Royal Geological Society of Cornwall

Volume 3 paper 2

II.-Some Observations on the Alluvial Formations of the Western Part of Cornwall.

By HENRY S. BOASE, M. D.

SECRETARY OF THE SOCIETY. .

(Read October, 1824.)

By the term Alluvial Formations, as used in its most general acceptation, is understood all those loose and unconsolidated layers which invest the Primary and Secondary rocks. These deposits have lately occupied the attention of the most distinguished Geologists; and their investigations have been rewarded by many curious and important discoveries.

The Alluvia of Cornwall, however, with the exception of some few cursory notices concerning the Stream Tin Works, have been hitherto almost totally neglected. They seldom exceed a hundred feet in depth, and are not therefore so extensive as those of some secondary districts in this country; neither do they abound in so great a variety of animal and vegetable remains; they are not, however, devoid of geological interest, and in an economical point of view they are very important, as affording siliceous and calcareous sands for agricultural purposes, a great variety of clays for the potteries and china manufactories, and lastly as the source of grain tin so indispensable in dyeing and some other arts.

It is not, however, my intention at present to describe all these alluvial deposits, but to confine my remarks to those which have been produced by causes now in action. These are the postdiluvia of Professor Jameson. The diluvia of the same author, or those which have resulted from causes no longer in action will form the subjects of future communications. Alluvion, as defined by Professor Buckland [Geological Traniactioni, vol. 5, page 533], does not embrace the untransported postdiluvium: the term alluvion is certainly more correct as it does not convey a meaning contrary to that of its derivation, but it is defective inasmuch as it does not include those untransported beds of earthy debris which are now in a state of actual formation. The nomenclature of this department of geology is very defective; which is much to be regretted, as a good

terminology is absolutely requisite for intelligible and accurate descriptions.

On taking a bird's-eye view of the western part of Cornwall, a range of hills will be observed stretching from north-east to southwest, more elevated than the surrounding country, and considerably inclined on its northern and southern slopes which in some places terminate in sea cliffs, but more generally they disappear under the incumbent lateral hills that stretch out their numerous undulating arms at various angles; sometimes, extending in the form of bold headlands into the ocean, and at others ending abruptly by uniting with one another. The sides of these hills descending towards each other form narrow and deep, or more extended and level vallies. Most of these rallies communicate with some outlet by which they are drained, and the point at which the water disgorges itself into the sea is usually a narrow defile. In some few upland vallies the rain water has no other escape than by infiltration and evaporation; consequently their surface is marshy in wet weather, and a boggy moor- ground in dry seasons: but the superficies of the adjoining hills is in no instance so extensive as to collect water sufficient for the formation of a lake.

The central chain of hills is formed of granite,and the lateral branches of clay-slate, and various other schistose rocks. The summits of the latter are rounded and covered by a luxuriant and cultivated vegetation, indicative of a deep and good soil: those of the former, on the contrary, have a sterile aspect being but sparingly covered with heath, furze, and other wild plants ; and from beneath the meagre soil huge masses of naked rock protrude themselves, by which the gloominess of the scene is considerably increased. These granitic masses, generally crown the highest and steepest summits, and are often so piled upon each other as to assume the appearance of dilapidated castles and other fantastic forms. Insulated rounded blocks of the same rock lie scattered on the slope beneath, and as they approach the slate formation they become fewer and more widely dispersed.

Wherever the granite is exposed, whether in situ, or in detatched boulders, the angles of the masses are rounded off', and the upper surfaces are generally hollowed out into a basin-like form. These concavities are rough, owing to the projection of angular pieces of quartz; particles of the same mineral also lie loose at the bottom in the form of coarse sand. The whole of the surface of this rock has a dull earthy appearance, even the large crystals in the porphyritic varieties having lost their characteristic lustre. Nor is this change superficial only, it extends into the body of the rock in proportion as the granite is more or less susceptible of decomposition. Hence the difficulty of obtaining perfect specimens of this rock, except in quarries and similar excavations. And yet notwithstanding this weathering, some kinds of granite, and more particularly their boulders, still retain a considerable degree of tenacity: instances of this occur at Lamorna Cove, and at Kerris in the parish of Paul, where the parent rock has a close firm texture, but the boulders are so much harder that they are with difficulty broken by a hand-hammer.

