Transactions of the Geological Society, 1st series, vol. 1/On the Geology of some parts of Hampshire and Dorsetshire
|←On an Aluminous Chalybeate Spring in the Isle of Wright||On the Geology of some parts of Hampshire and Dorsetshire
|On the Geology of the vicinity of Dublin→|
By J.F. Berger, M.D.
Honorary Member of the Geological Society.
Although the chalk hills form the most striking feature of Dorsetshire, Hampshire, and the Isle of Wight, they do not occupy the whole of the surface of those counties. Several other strata or mineral beds there occur, the general arrangement of which it is the purpose of this communication briefly to illustrate.
The chalk hills which appear on the south eastern-coast of the Isle of Wight traverse the interior of that district in a line nearly due west to the Needles; they are interrupted by the sea, and by the alluvial deposits on the eastern side of Studland Bay; but reappear at Corfe Castle, and on the coast at Lulworth, from which last place they may be observed passing towards Weymouth, still preserving their original direction, having left to the south of them nearly the whole of the Isle of Purbeck.
The breadth of this range of chalk is not very considerable, for the entire coast from Christchurch Bay to Poole lies to the North of it.
The dip of the strata varies from N.E. to S.E. but the point of the compass, towards which they all tend, is the east.
The outline of these hills, is characterized by gradual and successive swellings and depressions of the ground; they also offer natural scoops or semi-circular excavations on their acclivities. Though covered with a short grass, they may be called naked, being entirely destitute of timber.
They rise to a greater absolute elevation than the other beds by which they are accompanied; so that even at a distance, we may safely conclude the highest bare hills to be chalk.
This rock is harder than most of those with which it is associated, and in consequence wherever it appears on the coast, the sea-water is in such places, more transparent, and generally of a greater depth, on account of the cliffs being cut off more abruptly.
The cliffs of chalk assume often the shape of recesses or semicircular bays, the outline of which is well defined: such is Freshwater Bay, Lulworth Cove, the Bay of Weymouth, &c.
In the Isle of Wight, the chalk hills, as far as I could ascertain form a belt across the middle of the island, the greatest breadth of which may be five or six miles. As this belt proceeds westward, it contracts gradually to a point of land deeply indented on the north by Alum Bay, and on the south by Freshwater Bay. On that projecting tongue of land called High Down stands the signal post, the light house further on to the west, and the Needles lower down. I found by the barometer the elevation of the signal house to be 430 feet above the level of the sea; that of the light house 379; and of the furthest and most western part of the cliffs, above the Needles, 189 feet.
The chalk hills in the Isle of Purbeck lie in the prolongation of a line westward from the Needles: they pass through Corfe Castle, and establish a separation between two troughs, or basins. The one on the north has a gentle slope towards Poole Harbour, the other on the south is more horizontal, extending towards the sea, on the coast of which it presents a range of rather abrupt cliffs. The soil of the latter is very fertile, while that of the former is completely barren, and covered all over with heath.
These hills pass under particular names, such as Nine Barrow Downs, which I found by the barometer to be 625 feet above the level of the sea; Challer Hill, 390; Norden Hill, 369; and East and West Creach. Between Challer and Norden Hills, stands on the top of an hillock, the line ruin of Corfe Castle, the height of which is 207 feet. These hills consist of what is called hard chalk, perhaps from the greater quantity of sand which it contains.
Over the chalk lie several beds or strata of a later formation, the relative age of which I shall not now presume to determine, as their alternation with each other appears to be several times repeated.
This is the formation which prevails chiefly on quitting London, in travelling to the south-west. On the road to Southampton by Bagshot, Farnham, &c. I have traced it as far as to New Alresford; at this latter plane the chalk is first to be found in situ, at the distance of fifty-seven miles from the metropolis. We lose the chalk in the neighbourhood of Otterborne, five or six miles to the S.S.W. from Winchester, where we enter again into a loam mixed with flint gravel. In the Isle of Wight, the same flint gravel in loam occurs near Marsh Green in Brixton Bay, not far from the chalk hills called Brixton and Mottiston Downs.
The heath, which extends from Christchurch town to Poole, a distance of eight miles, has for its bottom the same flint gravel either in sand or in loam. It forms also the upper part of the cliffs in Christchurch Bay by Milford, Hordel, &c.
