Royal Geological Society of Cornwall

Volume 3 paper 1

I.-On the Changes which appear to have taken place in the primitive form of the Cornish Peninsula.

Bv JOHN HAWKINS, Esq. F. R. S., &c.

HONORARY MEMBER OF THE SOCIETY.

The theory of the formation of the earth, is a subject, which few men, however conversant with the laws of nature, or the phenomena of the universe, can approach without a sense of its insuperable difficulties. The labours of another age perhaps are required to prepare us for such an undertaking; those of the present I believe, must be confined to observation and arrangement.

Of the difficulties which successively present themselves in the contemplation of this subject, not one of the least is that, which arises from the inequalities of the earth's surface. We can trace the order, as far as it goes, in which the strata are disposed over each other, but there is none observable in their local distribution ; nor does there appear to be any balance in the relative masses of sea and of land, or between the higher and the lower portions of the latter, or in short any thing like a system of compensation.

If some slight indications of analogy can be pointed out in the direction of a few of the highest chains of mountains, they disappear in all the others; and we look in vain for the result of some regular law, that is universal in its operation. To add to our perplexity, we are told, that no small portion of the surface of our globe is formed by its own ruins.

Doubtless, the present form of the earth's surface, is not such as could have been the simple result of its original formation; for it bears every where the marks of great revolutions, the investigation of which is one of the principal objects of that new science, which is known under the name of Geognosy.

Even the lapse of ages in the present constitution of our globe, must have occasioned great changes; the effect of climate being manifest every where, in smoothing the rugged deformities, and in reducing the more elevated points of the earth's surface to a common level; while changes of an opposite character are taking place in those countries which are subject to volcanic eruptions. These last however are confined to a few particular situations, and are chiefly interesting, in as much as they point out the agency of fire in the formation of countries, where it is now either dormant or extinct.

It is obvious that the present physiognomy of the earth's surface, is in some measure the result of these secondary changes ; nor can we attempt so difficult a problem as the theory of the earth, without first entering into a consideration of their extent and importance. To what degree they have influenced the form of this peninsula in particular, I shall have occasion to enquire, when I have first called your attention to changes of greater magnitude which have preceded them, and which have evidently been produced by other causes.

Perhaps there is no country where the traces of these great revolutions are more conspicuous. It will be sufficient for me to notice the fact, that all the vallies of this county have been raised very considerably, by the accumulated ruins of the strata which compose the higher and distant levels. Nor is it possible for us to misunderstand the language of such records.-They point out those convulsions of the solid strata of the earth, and the powerful agency of those floods, which at a period antecedent either to history or tradition, have so materially altered the primitive aspect of this country.

To no other causes can we assign the formation of that vast mass of alluvial matter which is spread over our vallies; for there is nothing in the scriptural account of the Deluge, which can justify us in supposing that it was capable of producing so violent a change as we here contemplate; and every attempt to explain it by the ordinary operations of nature, or by causes connected with the present constitution of our atmosphere, must be fruitless ; successive depositions made under such circumstances, being wholly inadequate to account for the uniform occurrence of our stream tin at the lowest level of these depositions, and in immediate contact with the naked rock.

On the other hand, a great volume of water rushing down an inclined plane, with a force concentrated by its passage through narrow vallies, may well be conceived capable of transporting a great quantity of disintegrated rock to a considerable distance ; and in fact, all our observations tend to prove that in this instance, such has been the mode of its transportation.

To the same period in the history of the earth, but to a very different origin, we must refer I believe, those deep beds of decomposed felspar and granular quartz, which we find deposited at the foot of our moorstone hills. The most remarkable example of these, which is well known under the name of the porcelain-clay deposit, is situated in the parish of St. Stephens. The whole mass of this clay, if it may be so termed, is evidently derived from a very peculiar sort of Granite which is contiguous to it. The change which we here contemplate could neither have been the result of the slow atmospherical action which is constantly taking place on the surface of the earth, nor of any violent catastrophe ; but must have been produced by a strong solvent power, of transient existence. Nor could the fluid in which the particles of felspar were suspended, have had any rapidity of motion, for the subsidence of these particles seems to have been quick and undisturbed.

A different sort of clay occurs in the vales of the slate district, and more generally in the hollows which lie at the head of these vales; but this seems to have originated in the same way as the other, that is to say, from a solution or decomposition of the parent rock. It covers whatever portion of alluvial tin occurs in these situations.

But it would be imprudent to proceed farther in this enquiry, before we have disposed of a question of greater difficulty,-the origin of our vallies!

Admitting that the alluvial matter which covers these vallies, is chiefly the result of such a catastrophe as I have alluded to, the solution of the problem before us will mainly depend upon the mass of the matter so deposited. Now the cubic contents of the transported mineral, after a due allowance has been made for the escape of a portion of it into the sea, will be found to be by no means commensurate with the vacant space from which it has been removed: in other words, this mass of alluvium does not correspond in any instance with the size or capacity of the valley above, it.