This change in the composition of granite is one of the primary sources of earth or soil, which at first is only sufficient for the support of lichens, and some others of the cryptogamous class of vegetables: but after these have flourished for a while, if circumstances be favorable, the soil will increase in depth, a higher order of plants will succeed, and the rocks will be completely hid from view. To the action of the atmosphere must be attributed this decomposition and disintegration of the solid rock: but if this destructive process continue in operation after the accumulation of an earthy layer clothed with vegetation, it cannot then be ascribed to the same agency.

Professor Jameson to his description of Mountain Alluvia [Jameson's Manual of Mineralogy, page 424.] adds, " this bed, particularly when covered with vegetable matter, forms a coat which long protects the solid strata from the wasting influence of the weather." In the introduction to that excellent work, Outlines of the Geology of England and Wales, are observations to the same effect, when treating of those changes which are daily taking place under the operation of existing causes :- " And where circumstances are favorable to a greater degree of waste, still there is often a tendency to approach a maximum at which farther waste will be checked; the abrupt cliff will at last become a slope, and that slope become defended by a grassy coat of proof"[ Page xxxii] ' The progress of the disintegration of rocks,' observes Dr. Kidd, 'in many instances, may ' be arrested by means of vegetation [Kidd's Geological Essay, p. 187 ].' And lastly Mr. Bakewell remarks that " The diminution of rocks is constantly taking place by the incessant operation of the elements, until the loftiest eminences are reduced and covered with soil and vegetables, which protect them from further decay [Bakewell's Geology, p. 230.]." Many other authors might be adduced to shew that it is the general opinion that a soil covered with vegetation, by excluding the action of the atmosphere, arrests the decomposition of the subjacent rock. This, however, does not appear to be the case, at least in Cornwall: for on examining our granite at various elevations, it will be found that so far from being defended by such a covering, it is generally disintegrated to a much greater degree in vallies than on the sides of kills, and on the latter than on their summits: the rock therefore is most altered in those situations where the untransported postdiluvium is deepest, and consequently where the influence of the atmosphere is most perfectly excluded.

The nature and depth of granitic postdiluvium vary according to the kind of granite and its position. On the summits and steeply inclined sides of hills the soil is shallow, and consists for the most part of quartzose particles, the felspar and mica in such situations are entirely wanting. Advancing, however, towards the vallies, the argillaceous particles arising from the decomposition of the felspar and mica become more abundant, and the soil increases in depth. The tendency of the granite to undergo a chemical change also proportionally modifies the extent of this layer of earthy debris. On removing this perfectly decomposed stratum, the constituents of the granite are found in their original position ; but the felspar, although it still retains its form, is notwithstanding so much changed, that the whole mass can be easily dug through with a spade. This decay of granite is very extensive, particularly in those localities which afford the china clay. A fine example is exhibited in the upper part of Carclase, an open mine near St. Austle; and indeed the same may be witnessed in most granitic quarries. On the St. Just road, about three miles from Penzance, a very illustrative instance on a small scale also occurs, where this gravelly mass has been hollowed out into a large cavern. Similar excavations are to be frequently met with, as this untransported granitic postdiluvium forms an excellent material for laying on the surface of newly made roads. As the solid rock is approached, the felspar becomes less altered ; so that there is a gradual transition from the perfect rock, to the loose granitic soil. Such is the general appearance of the untransported postdiluvium in the granitic districts of this County: and the various kinds of clay-slate and horn blend rocks have undergone similar changes. At Lariggan in this neighbourhood, masses of greenstone occur naked above the soil in the form of tors or cairns. The schistose rocks seldom protrude, but are almost uniformly covered by a stratum of soil, except in precipices and sea cliffs, where all are equally exposed. These rocks are not at the present day so much exposed to the action of the atmosphere as granite; but when they are, decomposition also ensues, and likewise continues, when they are covered by soil and vegetation. On sinking a well in a slate district after passing several feet through clay, the latter becomes interspersed with fragments of decayed rock, corresponding in nature and position with the slate beneath; soon after these indications the rock itself is found, but so soft and decomposed as to yield to the spade, and for many fathoms beneath it may be worked with a pick. In short, as in the case of the granite, there is a regular gradation from the soil to the perfect rock. Examples of this species of decomposition in hornblende slate rocks, may be seen at Chyandour, and in Newlyn cliffs.

The only remaining postdiluvium is the alluvion properly so called, the fine mud and slimes which occur at the bottom of our rivers, and are accumulated at their mouths : but since all the rivers in this part of the county are very small, these deposits are confined to very narrow limits.

From these facts it appears, that rdcks are subject to a more rapid and extensive decay than that which is produced by the chemical and mechanical action of the atmosphere. Of these causes the chemical is by far the most efficient, and consists in the oxygen and carbonic acid gases of the air forming new compounds with one or more of the constituent parts of the rocks.