The flint pebbles found in this formation are rounded and much smaller than those which are imbedded in the chalk; they have no coat, but on the contrary, a sort of semi-transparency approaching that of amber. They have in all probability been worn by the sea.
It is always strongly impregnated with oxyd of iron; it presents sometimes all the varieties of colour imaginable, white, ochre-yellow, brownish-red, pink, green and black, but in all these instances the iron seems to be in a state of peroxide, as none of the rocks which the sand forms, act at all on the magnet, though the considerable quantity of this metal which they contain is readily shewn by the application of chemical solvents.
This sandy formation is to be met with in the Isle of Wight on the southern boundary of the chalk from Shorwell to Chale, but especially in Alum Bay, where it makes high, precipitous and very grotesque cliffs, remarkable both for their variety of colour and the multiplicity of short pyramids implanted one upon the other.
This loose sand is also the matter which fills up for the most part the inclined semi-trough, which from the shallow inner harbour of Poole extends to the northern acclivities of the chalk hills by Corfe Castle. It is there of a whitish-grey colour, covered with heath reduced in some places to the state of turf. To this formation of sand belong the following mineral substances:
(a) Coarse hard ferruginous sandstone, passing to a conglomerate, composed of rolled quartzose pebbles of different sizes, united by a ferruginous cement which does not effervesce with acids. This rock strikes fire, and has at first the appearance of slag, being rough, hollowed out, and covered externally with a crust of an earthy brown colour, arising from the oxidation of the iron. It has no action whatsoever on the magnet. It occurs in the form of flat scattered masses at the depth of a few feet from the surface of the sandy soil; the upper strata of loose sand being sometimes washed away, these pieces project, making a sort of cornices, and protecting the inferior strata of sand. I suspect this conglomerate to be of a very late origin, and indeed, daily forming by a process somewhat like that of cementation, viz. the percolation of water strongly impregnated with iron coming from the upper strata, and thus agglutinating the loose sand.
This rock is to be seen plentifully on the road from Shorwell to Chale in the Isle of Wight, and in. the sandy trough of Poole.
(b) Potter's clay.
It alternates with the loose sand in the trough of Poole, where it is found in beds of various thickness at different depths. It does not effervesce with acids, and from a cursory chemical examination which Dr. Marcet had the kindness to make at my request, we traced in it the existence of alumine, of lime, of magnesia, of oxyd of iron, and of silica. It feels greasy and smooth, its colour varies from ash-grey to blue, its fracture is a little shining and uneven. It contains sometimes cylindrical blue nodules (called pins by the workmen) of a more close texture, in which there is probably a greater proportion of oxyd of iron. This clay is sent to Staffordshire, where it is mixed with ground flints, and employed in the finer kinds of pottery. I found the specific gravity of one of the purest specimens from Threshers's clay-pit, 1.723. Mr. Kirwan states potters' clay to be from 1,8 to 2.
I have been informed by very competent persons that the beds of clay in the trough of Poole do not affect any particular direction.
The situation of the potters' clay in this present instance, is perfectly agreeable to Werner's opinion.
(c) Coaly bituminous matter.
Remains of vegetables, some of which still retain their texture and shew that they belonged to the tribe of aquatic plants, are to be found in a white quartzose sand impregnated however with oxyd of iron, in the cliffs of the south-western coast of the Isle of Wight: at the east of Freshwater bay, some scaly carbonated wood with iron pyrites, is also to be found in the sand.
An earthy brown-coal extremely friable, and which crumbles to pieces when put into water, underlies the potters' clay in the sandy trough of Poole, where it forms a seam of some thickness. It burns with a weak flame emitting a particular and rather fragrant smell of bitumen, somewhat analogous to that of the Bovey coal. They differ however in point of specific gravity, this being 1,153, while Bovey coal varies from 1,4 to 1,558. There are frequently pins of clay passing through this earthy brown-coal.
I have seen at West Lulworth, another brown-coal passing into pitch coal, in a black loose quartzose sandstone, in which it makes a very thin layer or seam. It hardly burns with a flame, but chars like wood, emitting an empyreumatic or subacid smell. The spec. grav. of the specimen I tried, was 1,340. This last species of combustible matter, as well as the former, is used by the poor people as fuel, and they may be both referred, I think, to the spurious coal of Mr. Kirwan; the range of the specific gravity which he gives, is that of 1,500 to 1,600.
