shore having been invoked in turn as best explaining the origin of the bank. Our own theory will, we believe, suit all conditions.

There is one point on which all must agree; it is that the bank could not have been formed without the huge natural groyne, or breakwater, of the Isle of Portland, which bounds it on the east, and stops the shingle in its easterly course; but beyond this we venture to differ from the explanations that have been given to account for the presence of the shingle in so anomalous a position.

The above theories rest on the supposition that the form of the neighbouring land, at the time of the formation of the bank, was much the same as now; and although the theory of Sir H. De la Beche would seem to allow that the beach might have been formed against land, and separated merely by the sinking of the land (in this case the shingle ought surely to have been driven back as the land sunk), yet the other theories imply that the bank was originally formed as a detached mass, separated from the land as now by a narrow channel of water, unlike other long tracts of shingle, which are formed against the land, and which, travel as they may, touch the land. On the other hand, the theory that we suggest needs no such supposition, but starts with the reasonable assumption that the Chesil Bank may have been formed at first in the same way as the ordinary shingle-beaches of our coast, and that what was once an ordinary beach, banked up against the land, has been since separated, as a bank or bar, by the denudation of the land behind it, such denudation having taken place in a way that would hinder the backward motion of the shingle, and would leave a narrow channel (the present Fleet) between the bank and the land.

In order to make our theory more easily understood, it will be well to give a short description of the Chesil Bank. In doing this, we shall avail ourselves of Mr. Coode's account, which has made needless any measurements on our part; at the same time we ought to state that both of us can speak from personal knowledge of the coast and of the bank, the first named of us having done the Geological Survey mapping of that district, while the other spent great part of a summer holiday in an examination of the Dorsetshire

coast.

The Chesil Bank (including under that name the whole of the continuous strip of shingle from Burton Bradstock to Portland) is the largest accumulation of shingle in this country, and more than fifteen miles long. On the N.W., for five or six miles, it touches the shore, but on the S.E., from Abbotsbury, it is divided from the main

1 We are aware that at the mouths of many rivers bars of shingle stretch a long way across from one side, sometimes indeed to such an extent as to turn the rivers along the shore (between the land and the shingle), in the direction of the prevailing set of the currents, for some distance. But these are not really analogous to the Chesil Bank, where the shingle-beach is far longer, and where there is no river emptying into the sea, but only a succession of very small streams. There are also cases of shingle-banks completely damming up streams, and with a marsh or expanse of fresh-water on the land side, as at Slapton Sands, South Devon, and Cuckmere, in Sussex.

land by a shallow estuary about eight miles long, of variable breadth, but nowhere more than two-thirds and often less than a quarter of a mile wide, which is known as "the Fleet" or "Backwater" (Plate XV.); and for the remaining two miles, nearest to Portland, it has the sea on either side. It is with the last ten miles that we are now chiefly concerned, and to this part the following paragraph refers.

The average width at the base is 170 yards near Abbotsbury, and 200 yards at Portland. The height increases from N.W. to S.E, but the inclination of the crest is not uniform. At Abbotsbury, the crest is over 22 feet, and at Chesil over 42 feet high. Borings made down to high-water level, and sometimes lower, passed through nothing but beach (except at one place, where clay was met with deep down): ten or fifteen feet from the surface, the shingle was generally mixed with a little sand, and the quantity of the latter increased with the depth until the whole was found to be very compact. (Coode.)

The largest pebbles are at the eastern end, and gradually decrease in size westward, until near Burton the beach consists of sand and very fine shingle. The accompanying map will make the above description clearer. (See Plate XIV.)

Westward from the end of the Bank the coast gradually gets higher, and soon there are high cliffs of Liassic, Jurassic, and Cretaceous beds. These cliffs are cut through at Burton and Charmouth by valleys with small streams, such as the Char and the Bredy, which, of course, flow seaward, that is in a south-westerly direction.

These streams (excepting the Bredy) do not breach the shingle of the bays where they flow into the sea, but turn eastward (the direction of the general set of the current) for a short distance, between the beach and the land, and then filter through the shingle. There is an interval of some miles between each of these successive streams, in which the cliffs are not breached by valleys, or only by such as are cut off at some height above the sea. On the other hand, along the low shelving shore eastward of Abbotsbury, the least approach to a cliff is a great rarity, and a cliff ten feet high is a marked object: here, therefore, the streams are much closer together, each hollow in the ground sending in its share of water to the still channel of "the Fleet."

