Volume I Part 24 (2/2)
Some nodules in chalk-beds consist of sh.e.l.ls of echini filled up with chalk, the animal having been dissolved away by putrescence in water, or eaten by other sea-insects; other sh.e.l.ls of echini, in which I suppose the animal's body remained, are converted into flint but still retain the form of the sh.e.l.l. Others, I suppose as above, being more completely fused, have become flint coloured by the animal flesh, but without the exact form either of the flesh or sh.e.l.l of the animal. Many of these are hollow within and lined with crystals, like the Scot's-pebbles above described; but as the colouring matter of animal bodies differs but little from each other compared with those of vegetables, these flints vary less in their colours than those above mentioned. At the same time as they cooled in concentric spheres like the Scot's-pebbles, they often possess faint rings of colours, and always break in conchoide forms like them.
This idea of the production of nodules of flint in chalk-beds is countenanced from the iron which generally appears as these flints become decomposed by the air; which by uniting with the iron in their composition reduces it from a vitrescent state to that of calx, and thus renders it visible. And secondly, by there being no appearance in chalk- beds of a string or pipe of siliceous matter connecting one nodule with another, which must have happened if the siliceous matter, or its acid, had been injected from without according to the idea of Dr. Hutton. And thirdly, because many of them have very large cavities at their centres, which should not have happened had they been formed by the injection of a material from without.
When sh.e.l.ls or chalk are thus converted from calcareous to siliceous matter by the flesh of the animal, the new flint being heavier than the sh.e.l.l or chalk occupies less s.p.a.ce than the materials it was produced from; this is the cause of frequent cavities within them, where the whole ma.s.s has not been completely fused and pressed together. In Derbys.h.i.+re there are ma.s.ses of coralloid and other sh.e.l.ls which have become siliceous, and are thus left with large vacuities sometimes within and sometimes on the outside of the remaining form of the sh.e.l.l, like the French millstones, and I suppose might serve the same purpose; the gravel of the Derwent is full of specimens of this kind.
Since writing the above I have received a very ingenious account of chalk-beds from Dr. MENISH of Chelmsford. He distinguishes chalk-beds into three kinds; such as have been raised from the sea with little disturbance of their strata, as the cliffs of Dover and Margate, which he terms _intire_ chalk. Another state of chalk is where it has suffered much derangement, as the banks of the Thames at Gravesend and Dartford.
And a third state where fragments of chalk have been rounded by water, which he terms _alluvial_ chalk. In the first of these situations of chalk he observes, that the flint lies in strata horizontally, generally in distinct nodules, but that he has observed two instances of solid plates or strata of flint, from an inch to two inches in thickness, interposed between the chalk-beds; one of these is in a chalk-bank by the road side at Berkhamstead, the other in a bank on the road from Chatham leading to Canterbury. Dr. Menish has further observed, that many of the echini are crushed in their form, and yet filled with flint, which has taken the form of the crushed sh.e.l.l, and that though many flint nodules are hollow, yet that in some echini the siliceum seems to have enlarged, as it pa.s.sed from a fluid to a solid state, as it swells out in a protuberance at the mouth and a.n.u.s of the sh.e.l.l, and that though these sh.e.l.ls are so filled with flint yet that in many places the sh.e.l.l itself remains calcareous. These strata of nodules and plates of flint seem to countenance their origin from the flesh of a stratum of animals which perished by some natural violence, and were buried in their sh.e.l.ls.
7. ANGLES OF SILICEOUS SAND.
In many rocks of siliceous sand the particles retain their angular form, and in some beds of loose sand, of which there is one of considerable purity a few yards beneath the marl at Normington about a mile south of Derby. Other siliceous sands have had their angles rounded off, like the pebbles in gravel-beds. These seem to owe their globular form to two causes; one to their attrition against each other, when they may for centuries have lain at the bottom of the sea, or of rivers; where they may have been progressively acc.u.mulated, and thus progressively at the same time rubbed upon each other by the das.h.i.+ng of the water, and where they would be more easily rolled over each other by their gravity being so much less than in air. This is evidently now going on in the river Derwent, for though there are no limestone rocks for ten or fifteen miles above Derby, yet a great part of the river-gravel at Derby consists of limestone nodules, whose angles are quite worn off in their descent down the stream.
There is however another cause which must have contributed to round the angles both of calcareous and siliceous fragments; and that is, their solubility in water; calcareous earth is perpetually found suspended in the waters which pa.s.s over it; and the earth of flints was observed by Bergman to be contained in water in the proportion of one grain to a gallon. Kirwan's Mineralogy, p. 107. In boiling water, however, it is soluble in much greater proportion, as appears from the siliceous earth sublimed in the distillation of fluor acid in gla.s.s vessels; and from the basons of calcedony which surrounded the jets of hot water near mount Heccla in Iceland. Troil on Iceland. It is probable most siliceous sands or pebbles have at some ages of the world been long exposed to aqueous steams raised by subterranean fires. And if fragments of stone were long immersed in a fluid menstrum, their angular parts would be first dissolved, on account of their greater surface.
Many beds of siliceous gravel are cemented together by a siliceous cement, and are called breccia; as the plumb-pudding stones of Hartfords.h.i.+re, and the walls of a subterraneous temple excavated by Mr.