This rock occupies a pretty large extent along the coast: I have observed the four following varieties.
(a) Marl of an earthy semi-indurated texture, which assumes spontaneously polyhedral forms, and contains nodules or kidneys of sulphuret of iron, some of which have undergone a partial decomposition. It has no lustre, the fracture is coarse, the colour of a bluish-grey; it contains numerous specks of mica, it does not effervesce with acids, though the presence of lime is readily shewn by chemical tests; it is composed also of alumina and of a good deal of oxyd of iron; it adheres slightly to the tongue, and is friable when immersed in water, it soon falls into a powder which is rather rough and dry, it decrepitates on the first impression of the fire, becomes hard, and when heated to redness, turns greyish-yellow. It forms a bed of several fathoms in thickness on the south-western coast of the Isle of Wight, where it lies between a stratum of loose quartzose sand, and one of calcareous freestone with chert, above the source of an alum chalybeate spring. It is found also to the north-east of Alum bay, where it alternates with sand; at High cliff in Christchurch bay, where it constitutes most part of the cliff, and is covered only by the flint-gravel; and between Encombe and St. Aldham's Head in the Isle of Purbeck.
(b) Marl of a bluish-grey colour, intermixed with iron-pyrites. It has a degree of lustre, its texture is slaty, it feels smooth and greasy, does not contain mica, and is not acted upon by acids; it falls in water into a very fine powder, but not so readily as the former variety: when heated it becomes harder, of a yellowish colour, and its structure appears more evidently foliated. I have observed it in the bed of a rivulet which runs to the sea near Compton-field, on the west-south-western coast of the Isle of Wight, not far from Brixton bay. The flint gravel lies over it.
(c) Marl of a greyish-white colour, harder than the two former varieties, effervescing with acids, and adhering more strongly to the tongue. In water it does not fall into powder; the fracture is earthy ; no specks of mica are visible in it; when heated to redness it does not decrepitate, but becomes harder, and of a reddish-brown colour. The surface is often covered with compressed nodules of sulphuret of iron; it alternates with the preceding kind.
(d) Marl of a bluish-grey colour effervescing with acids; its fracture is dull and earthy; it contains a great many shells and a few specks of mica. In water it does not fall into powder; it hardly adheres to the tongue; when heated, it decrepitates slightly, and the texture becomes more evidently slaty ; by a red heat it acquires a white-reddish tinge, and gives out a bituminous smell analogous to that of the Kimrridge coal, or to express myself more correctly, with the slate-clay, or shale (Schiefer-thon of Werner) with which it alternates.
This is coarse-grained, loosely aggregated, of a yellowish-white colour, with small brown grains of siliceous sand, and some specks of mica; it effervesces briskly with acids; it is disposed in strata of several yards in thickness which alternate with coarse shelly limestone and thin layers of chert; it enters into the composition of the cliffs of the south-western and southern coast of the Isle of Wight. As it is easily acted upon by external agents, it is often hollowed out, leaving the interposed strata of coarse shelly limestone, and the layers of chert, like shelves or overhanging cornices projecting from the cliffs, till, the sandstone giving way to a great extent, the upper strata fall down, and take in settling all degrees of inclination. This is the only way to account for those large and numerous blocks which have encumbered the under cliff in the Isle of Wight. I have observed in many instances the passage of the sandstone to chert, and of this latter to a beautiful transparent chalcedony.
This kind of rock I am rather anxious to introduce here, because it seems to constitute a formation by itself, which, I believe, was first pointed out by M. Brongniart, who has given us some very valuable information on the subject. This variety of limestone was thought to be very scarce in England, but I am now disposed to believe, that upon a further examination, it will be found to occupy a great extent of country, and will, by many persons perhaps, be considered as a continuation of the same strata or beds which exist on the opposite coast of France. I have observed it in the southern part of the Isle of Wight called the under cliff, in several places of the Isle of Purbeck, in the Isle of Portland, and, from some specimens which I have seen at the Geological Society's apartments, I have but very little doubt, that it exists also in the counties of Surry, Sussex, Oxford, Rutland, and Somerset. In France, it forms the bottom or basis of several extensive plains; such as that of the neighbourhood of Paris, and that of Caen in Normandy.