Frost is also in some cases an active agent, particularly in schistose rocks; the moisture between their laminae becoming frozen, expands and rends them asunder, producing thereby a talus of debris at the foot of the precipice, there to undergo a more speedy decomposition than in its former position.

All atmospheric agents, however, are thrown out of action when the rocks have acquired a coat of soil and vegetation, but a more powerful one is then called into operation. The rain water, which must have run off rapidly into rivers and other natural drains when the surface was a bare rock, is now absorbed, and percolating downwards penetrates through the solid strata.

It is well known that the air which rain-water holds in solution contains about thirty-five per cent of oxygen, but atmospheric air only twenty- one per cent; now the oxygen of the latter is applied only to the surface of the rock, and that too in a gaseous state; whereas that of the rain-water is in a condensed liquid form, and intimately brought into contact with the constituents of the rocks. It is not therefore to be wondered at, that in this case decomposition should proceed more rapidly and more extensively, since the same chemical agent is applied under more favourable circumstances. Besides, it is well known that stones when first raised from the quarry are moist, but by exposure to the air the water evaporates, and they become more indurated; this diminution of the cohesion of rocks by the presence of water also assists the chemical action of the oxygen.

It may have been often noticed, that those parts of stone crosses and other ancient remains which are buried in the ground, have become so decayed that they can be easily broken off, whilst the upper exposed parts have still a considerable degree of cohesion. The same thing may be observed in many gate-posts formed of the fine grained granite; these are indeed more recent, but being more easily decomposed afford examples equally instructive. The bases of most of our cairns and tors also illustrate the same fact " Much of the soil on the coast of Antrim," observes Mr. Brand, "is thus derived from " the decomposition of basalt which however in other cases singularly resists change as in Staffa, where the columns though exposed to the violence of the ocean, retain a sharp angularity and black colour. This difference depends upon the degree of induration of the basalt," I should be inclined rather to attribute its durability to the same cause which protects the tors and crosses; the position is unfavourable to the percolation of water through their substance, the water drains off rapidly, and the moisture left on the surface is dissipated by evaporation. The shallowness of the soil on the summits and sides of our granite hills must also be ascribed to their position. The water, owing to the great inclination of the hills, runs- down, carrying with it the light argillaceous particles of the decomposed felspar, which impart to the torrents and brooks of these districts a milky appearance. This gradual removal of the clay leaves the soil siliceous and sandy, and being badly adapted for the retention of water, the drainage is thereby facilitated, and consequently the decomposition of the subjacent rock is greatly interrupted in its progress.

The decomposition and disintegration of granite appears to arise principally from the action of oxygen and carbonic acid on the alcali of the felspar, but in clay-slate and hornblende rocks it is produced by the black oxide of iron, of which they always contain a portion, being converted into the peroxide by absorption of oxygen, and the colour of the soils resulting there from approaches more or less to red, according to the quantity of iron that enters into their composition. The greenstone rocks afford the reddest soil in the district, except that which is produced by a superficial cross-course of ironstone ; this occurrence, however, is very rare and partial. When the soil is very ferruginous it is generally shallow, and comparatively unfertile, for though some iron is favourable to vegetation, yet a large portion, like salt, appears to be pernicious.

On the examination of a piece of rock, it can be pretty certainly predicted what kind of soil would be produced by its decomposition; and rice versa, by the appearance of the ground alone it is not difficult to trace the line of junction of the granite with other rocks, because this untransported postdiluvium is in the place where it was originally produced. On this principle a geological map might be constructed; I know of but two objections to this method: 1st when transported alluvia, as mud, sand, and gravel, constitute the subsoil; and 2nd. when the granite soil overlaps that of the slate, which it doss even to the extent of a hundred yards or more, when the latter rock joins the former at- the foot of a steep hill, as at Carbrea near Redruth. Another interesting position of this untransported postdiluvium must not be omitted, viz, its occurrence as a substratum to diluvial sand and gravel.

This might a priori have been expected, still the fact is important as a confirmation of its origin from the action of rain-water; its interposition between the rock and the diluvian deposits is also a curious fact, as affording another exception to the doctrine, that the nearer the centre of the earth is approached, the older are the strata, whether they be rocks, or alluvia. This phenomenon may be seen at Hayle, in the sections made in forming a new road for the Causeway, where the calcareous sand rests on a clayey soil which gradually passes into the rock. Under the sand banks in Mount's-bay, below the sea level, also occurs clayslate postdiluvium. It may on the other hand, however, be urged that these examples are not conclusive, for at Carnon, Pentuan, and some other stream-works, the diluvium, containing the tin, rests on the solid rock: but in these instances it is possible that the numerous alternating beds of clay and vegetable remains, covering the rock to the depth of from 50 to 70 feet, may be sufficient to intercept the oxygen held in solution; or other circumstances may be found adequate to produce this effect on a more careful examination of these interesting deposits.