Our vallies therefore cannot be said in a general way to owe their origin to the rupture and removal of the strata, although, they may have been deepened and enlarged by these causes. Nor is it probable, when we reflect on the diversity of the materials which compose the solid crust of the earth, that the primitive surface of this country could have been otherwise than uneven. It may have been furrowed with primitive vallies of some extent, and the form of those countries through which at this day, no perennial streams are running, fully justifies this supposition.

I have always been of opinion, that the very uneven surface of some portion of the slate district in Cornwall and Devon, was connected in no small degree with its interior structure; having constantly observed that those parts of the country which were most uneven, were those of which the slate beds were the most capricious both in their direction and inclination. They are more particularly distinguished by this conformity between their outward form and their interior structure, the farther they recede from the central hills of moorstone, and hence it is perhaps, that in some maritime parishes, there is hardly a rood of level ground.

But however uneven the primitive face of this country may have been, we have no reason to suppose that it presented an abrupt or rugged aspect; for the rocky precipices which we observe on the sides of many of our moorstone vallies, and the tors, or piles of rocks which occur here and there on the surface of the hills, are evidently the result of some revolutionary violence. They could have been produced by no cause which we can reconcile with the ordinary course of nature. As for the bold cliffs which bound the peninsula on three sides; these, although they probably owe their origin to the same early catastrophe, have nevertheless derived their present appearance from a power which is so regular in its action, as to produce, what may with more propriety be denominated decay, than destruction.

But even these cliffs which owe so much of their stupendous height to the incessant action to which they are exposed, rise no where to the level of the adjacent country. A considerable slope usually intervenes, the remnant of its original outline. This line is still preserved almost entire in many parts of the sea-coast, and in all our estuaries. The very low undulating country around Hayle, extending as far southward as the alluvial plain of the Mount's-bay, is particularly illustrative of the fact, to which I am drawing your attention. Whatever changes this district may have undergone, they are evidently not such as denote any revolutionary violence; and its present aspect may be conceived to represent not unfaithfully what it was at its formation. Let me here add the remark, that our principal vallies usually terminate in these estuaries; and it is difficult to draw any distinction between their geological characters; both evidently deriving their form from a cause wholly different from revolutionary violence or gradual abrasion. Nor do the more level and upland parts of this country appear to have suffered any material degradation. Were there no other reasons for such an inference, the bed of quartz which is generally found in the killas- districts immediately beneath the vegetable mould, and unquestionably in situ, would alone be conclusive.

Having marked the distinction which it is so necessary for us to admit, in the nature of those changes which have taken place in the surface of this country; and having explained more fully those which may be referred to a very early period; I shall now proceed to the consideration of what may be called the secondary changes, most, if not all of which, are still in a state of progression.

These secondary changes are the natural result of the present constitution of our atmosphere ; and they are consequently familiar to our notice. A changeable temperature, constant humidity, the combined forces of wind and rain, the electric fluid, the action of violent floods, even the silent force of vegetation, and lastly the heavy swell and the boisterous surge of the Atlantic, have evidently wrought great changes in the form of our Peninsula.

The naked summits of our moorstone hills bear witness to the influence of some of the causes here enumerated; nor can the solid rampart of rocks which is opposed to the Western Ocean, wholly resist the constant impulse of its waves. In both however the change is gradual. First, the most exposed parts of the strata feel the action of the elements, they become disjointed, next insulated, and at length removed ; while the least consolidated undergo dissolution. Under such circumstances, the natural fissures of the rock, and the metallic veins, and slides, and heaves, which are of such frequent occurrence in this country, greatly contribute to their subversion.

In length of time, the cliffs so exposed to the fury of the waves are undermined; and not unfrequently immense masses of rock are detached by their own weight from the higher levels. The long reaches of rocks which are exposed to our view at low water, mark the extent of these depredations, and the rocky islets which range along the coast, must be regarded as the wrecks of some advanced portions of high land. Even the Mount, that noble ornament of our coast, must be placed, by the geologist, in this class of phenomena.

If the age of man be too short to measure the decay of that portion of this country, which is exposed to the combined force of the elements; much less is it able to form an estimate of that which is silently taking place on its surface; for it is so slow as to be almost imperceptible. It is natural to infer that things must have been always in this state: but although as far as our climate is concerned, the supposition may not be ill founded; the descent of the waters alone will in a great measure remove this difficulty, it being easy to perceive that at a remote period in the history of the world, their descent must have been infinitely more rapid, insomuch as to effect very great changes in the face of the country. The rapidity however diminishing as the bed of every stream became more deeply worn and enlarged, these changes must have gradually ceased to be of any importance.