I have seen it alternating with a calcareous sandstone, with the oolithe, (oviform limestone of Kirwan), and with marl: passages of it may be traced on one hand to the calcareous sandstone, and on the other to the oviform limestone. At the quarry of Tilly Wym by Seacombe Cliff, to the east of St. Aldham's Head, and at Chapman's Pool near Encombe, as also in the Isle of Portland, it includes patches of a compact limestone of a greyish, or dark blue colour, which becomes harder as it passes gradually into a state of complete flint or chert. Although these patches of compact blue limestone effervesce with acids, they are however hard enough to strike fire with steel. I have seen at the apartments of the Geological Society a series of specimens from the County of Rutland (Nos. 890 to 896), so very much like those of Purbeck and Portland, that they might be taken one for the other. But generally speaking, the texture of the coarse shelly limestone is uneven and rough; it contains a great many shells, which, according to M. Brongniart, belong mostly to the tribe of the littoral shells. Sand also occurs either filling up the cavities of the shells, or dispersed through the substance of the limestone. It is generally calcareous, and of a dirty-yellow colour; sometimes it is siliceous, and then appears under the form of very small brown grains. In the coarse shelly limestone of Swanage, the colour is yellowish grey, and the texture somewhat resembles that of a pisolite; (var. of the oviform limestone of Kirwan). In the quarry of Tilly Wym and in that of Wind Spit, a little westward of the former, it is mostly composed of shells of oysters, which have lost their outside coat. In the Portland stone, judging from the quarry which lies on the north-east of the island, the greatest part of the remains included within it, are casts of a species of Trigonia of Lamarck (Hippocephaloides of Plott) as I am informed by Mr. Parkinson; a genus of which Mr. Péron has found a living species in the Southern Seas. This stone is rather rough, on account of the many cavities left by the casts of the shells, and which cause the air contained within, to oppose a resistance to the hammer, in the manner of the porous lavas: some of those cavities are however lined with crystallized calcareous spar.
The specific gravity of the different specimens of this limestone, I have found to be as follows:
|From the Under Cliff in the Isle of Wight||2,666|
|From Steeple Ashton, Oxford||2,624|
|Wind Spit quarry||2,592|
|Portland N.E. quarry||2,568|
|Tilly Wym quarry, passing to a calc. sandst.||2,466|
|Mean spec. grav.||2,579|
The ratio between the extreme points, is as 92 : 100; a range much less than is stated by M. Brongniart from Rondelet's experiments, viz. of 24 : 17.
The specific gravity of the compact hard bluish limestone found in patches in the coarse shelly limestone, I have ascertained to be as follows:
|From Tilly Wym quarry||2,501|
Lastly, the specific gravity of a dark-blue flint or chert, from Portland, connected with the bluish compact limestone, and passing gradually into it, is 2,545.
In the quarry of Tilly Wym, the flint or chert is mostly under the form of detached masses or nodules; while in the under cliff in the Isle of Wight, and in the Isle of Portland, it forms continuous layers; in the latter place, their direction is that of the strata, viz. nearly from north to south, their dip being east by south.
With respect to the existence of chert: or flint in the coarse shelly limestone, M. Brongniart is not very consistent with himself : “ Il paroit que les silex si abondans dans la variété suivante (la craie), and qui se voyent aussi dans la précédente (l'oolithe), ne se trouvent presque jamais, ou peut être même jamais dans la chaux carbonaté de cette furmation.”
While on the contrary, speaking of the silex pyromaques, he says, “ Les silex pyromaques blonds se trouvent aussi en couches minces continues, ou presque continues, entre les bancs de chaux carbonatée grossiére, & au milieu d'un sable grossier.”
I have observed two beds or strata of this rock: one is in the quarry of Wind Spit in the Isle of Purbeck. This quarry is open in two places. To the east, where the rock has been excavated on a very large scale, there is nothing but the coarse shelly limestone before mentioned, which as it extends to the western quarry, passes into oviform limestone. The grain of the rock quarried at this latter place, is more close, the colour uniformly white, the texture less coherent; it still however retains some fragments of shells, but they are small and broken. I found the specific gravity of one specimen 2,539.