In favour of the recent origin of the beds of decomposed rocks in the above-mentioned instances at Hayle and Mount's-bay, it may also be stated that no vegetable fibres are to be found on their upper surface; now if they had existed previous to the transportation of the sand, a layer of vegetation would have clothed their surface, and consequently some traces of these remains would have been preserved. Again, at Kerris stream-work a decomposed granitic bed is interposed between the tin stratum and the granite, and on this bed, and accompanying the tin-ore, are large rounded masses of the same rock, many of which would weigh more than a ton. It may be presumed that a power equal to the transportation of tin- ore and these huge boulders would have entirely swept away any loose soil. We may therefore be justified in concluding that the tin stratum was deposited here originally on the solid rock, as in the eastern stream-works, and that the earthy bed at present intervening between them has been formed posterior to the diluvial deposit.

Such is the nature of the postdiluvia which now cover our rocks. It is difficult to make a calculation of their depth, for in every place it varies; but if the disintegrated rock be also included, it cannot average less than from 15 to 20 feet, nor is it easy to predict to what extent this mass will be increased; but since it is produced by causes still in operation, it will slowly but constantly accumulate, until its progress is arrested by a deluge, or some other convulsion of nature. If we consider the probable effects of an immense torrent of water sweeping over this district, it will be apparent that the whole of this earthy stratum would be carried away, and the rocks denuded; the masses of stone which crown the summits of the hills would be thrown down, and scattered on the hills beneath, in the form of boulders; the vallies would be deepened thereby, but the sides and summits of the hills would be less changed; all indeed would be affected only in proportion to the decomposition which they had previously undergone. In this manner it can be easily conceived that vallies of denudation have been formed; but it requires no little effort of the imagination to suppose that the waters of the deluge possessed such stupendous power, as to be able to tear up and sweep away the solid strata of our globe.

The vast mass of decayed and disintegrated rock thus swept away would sooner or later subside from the waters, and its various ingredients would be deposited according to their respective specific gravities, the heavier parts would occupy situations near their source, and the lighter would be transported to a greater distance. That this has been the case heretofore is exemplified in the position of the diluvia of the present day; for boulders of various rocks and tin-ore occur on the sides of the hills and in the vallies; beds of gravel and sand cover the lowest vallies, particularly in the vicinity of the sea; but the fine argillaceous particles appear to have been carried afar off in a state of suspension, and they have probably been deposited somewhere in the form of beds of clay, which, together with the beds of sand and gravel, may after the lapse of ages be consolidated into secondary rocks. This decomposition of rocks must not therefore be considered in the light of a destructive process; it is only one of those stages of transformation and emigration, of action and reaction, which characterise all the works of the creation.

Several rocks of the secondary class are also subject to similar changes, and more particularly those which contain the black oxide of iron, such as basalt, greenstone, and other hornblende rocks. This circumstance is worthy of remark, because several kinds of clay-slate, sand-stone, clay-iron-stone, and many others of this class, contain the red oxide, or peroxide, of iron,which, together with the water-worn appearance of their particles, and the imbedded organic remains, clearly point out their origin: but the presence of the black oxide of iron in basalt and greenstone, appears to indicate that they have not been formed from the debris produced by the decomposition of rocks at the surface of the earth. When therefore to this is added, their total want of conformity with the surrounding rocks, and their resemblance in structure and composition to the hornblende rocks of primitive districts, and to those of undoubted volcanic origin, there appears strong presumptive proof of their having been protruded from beneath. These rocks have already been the subject of much curious and interesting speculation, and are deserving of further investigation.

Before concluding, I must again revert to the necessity of a new nomenclature, and one too that should rather be descriptive of the objects themselves, than of their supposed origin, for what can be more unsatisfactory than the present terminology? In examining the Kerris stream-work, for example, before the earthy layer interposed between the rock and tin- stratum can receive a name, we must decide whether it be diluvial or postdiluvial: thus we are in a measure compelled in limine to pronounce on its nature and origin, a mode of proceeding, which in Geology, or indeed in any other science requiring patient and cautious investigations, cannot be too much avoided.