Here then is a power, the action of which on the surface of the earth, as well as its capability of producing great changes on it, cannot be disputed. Of the extent to which this power has been carried in the excavation of our smaller vallies, there is a proof I think which rests upon a fact, to which I shall now call your attention.

It may not be very generally known that alluvial tin-stone, and alluvial gravel, occur in small quantities even on the surface of our hills, as may be seen exemplified on St. Agnes beacon; and perhaps there is no portion of upland ground within a certain distance from the mining district, where some minute traces of tin may not be found deposited in the hollows; a circumstance the more difficult to account for, as small vallies often intercept the communication with the country from which this tin must have been transported. We are thus compelled to admit the posterior formation of these smaller vallies, by the abrading power of running water, in the ordinary constitution of our atmosphere.

The same power aided by frosts and other causes, may have occasioned the enlargement of our vallies, which contain the great alluvial deposits of a remoter epoch; although it is not in our power to estimate the extent to which it has been carried.

Some notice ought to be taken in this place, of a very singular revolution which is taking place on the northern coast of Cornwall, where many thousands of acres have been covered with sea-sand. The particles of this sand have been wafted thither by the north-westerly winds which blow with such force on that side of the county, and this deluge of sand is said to be progressively extending. The medium of its transportation is supposed to be the salt-spray, which in stormy weather is known to be wafted to a great distance. The sand consists chiefly of finely comminuted shells, which when once deposited by these natural air-balloons on the higher grounds, are continually drifted to the leeward.

And here I would have closed the investigation of a subject which is both new and interesting, but by no means exhausted; had not some popular opinions, at variance with my own, rendered it necessary for me to examine the foundation upon which they rest.

There is a very general disposition among the inhabitants of the Mounts-bay and the Lands-end, to believe that the sea, within no very remote period, has made great encroachments. It has sweeped away, as they say, a great deal of land in the bay, and submerged a considerable tract of country between the Lands-end and the Scilly Islands. Trees that have been fished up from the sea bottom, are cited by way of proof of one of these supposed revolutions; and houses that are said to have been perceived there in calm weather, in confirmation of the other.

Even a regular survey of the date of Edward the First is appealed to, to shew that the whole county contained then twice the the number of acres, that are now found in it.

I have not been able to discover where any authentic copy of such a regular survey is to be found, or whether it be in existence; but the authorities for the two other facts are well known, and they will not bear the test of critical examination. They are in fact traditions that have been handed down to us by monkish writers.

These tales of wonder have nevertheless acquired a degree of popular credit from having been first quoted by so respectable a writer as Carew; and having circulated so long without contradiction, they are become engrafted as it were on the ideal stock of the country, and mixed up with its local prejudices. Antiquaries are too apt to fall in with this tide of popular feeling, and to foster such prejudices; geologists alone treat them with ridicule. Opinions however which are consecrated by their antiquity are entitled at least to a serious refutation.

Dismissing therefore the historical foundation upon which these opinions rest, as wholly untenable, or considering that in these instances as well as in so many others, the vague traditions of a monkish age have been substituted for authentic history, let us submit the question of this reported loss of land to the test of sober enquiry. To begin with that which is recorded of the Mount's-bay. What reason have we to infer from the natural circumstances of this country, that the sea has made here any encroachments? A submersion of any considerable tract of land, either here, or to the westward of the Lands-end, could only be effected by a rise of the sea-level; a phenomenon which must have been noticed not merely in one spot but in every part of our hemisphere.

As for the discovery of the submarine forest, which is brought forward in support of this popular notion of the loss of land, the geologist can draw no other conclusion than that the catastrophe to which it owes its submersion, must have preceded the period of authentic history. It must be referred in fact, to a period in the natural history of our globe, when the constitution of our atmosphere was still unsettled.

Fossil trees have been found, in various other parts of the coast of England, exposed to view by extraordinary low tides, but unfortunately for the hypothesis of our antiquaries, they have been observed to belong generally to the fir tribe, which are not the natives of this country.

Those trifling alterations which are occasionally made by the concurrence of high tides and strong gales of wind on the sandy beach at the head of the bay, prove neither one thing nor another; and it would be contrary to al! analogy to suppose that they do not rather add to the accumulation of dry sand, than diminish it. I presume that it will be unnecessary for me, after what has been here said, to take any farther notice of the sunken or submerged land, known under the name of Lionessa.

I have relied chiefly upon geological arguments for the refutation of these popular opinions, but historical ones are not wanting. There might be opposed to these vague traditions, which have so long been current among us, the authority of a Greek writer which is quoted by Diodorus; from which we learn that the circumstances which characterize the situation of the Mount were, about 2,000 years ago, not materially different from what they are at present. It was a Peninsula then, as it is now, and was connected only with the main land at low water.