The quarry to the north-west of the Isle of Portland, which is the most extensive, is composed almost entirely of the oviform limestone, containing but very few shells; the texture of the work is granular, the nodules very small, and united by a calcareous cement. It crumbles to pieces much more readily than the coarse shelly limestone which lies on the opposite coast of the island. The specific gravity of one specimen I found 2,464; thus giving for the mean specific gravity of the two specimens here mentioned 2,5015.
“ Les oolithes,” says M. Brongniart, “ se trouvent en bancs ou en masses considerables au pied des collines ou des montagnes, dans le passage des terrains de crystallisation, aux terrains de sédiment : elles sont rares dans la chaux carbonatée compacte : on ne les a jamais vues dans la craie proprement dite; il paroit qu`elles sont particuliéres à la chaux carbonate grossiére.”
Of the bay where this coal is found, I have seen but the eastern part, about a mile to the east of Little Kimeridge, where the cliffs are cut down rather abruptly. These are composed of a slate-clay (Kirw.) of a greyish yellow colour, finely slaty, containing both animal and vegetable impressions. The leaves of which the rock is composed, become much more evident, after it has undergone some decay, or, when sound, after it has been exposed to the fire. It divides spontaneously into large tabular masses. The fracture of the rock is earthy, with many small specks and nodules of indurated clay. The outside of the rock is covered with a thin layer of calcareous spar. The specific gravity of the specimen I tried, was 2,052. The mass effervesces with acids, but the nodules of indurated clay do not.
This rock passes gradually to a bituminous shale. The first transition is to a slate-clay of a lighter or darker colour, the joints of which are covered with iron pyrites. It burns with a yellowish flame, giving out a sulphurous smell, and becomes afterwards of a light grey colour. The second transition is to a bituminous shale called Stony-coal, the spec. grav. of which is 1,319. Its colour is dark-brown without any lustre: it effervesces slightly with acids, contains no iron pyrites and burns readily with a yellowish rather smoky and heavy flame. The smell is bituminous but not sulphurous. The top and bottom of the Kimeridge coal consist of the slate-clay first described. At little Kimeridge on the shore, I saw several large blocks of a very hard, compact brown limestone, having a conchoidal fracture, and displaying a few specks of indurated clay: it had an external covering of a grey earthy colour, owing no doubt to the oxidation of the iron, though the rock itself does not act in any sensible manner on the magnet. This limestone has a superficial resemblance to iron-stone and to some basaltic rocks; passages however might be traced to the slate-clay just mentioned, with which perhaps it alternates. I found its spec. grav. 2,641. It was not in situ, but comes very probably only from a little distance.
On taking a general view of the district here described, we shall find that of the two islands which it comprehends, one, the Isle of Wight, has its greatest dimension from east to west, while the other, the Isle of Portland, extends longitudinally from north to south, or nearly so: that in the Isle of Wight, the shelving of the land, independently of the particular slopes of the hills, is from south to north, as is clearly shewn by the rise of the Medina river, and by the elevation of Niton, one of the most southern villages, situated on the back of St. Carherine's-hill. In the Isle of Portland, on the contrary, we have an uninterrupted plain, with a gradual and uniform slope from north to south, of nearly four hundred feet in a distance of five miles. No river, that I am aware of, waters the Isle of Portland, but the inhabitants are plentifully supplied with fresh water, by two very line and abundant springs.
In the Isle of Wight, the tract of land to the north of the ridge of chalk, from Newport to Cowes, is but little elevated, being indeed almost flat: in Portland, the abruptness of the cliffs on all sides is rather considerable, and as far as I have been able to judge, pretty nearly the same, somewhat less perhaps to the south.
St. Aldham's Head, the most projecting part of the peninsula of Purbeck, lies exactly on the same parallel as the southern part of the Isle of Wight, and both belong to the same formation, as does also the Isle of Portland, which projects still further to the south.
In following that part of the coast step by step, the attention is strongly drawn towards the considerable wearing away of the land and of the solid strata which is daily taking place. Between Rockenend and Blackgang Chine on the S.S.W. coast of the Isle of Wight, a land slip happened in 1799, the fragments of which cover a space of near half a mile in diameter.
