where to be seen on the inner or land-side, and the series of deposits which it overlies show that, with reference to past conditions, the most recent change has been one of slight depression.

3. Marine silt. On the land-side of the shingle ridge is an expanse of salt-meadows, which at present are but occasionally, partially, and for short intervals covered with water; their level is at the very upper limit of the Bristol Channel waters. From the materials thrown out from the dykes cut across this flat, as also in the sections thus exhibited, the nature of the subsoil is well seen, consisting of mud-deposits. Of these the uppermost is of a yellowishbrown colour, and contains the shells of Scrobicularia piperata, with the valves united, and of all ages. The presence of these shells shows that the lower part of the Porlock valley was at one time in the condition of a mud-flat, wholly covered by the sea at every tide, perhaps even permanently covered that they were then at or near the low-water level, or that the area was then depressed to the extent of the difference between high and low water; the range of Scrobicularia piperata being from low water to four fathoms beneath.

Beneath the Scrobicularian mud of the meadows is a band of vegetable matter, and under that a dark tenacious clay; these are better seen on the coast.

4. Surface of Plant-growths.-At low water, and when the coast has been swept clean of shingle, there is presented an expanse of mud-deposits, with the stumps of trees studded about. The muddeposits occur in patches, owing to the action of the breakers, which cut out portions; around these patches are good sections.

The uppermost mud-deposit, that with the Scrobiculariæ, is not very resisting, so that it occurs only occasionally over the area left by the tide; enough, however, remains to show that it was at one time spread out continuously, with a like composition and under the same conditions as are presented by the beds beneath the meadows; as such it passes down beneath the present lowest water-level. Under the yellow mud-deposit is a dark band, and when the mud has been removed, the surface presents a layer of matted vegetable matter, seemingly composed of roots. This seam is in places several inches thick; on the underside roots descend into the subjacent clay, showing that it has been an old surface of plant-growths.

The stools of trees project slightly above the level of this old land surface; and upon it lie the trunks of trees. In every instance that I examined, the surface of plant-growths was interposed between the prostrate trunks and the underlying blue clay. There were stems measuring from 10 to 20 feet and more in length. They were mostly, if not altogether, the remains of trees which had died, and become bare of bark, some much decayed before they fell; they projected above the surface, and had not sunk into it, or the surface was firm, and not in the condition of a soft morass, when the trees fell upon it.

The roots and long leaves seemed to belong to an Iris, such as the common" Yellow Flag."

5. Blue Mud-deposit.-This is a very tenacious mass, resisting the action of the sea. It is of variable thickness, increasing as it passes down seawards. After most diligent search I was unable to find any evidence as to the nature of this deposit, whether freshwater or marine; in this respect it resembles the blue-clay deposit of the Bridgewater levels; from the abundance of diffused vegetable matter it has the appearance of being of freshwater origin.

It might seem at first sight, as indeed it did to Sir H. De la Beche, that the trees of the submerged forest had grown in this blue mud, from the manner in which they stand out of it; but the muddeposit surrounds the stools of the trees, which never send roots into it; and wherever the mud has been removed the trees are seen to be rooted in the beds beneath. The accumulation of the mud has been subsequent to the forest-growth. The uniform level at which the trees have been cut off may, perhaps, indicate the level of the water which deposited the blue mud, and killed the trees at the same time.

6. Submerged Forest.-The interval left bare by the tide may, under favourable conditions, be seen studded thickly by the stools of large trees some bare, some covered by a thick growth of seaweed, some just projecting above the mud-beds. Some are of large size; one measured 2 feet in diameter. The largest trees were the oaks, which may be distinguished by the black colour of the wood; others, when split open, are red, probably alders; from their dimensions both must have grown under favourable conditions *.

The prostrate trunks lay generally N.W., S.E., or away from the opening of the valley; they had been broken off without tearing up their roots.

