of the great geological story that this journey unfolds. But I trust I have not wearied you, and that thus running along you have been able to read something of the main geological features of the country through which this most interesting line passes.

In conclusion a few words may perhaps be said as to the distribution of the chalk in England. From its western outcrop in Antrim, in Ireland, the cretaceous sea, shallow in the west of England, deepened to the east and south, so that in the Isle of Wight the chalk reaches 1700ft., its maximum thickness in England. No well in Marlborough has gone through the upper cretaceous beds, and we do not know what lies beneath that formation in the Vale of Pewsey. A boring there would be a most interesting experiment geologically, and might yield most important results from an industrial point of view, as the upheaval there may have brought up the older rocks, with perhaps the coal-bearing strata, to within no very great distance from the surface, near enough, possibly, in the event of coal being present, to allow of its being worked.