As we walk along the cliffs, we see every where the surface of the soil rent by deep fissures; but a circumstance particularly remarkable is, that in this district, the decay seems to begin with the upper strata, which are gradually removed in succession. At Freshwatergate and at the Needles, those standing pyramids of chalk present us with a striking illustration of this supposed mode of decay. At the furthest extremity of High Down below the light house, there is a gap now interrupting in its upper part the continuity of that projecting tongue of chalk. Without attempting to estimate within how many years such a portion of the solid strata will be completely broken asunder, and make a Needle by itself, no one will deny that it is one step towards such an event. Conformably with this and other similar appearances, in attempting to account for the separation of the Isle of Wight from the opposite coast of Hampshire, I should be more disposed to ascribe it to the continued action of causes, the effects of which we may ascertain and even almost calculate every day, than to a sudden subsiding of the strata at a period of time far beyond the reach of all historical monuments.
If the chalk hills constitute ridges separated by low troughs or vales which have been filled up by alluvial depositions, may not the bottom of one of these vales have once existed in the space now occupied by the Southampton channel; and thus the separation of the Isle of Wight have taken place in consequence of an encroachment of the sea on a portion of little elevated land, the loose materials of which besides, could have presented but a feeble barrier to the repeated assaults of the sea?
May not that narrow arm of the sea, which from Cowes-harbour extends four miles inland to Newport, and which is improperly called Medina river, be owing to a cause of that kind; as also that other still deeper arm of the sea which from Yarmouth runs to Freshwater-gate, and makes almost a complete island of that portion of the land which lies westward?
The shape of this channel, and its slanting declivity on both sides, affords also a further presumption of the truth of this hypothesis.
That I have not gone beyond the warrant of the facts in admitting such a disposition, as has been described, of chalk-hills with vales between them, may, I think, be clearly demonstrated from actual observations.
In Kent and Sussex are two ranges of chalk hills, the north and south downs, with an alluvial vale between them. In Hampshire, near Alresford, where the chalk begins to crop out, we pass over a ridge of this rock in a transverse direction to its length; till near Otterborne, on the southern slope of that same ridge, we lose the chalk, because, the country lowering, we enter into an alluvial basin; in which lies Southampton, and which probably extends as far as to Carisbrook a little beyond Newport, in the Isle of Wight. There we meet with the belt of chalk hills before mentioned, and again on the southern slope we enter at Shorwell, a sandy basin, till we come no the southern coast of the isle which is chiefly composed of calcareous sandstone, chert, and coarse shelly limestone.
We shall find that the same arrangement prevails about Corfe Castle, and there indeed we may fairly say that the shallow inner harbour of Poole lies in the bottom of the trough of sand which rests on the acclivities of Corfe Castle chalk-hills. Were the sea to force itself a passage somewhere between Lulworth and Wareham, (situated at the head of Poole harbour) would not then the Isle of Purbeck improperly called so now, become a true island? and would not then its formation be owing to a cause exactly like that which I have ventured to suppose, has formed the present Isle of Wight?
I shall close these observations by saying, that if we take a comprehensive view of the southern counties of England, from the east of Kent to the Land's End, we may safely assert, that there are very few countries which, within such limits, can boast of so varied and regular a succession of rocks, from those which are reckoned by most geologists to be of the latest formation, to those which belong to the oldest. Elevation of some places in the Isle of Wight, and the County of Dorset.
|By the Barometer, Height above the level of the sea||From the trigonometrical Survey.|
|Isle of Wight||Feet|
|St. Catherine's Sea-Mark, or Signal-house||750|
|Alum Chalybeate Spring, below St. Catherine's, S.S.W. Coast||130|
|Shanklin-Hill, S.S.E. the chalk begins to crop out at Steep-hill||769|
|Bembridge Down, E.S.E||400 from Sir H. Englefield. Vide. vol vi. of the Linn. Trans.|
|Motteston or Mottiston Downs W.S.W.||698|
|The furthest and most western part of High Down above the Needles||189|
|Isle of Purbeck|
|A hillock on Poole heayh, half a mille W. by S. of the town of Poole; and S.W. by S. of branksea Castle||102|
|Nine barrow Down W. of Studland||625||642, reckoning the level of the sea from low-water mark. Vide Maton's Obs. on the West Count. vol i. p. 18.|
|Corfe-Castle (village) Ship-inn||105|
|The hill on which Corfe castle stands||207|
|Challer hill, E. of Corfe castle hummock||390|
|Norden hill, W. of Corfe-castle hummock||369|
|St. Adhelm's, St. Alban's or St. Haldham's head||669|
|Swyer hill (top) W.N.W. of Encombe||669|
|Isle of Portland|
|Signal-house, a little to the N.N.E. of the village of Chesil||458|
|Southwell (village) S.S.W. of the island||99|
- In the S.S.W. coast of the Isle of Wight, below St. Catherine's Sea-mark, the dip of the strata is E.N.E. In the southern coast called the Under Ulm the dip is N by N.E. in Freshwater Bay N. by N.E. towards the Needles at High Down, N.E. in Hampshire, near Alresford, S.S.E. at Kimeridge Bay, and generally along the coast of the Isle at Purbeck, E.N.E. in the Isle of Portland, E. by S.E.