7. Angular detritus.-The flooring upon which the blue mud-deposit lies, for as far out as it can be traced, is of coarse angular rocks, instead of some form or other of water-worn materials, as might have been expected; these consisted only of fragments of the splintery quartzose rocks of Dunkerry, of all sizes, with their edges and points as sharp, their surfaces as clean as if just broken; all thrown together in the greatest confusion. Part of this rugged appearance may have been produced by the tides having washed away some of the finer materials. All the trees are rooted in this detritus; it was the surface on which they grew, and had established themselves antecedently to the changes here noticed. The nature of this accumulation, if I am right in my explanation, renders the Porlock Bay forest-beds of more geological interest than the more extensive tracts of Bridgewater or Swansea.

A thick coating of angular débris covers the surface of all the hill-ranges of North Devon and Somerset; it is always strictly local, and is simply the product of the breaking up of the surface to a great depth. In places the mass of débris is in situ,-merely detached; but along some of the new roads which have been cut about the sides of these ranges, sections are exposed, showing that

Large oak and alder characterize the Porlock valley, at present higher up as along the course of the Horner stream.

great masses or "trainées" of the débris have come down from above, cutting deep channels through the detritus at lower levels.

These accumulations of angular detritus along the lower slopes are of great thickness; the materials have also been carried forward to considerable distances, more particularly in advance of deep glens. The high ground from Countesbury to Dunkerry (1678) and Porlock Hill has everywhere good illustrations of the character of these masses of detritus.

A section of such beds is to be seen in a low cliff extending eastwards from Porlock Quarry, and serves to connect the detrital materials of the interior of the country with the beds beneath the bay. It is a section of part of the under-terrace already noticed.

This section, which has a depth of from 10 to 12 feet above the level of the shingle, shows an accumulation of earthy materials, angular fragments of all sizes, all derived from the rocks of Porlock Hill. There is to be noticed a sort of horizontal arrangement resulting from interbedded lines of finer materials, and showing successive accumulation. On either side of the place at which a stream comes down to the coast, cutting through the thickness of angular materials, there are to be seen, mixed with it, some water-worn, partially rounded blocks; these indicate the course of a torrential stream from the high grounds above (as at present), but occasionally of greater volume.

This accumulation of angular materials is referable to like bygone conditions as have been already indicated for the whole of the rest of the West of England (Quart. Journ. Geol. Soc. vol. vii. pp. 121-131). It is a condition of surface presented everywhere by that portion of this country, and of Europe, which was not submerged during the great subaqueous depression of the Northern hemisphere. In geological history it belongs to the subaerial phenomena of the "Glacial Period," and represents the whole of the variable conditions of that long interval of time.

III. CONCLUSION.

Reversing the order of superposition, the sequence of change at this place is as follows:- :

1st. The formation of angular detritus, and its accumulation at lower levels. Highest relative level.

2nd. Forest-growths established on detrital beds.-Trees of great age.

3rd. Accumulation of freshwater mud, resulting probably from a depression of the level of the land.-Trees killed.

4th. Surface of water-plant growths on mud-deposit, or nearly dry surface, on which the trees fell.

5th. Deposit of sea-mud, with Scrobicularia.—Area depressed below the sea-level.

6th. Conversion into meadow, at the level of the highest springs at present.

7th. Shingle.

It may be inferred from Sir H. De la Beche's description that he had not had opportunities of distinguishing the two surfaces of plantgrowths, nor the remarkable floor of angular materials on which the forest-bed is rooted. It is not stated whether the "silt and sand" of the "inclined plane" was freshwater or marine.

The succession of changes here indicated corresponds with that of numerous other localities at which I have had opportunities of examining the evidence in the West of England. The Porlock example, in addition, seems to fix a relative date for a part, namely, as being subsequent to the great subaerial weathering of the surface during the Glacial Period. The elevation of the land was greater at that time than at present, but by how much we have no means of determining.

The difference of level within which the land has oscillated since then need not be estimated at more than 40 feet, the range between high and low water on the Bristol Channel coast being taken at 33 feet. It is certain that such forest-growths as pass on all sides beneath the line of low water could not have lived at the high-water level, but a rise of 50 feet would convert the upper part of the Bristol Channel into land-surface. The greatest depth at which submerged land-surface has been ascertained is about 120 feet; a rise of such amount would place the whole of the Bristol Channel in the condition of dry land, and such probably it was at the time of the forest-growths.