Let us think now what would happen if in former times this latter part had been in the normal state of a beach skirting a low coast. The streamlets flowing down the small valleys would act just as those on the coast to the westward do; that is to say, they would turn eastward for some distance before filtering through the shingle, and most likely they would run the further between the beach and the mainland, by reason of the former being so much broader, higher, and more compact than it is on the coast. Now, as the streams are near together, instead of being separated by miles of unbreached cliff, it is quite possible (and we think most likely) that some stream might continue its easterly course between the shingle and the shore,

until it reached the next stream. This might take place with several streams, where two were near together, and the increased force of two united would give the resultant stream greater power to continue its eastward course and to join on to the next. The consequence of the continuance of this process would be, that all the streams would at last join to form one long channel between the beach and the mainland, as in the subjoined Woodcut, which is on a larger scale than the Map (Plate XIV.), with the streams continued to the beach.

NW.

S.E.

Sea

Plan of streams flowing to a shore where there is a current chiefly in one direction. It seems well within the limits of possibility that this body of water would have power enough to keep its channel free, and to prevent the firm compact shingle from being driven back against the mainland. In this case the land would be protected from the cliffforming action of the sea, but on the other hand would be subject to the scooping action of land-waters, which would cut it back in gentle slopes, and irregularly widen the channel, according to the

nature and hardness of the different beds acted on.

Of course, on any theory, the Chesil Bank could not have been formed but for the existence of the natural breakwater of Portland; and, according to the theory now brought forward, either the narrow neck of land which must once have connected Portland with the mainland, has been breached by some means or other, inwards towards Weymouth Bay (instead of the beach being breached outwards to the sea); or, on the other hand, if at first the beach was broken through, and the stream flowed out to the sea in a southerly direction, that breach must have been slight enough to have been filled by the heaping up of shingle, when the land in its rear was worn away and Portland was separated from the mainland. As the connecting isthmus must have consisted of Kimmeridge Clay, its destruction would be no hard matter to the sea on one side (N.), and the stream on the other; and as large breaches in the Chesil Bank are known to have been refilled in a very short time, there is, perhaps, no great difficulty in accounting for the phenomena on either supposition.

The destruction of the land between Portland and the mainland on the north, has been made easier by the beds having been thrown into a sort of arch, with a sharp northerly and a gentle southerly dip; the arch being of course the form that helps denudation, both by fissuring the beds, and by giving them a tendency to fall outwards. In this particular case, moreover, the lower beds that have been brought to a higher level by the said arch, so as to be within

reach of denuding actions, are of a softer and more destructible kind than those that overlie them.

There are three facts that seem to confirm the theory that the channel in the rear of the Chesil Bank has been formed since the heaping up of the shingle-they are (1) that the isolated part of the bank is also the largest and strongest, the best able both to withstand the sea, and to stop the streams from flowing directly into the sea (2) that the very irregular shape and cliffless character of the shore of "the Fleet" are not such as one would expect to be caused by the action of the sea along such a coast, whilst they are just what should be produced by the action of streams: and (3) that the Channel ends where the streams end. Westward of Abbotsbury, where there are no streams, the beach is not separated from the land; eastward of Abbotsbury, where there are streams, the beach is separated from the land.

Whether there may have been a slight rising or sinking of the land during the formation of the beach, would we think make little difference, on the theory which we have brought forward. Whilst we are far from asserting dogmatically that the Chesil Bank must have been formed in the way described, yet we think that our theory, or explanation, involves less supposition, and tallies more with observed facts, than any other does, and that, therefore, it should be accepted until replaced by a better, or disproved.

Lastly, we wish to draw attention to the confirmation given to the theory of Subaërial Denudation by our explanation of the origin of the Chesil Bank. It was not until we were convinced of the truth of the former that we saw our way to the latter; but when we began to see how great has been the share of rain and rivers in wearing away the land, and in cutting out hills and valleys, then we were enabled, by the new light thus gained, to explain the origin of a very uncommon phenomenon, which before we could not understand or account for. What had previously been a mystery, and looked like a freak of nature, became clearly intelligible, and was seen to be the natural result of ordinary causes and existing agencies. Postcript. In the discussion of this paper, Mr. J. Evans, F.R.S., suggested that tidal action may have assisted materially in the formation and widening of the Fleet." This we are far from denying, although we omitted to notice in our paper the assistance that may have been given by that action when it was enabled to come into play.

II. ON A RAISED BEACH AT PORTLAND BILL, DORSET.

By W. WHITAKER, B.A. (Lond.), F.G.S., of the Geological Survey of England. [PLATE XIV.]

[A paper read before the Geological Society of London, May, 26, 1869.]

N 1850, Mr. H. W. Bristow recorded, on sheet 17 of the Geological Survey Map, the existence of "conglomerate" and "recent stone" at Portland Bill.-(See Plate XIV. herewith.)