- It would be interesting to trace the characters of this rock more distinctly, and especially to ascertain whether it inclosed fossils which are not to be found in the common and soft variety. Chaux écrivante of the French.
- Elements of Mineralogy, vol. i. p. 130.
- Brochant, Traité de Minéralogie, tome i. p. 325.
- Kirwan's Mineralogy, vol. ii. p. 61.
- Ditto vol. ii. p. 57.
- Though the words flint and chert are pretty often used indiscriminately, I do not by any means consider them as synonymous: chert, I believe, is a kind of hornstone, the fracture of which is between scaly and flat conchoidal: it has a somewhat drier aspect, and is more generally of a greyish colour, or variegated white and brown: such are some of the characters clearly made out by Mr. Kirwan. Elements of Miner. vol. i. p. 303. There is besides, I think, a geological character, viz. that chert is not generally to be found in distinct globular masses as flint is, but rather in continuous layers, separating thicker strata of rocks.
- Chaux carbonatée grossière; pierre à batir ; pierre de taille when in large blocks, and moëllon, when it is in smaller masses. Brongniart, Traité de Minéralogie, tom. I. p. 204.
- Traité élémentaire de Minéral. tome i. p. 208.
- Traité élémentaire de Minéral. tome i. p. 206.
- Traité élémentaire de Minéral. tome i. p. 315.
Since the publication of his Traité de Minéralogie, M. Bronguiart, with the able assistance of M. Cuvier, has given the following more pointed characters—“ Les pierres siliceuses du terrain d'eau douce sont tantôt un silex pyromaque pur et transparent; tantôt un silex a cassure résineuse, transparent ou opaque; tantôt un silex opaque à cassure terne, largement couchoide et semblable à celle dnujaspe; tantôt c'est un silex carié, opaque, à cassure terne et droite qui a tous les caractères de la meuliére proprement dite, mais qui est géneralement plus compacte que la meuliere sans coquille; tantôt enfin c'est on vrai grès à grains plus on moins fins, disposés en rognons ou en couches minces.”
“ Les caractères de ces pierres siliceuses ne diffèrent donc pas de ceux qu'on leur connoit ; leur origine n'est décelée que par celle du terrain au milieu duquel il se trouvent, ou par les coquilles qu'ils renferment.”
“ Mémoire sur des terrains qui paroissent avoir été formés sous l'eau douce ;” p. 3, Paris, Juillet, 1810.
“ By the term terrain d'eau douce, Messrs. Cuvier and Brongniart understand those mineral beds which, according to their theory, were deposited by fresh-water lakes (in closing the vegetables and animals that inhabited their banks or waters), at the same period that marbles and schists were formed by depositions from the sea. Is the calcaire grossier, than, anterior to or coeval with the terrains d'eau douce?”
- Traité élement. do Mineral. tome i. p. 203.
- The Kimeridge coal is used as fuel by the poor people in the neighbourhood, and the ashes spread over the meadows, are considered as a good manure.
- It may not be amiss perhaps to observe, that in the Isle of Wight, and along the coast of Hampshire, the rivulets which empty themselves into the sea, go by the name of china, which is synonymous with gully, and the beds of those streams or brooks widen very much at their approach towards the sea.
- One of them, called Fortune's well, is situated a little above the village of Chesil, on the way to the signal-house. The other spring is in the south of the isle, at the village of Southwell, a name derived no doubt from the spring.
- “ The ebb, at low water, between the coast of Hampshire and the Isle of Wight; runs so strong that it shoots into Poole harbour, (which lies in the line of its course) so that when it is low water at Hurst-castle, it is high water here.” Maton's Obs. on the West. Count. vol. i. p. 28.