The line of the Bristol Channel is that from which the amount of depression of the British area in the West increased progressively northwards, corresponding to the line of the Thames valley in the East. There is clear evidence that the line of the English Channel was occupied by sea during the "Cold Period" (Quart. Journ. Geol. Soc. vol. vii. p. 135); and at the Newcastle Meeting of the British Association I indicated to what extent this unsubmerged area of the South of England was affected by that depression. Like evidence may

now be derived from the coasts of North Devon and Somerset.

The lowest and oldest beds beneath the Bridgewater levels are of sands and subangular gravel, made up of all the materials of all the hill-ranges which send their streams towards that depression, from the grauwacke of Exmoor and the Quantocks up to the Chalk inclusive.

From the great extent of the old alluvia of this area of drainage, the volume of the rivers must at some time have been very great, and as no part of it was comprised by the line of circumpolar submergence, these alluvia are referable to the subaërial conditions of the whole of the Glacial Period. The great gravel-beds of the Bristol Channel, noticed by Dr. Buckland as proofs of his Diluvial theory, are the accumulated glacial alluvia of all the rivers of the West, from the Severn to the Tone: they are the equivalents in age of the great accumulations of angular débris.

Over the whole of the West of England the remains of the great Pachyderm fauna occur abundantly in, more generally beneath, the old alluvia, as also beneath angular débris, at various elevations,

about the Mendip, Quantock, and Exmoor ranges; but there is no evidence whatever that the great characteristic part of that fauna was in occupation of the area at more than one period. It became extinct, even over the area of the South of England, which was not submerged; indeed the extensive subaërial glaciation which that area has undergone from Cornwall to Kent and Sussex, is inconsistent with the existence of such a fauna.

Lastly, with reference to the age assigned to these later changes, subsequent to that of extreme cold, a relative date is arrived at for many of the so-called "Raised Beaches" occurring on our western coasts. The example nearest to Porlock in the West is that between Braunton and Baggy Point, long since so well described by Sir R. Murchison and Professor Sedgwick. At this place there is the evidence of the more recent changes at low levels, the older seabed, at an elevation of 60 feet for its higher portions, being covered up by an enormous accumulation of angular debris-bed *.

I have already called attention to the great thicknesses and other characters of the angular débris overlying old sea-bed (Quart. Journ. Geol. Soc. vol. vii. p. 118), and would also refer to the observations of Sir H. De la Beche (Report on Cornwall, &c., chap. 13).

In a modified sense these sea-beds or raised beaches are preglacial, or older than the period of deep surface disintegration. But just as the amount of northern depression increased from South Northwards, so the progress of that depression was in the contrary direction, or from North to South; the line of the Bristol Channel was nearly the limit of this submergence, and was the last reached. The interval of time which separates these more recent changes of relative level from that of the great depression of the Northern Hemisphere, is possibly very great, not so their distance from the present. The distinctive features of the newer deposits are, that they belong to the time of our existing assemblage of animals and plants, that they indicate changes of small vertical amount, and are remarkably uniform.

2. On the MARINE ORIGIN of the PARALLEL ROADS of GLEN ROY. By the Rev. R. BOOG WATSON, F.R.S.E., F.G.S.

(Abstract.)

6

THE Parallel Roads of Glen Roy have been described by several observers, who have also offered explanations of the manner of their formation. In the author's opinion, Mr. Robert Chambers has solved this problem in his Ancient Sea Margins,' and in this paper he supported the view there advocated, and offered some objections to the theory of an ice-dam, originally put forward by Prof. Agassiz, and recently illustrated by Mr. Jamieson†.

After noticing the strong points of the ice-dam theory, Mr. Watson remarks that, though Prof. Agassiz and Mr. Jamieson agree

* Bed a of Section. Geol. Trans. 2nd ser. vol. v. p. 279.

+ Quart. Journ. Geol. Soc. vol. xix. p. 235.