Curzon, at Hagley near Rugely in Staffordfs.h.i.+re; these may have been exposed to great heat as they were immersed in water; which water under great pressure of superinc.u.mbent materials may have been rendered red- hot, as in Papin's digester; and have thus possessed powers of solution with which we are unacquainted.
8. BASALTES AND GRANITES.
Another source of siliceous stones is from the granite, or basaltes, or porphyries, which are of different hardnesses according to the materials of their composition, or to the fire they have undergone; such are the stones of Arthur's-hill near Edinburgh, of the Giant's Causway in Ireland, and of Charnwood Forest in Leicesters.h.i.+re; the uppermost stratum of which last seems to have been cracked either by its elevation, or by its hastily cooling after ignition by the contact of dews or snows, and thus breaks into angular fragments, such as the streets of London are paved with; or have had their angles rounded by attrition or by partial solution; and have thus formed the common paving stones or bowlers; as well as the gravel, which is often rolled into strata amid the siliceous sand-beds, which are either formed or collected in the sea.
In what manner such a ma.s.s of crystallized matter as the Giant's Causway and similar columns of basaltes, could have been raised without other volcanic appearances, may be a matter not easy to comprehend; but there is another power in nature besides that of expansile vapour which may have raised some materials which have previously been in igneous or aqueous solution; and that is the act of congelation. When the water in the experiments above related of Major Williams had by congelation thrown out the plugs from the bomb-sh.e.l.ls, a column of ice rose from the hole of the bomb six or eight inches high. Other bodies I suspect increase in bulk which crystallize in cooling, as iron and type-metal. I remember pouring eight or ten pounds of melted brimstone into a pot to cool and was surprized to see after a little time a part of the fluid beneath break a hole in the congealed crust above it, and gradually rise into a promontory several inches high; the basaltes has many marks of fusion and of crystallization and may thence, as well as many other kinds of rocks, as of spar, marble, petrosilex, jasper, &c. have been raised by the power of congelation, a power whose quant.i.ty has not yet been ascertained, and perhaps greater and more universal than that of vapours expanded by heat. These basaltic columns rise sometimes out of mountains of granite itself, as mentioned by Dr. Beddoes, (Phil.
Transact. Vol. Lx.x.x.) and as they seem to consist of similar materials more completely fused, there is still greater reason to believe them to have been elevated in the cooling or crystallization of the ma.s.s. See note XXIV.
NOTE XX.--CLAY.
_Whence ductile Clays in wide expansion spread, Soft as the Cygnet's down, their snow-white bed._
CANTO II. l. 277.
The philosophers, who have attended to the formation of the earth, have acknowledged two great agents in producing the various changes which the terraqueous globe has undergone, and these are water and fire. Some of them have perhaps ascribed too much to one of these great agents of nature, and some to the other. They have generally agreed that the stratification of materials could only be produced from sediments or precipitations, which were previously mixed or dissolved in the sea; and that whatever effects were produced by fire were performed afterwards.
There is however great difficulty in accounting for the universal stratification of the solid globe of the earth in this manner, since many of the materials, which appear in strata, could not have been suspended in water; as the nodules of flint in chalk-beds, the extensive beds of sh.e.l.ls, and lastly the strata of coal, clay, sand, and iron-ore, which in most coal-countries lie from five to seven times alternately stratified over each other, and none of them are soluble in water. Add to this if a solution of them or a mixture of them in water could be supposed, the cause of that solution must cease before a precipitation could commence.
1. The great ma.s.ses of lava, under the various names of granite, porphyry, toadstone, moor-stone, rag, and slate, which const.i.tute the old world, may have acquired the stratification, which some of them appear to possess, by their having been formed by successive eruptions of a fluid ma.s.s, which at different periods of antient time arose from volcanic shafts and covered each other, the surface of the interior ma.s.s of lava would cool and become solid before the superinc.u.mbent stratum was poured over it; to the same cause may be ascribed their different compositions and textures, which are scarcely the same in any two parts of the world.
2. The stratifications of the great ma.s.ses of limestone, which were produced from sea-sh.e.l.ls, seem to have been formed by the different times at which the innumerable sh.e.l.ls were produced and deposited. A colony of echini, or madrepores, or cornua ammonis, lived and perished in one period of time; in another a new colony of either similar or different sh.e.l.ls lived and died over the former ones, producing a stratum of more recent sh.e.l.l over a stratum of others which had began to petrify or to become marble; and thus from unknown depths to what are now the summits of mountains the limestone is disposed in strata of varying solidity and colour. These have afterwards undergone variety of changes by their solution and deposition from the water in which they were immersed, or from having been exposed to great heat under great pressure, according to the ingenious theory of Dr. Hutton. Edinb.
Transact. Vol. I. See Note XVI.
3. In most of the coal-countries of this island there are from five to seven beds of coal stratified with an equal number of beds, though of much greater thickness, of clay and sandstone, and occasionally of iron- ores. In what manner to account for the stratification of these materials seems to be a problem of greater difficulty. Philosophers have generally supposed that they have been arranged by the currents of the sea; but considering their insolubility in water, and their almost similar specific gravity, an acc.u.mulation of them in such distinct beds from this cause is altogether inconceiveable, though some coal-countries bear marks of having been at some time immersed beneath the waves and raised again by subterranean fires.
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