POSTSCRIPT.
I have to acknowledge my obligations to various gentlemen for assistance in the progress of this work, but especially to the following:
To Richard Taunton, Esq. M.D. for his loan of Mr Hals’s Manuscript.
To the late Right Honorable Francis Basset, Baron De Dunstanville, for his supplying me with Mr. Tonkin’s Manuscript.
To Henry S. Boase, Esq. M.D. for his most liberal communication of an abstract for each Parish, taken from an accurate and minute
Geological Survey of the whole County, made by himself.
To the Right Honorable Heneage Finch, Earl of Aylesford, for his allowing me the use of a Transcript of Mr. Hals’ Parochial History as it now exists, formerly belonging to the late Mr. Lysons.
To Gordon William Francis Gregor, Esq. for his supplying me with an emblazoned copy of the last Visitation of Cornwall by the Heralds in 1620.
To Sir Henry Ellis, K.H. F.R.S. Sec. S.A. for the communication of his MS. additions to Tanner’s Notitia Monastica for Cornwall.
To the Rev. Josiah Forshall, M.A. F.R.S. late Fellow of Exeter College, Oxford, and now Keeper of the Manuscripts in the British Museum, for his extracts from a very ancient Manuscript relative to the See of Bodmin.
I have to acknowledge, with my best thanks, the assistance of Thomas Hingston, Esq. M.D. afforded by his arrangement of Extracts relating to Cornwall, from the Itinerary of William of Worcester; and by an original communication on the Etymology of Names of Places within the county.[1]
The only existing Manuscript of Mr. Tonkin’s Work having remained for some years in the possession of Mr. Whitaker, he added to it various
notes and ilustrations, frequently interlined, or blended with the original writing, so as to render the task difficult, in many cases, to distinguish the one from the other. In almost every passage of any length, Mr. Whitaker’s additions are marked with a [W.]
Geology having pressed forward during the present century, at a pace unexampled in other sciences, may reasonably be expected to reach new discoveries in comparatively short intervals of time; these have been reduced, however, almost to instants in respect to Cornwall. After the very able, minute, and laborious investigation made by Doctor Boase, of every district, of every parish in the whole county, the work of discovery would seem to have been completed, at the least for several years; but Mr. de la Beche came soon afterwards into Cornwall, under the sanction of Government, assisted by officers of the Engineers employed on the great Trigonometrical Survey; and this eminent Geologist has, in consequence, been enabled to lay down the various main lodes, the cross courses, the elvans, &c. together with the junctions of granite, greenstone, and killas, with an accuracy and discrimination never before attained, nor ever approximated to, except by Mr. Richard Thomas, in his survey of the mining district, made about twenty years ago.
Mr. De la Beche has also been enabled to deduce several general laws observed in the direction of cleavages, in the dip of strata, in the heaves and slides of lodes, all of which will be detailed in an eagerly expected volume, together with a discovery
most unexpected. The saxa metallifera of Cornwall had always been supposed referable to a very remote period of geological epochs, far anterior to the age of organic remains; till this opinion became shaken by the discovery of shells, or of their impressions, in the hard schist rocks near Tintagell: others were subsequently found more to the south and west; till at last Mr. De la Beche has detected the remains of organized life adjacent to a productive Copper Lode.
Two other eminent geologists, whom it would be equally idle and presumptuous for me to praise, have established the fact of a formation in the northern parts of Cornwall and Devon, not less unexpected than the discovery just noticed.
Professor Sedgwick and Mr. Murchison having investigated the deep-seated rocks in Wales, and in the adjacent districts, have finally traced the carboniferous series under the Severn, and so far west as the level ridge of land, extending from near Launceston to the sea coast between St. Gennys and Botreaux Castle, along which plane the escarpment manifests itself in a very conspicuous manner.
While these discoveries may be considered as in progress, a Cornish gentleman, but one whose genius does honour to the nation, Mr. Robert Weare Fox, has deduced from galvanic action on metals, on their oxides, on their sulphurets, and on their saline solutions in water, the only theory that has yet accounted for the various phenomena observed in metallic lodes; and extending still further his investigations to the recently discovered connection
between electric energies and terrestrial magnetism, Mr. Fox has been enabled to give more than probable reasons for the extraordinary fact of some metals usually selecting, in all parts of the world, lodes or fissures running nearly east and west, and why other metals prefer rents at right angles to the former; and in respect to the fissures themselves, Mr. Fox has remarked appearances inducing him to believe that lodes of considerable breadth have not been formed by any one great and sudden rending of the earth; but that, in a manner similar to the rising or to the sinking of land, by the gradual action of causes now well known to exist, those clefts have been enlarged from time to time, and have as frequently received additional deposits, easily discriminated from each other.
Mr. Fox appears also to have settled beyond the possibility of doubt, the long-agitated question respecting the temperature of mines, by establishing a general relation between increases of heat and depth; although the ratio cannot be reduced to any definite formula, being liable to vary with the presence of more or less water, and with the different conducting power of rocks, since mines in granite and in killas differ by several degrees of heat at the same level: yet the increase corresponds so generally with greater descents into the earth, that elevation of temperature, and not the expense, nor the difficulty of exhausting water, appears likely to oppose the final limit to the progress of mines in depth.
In continuation of the same trains of reasoning
and of thought, Mr. Fox has been led to investigate the important elements of variation and dip of the magnetic needle; and in pursuing these inquiries, he has invented an instrument possessed of far greater accuracy than any one previously employed, and which at this moment is in actual use, through the enlightened liberality of our own and other governments, in various and distant portions of the globe.
Mr. Henwood is about to lay before the Public a Work containing the results of more extensive and scientific researches into the nature of Lodes and Fissures, than have perhaps been ever executed by any individual. Mr. Henwood is well known to geologists: I shall therefore only add what I think myself bound in duty to notice, that an original appointment in the Stanneries, and a subsequent promotion, have been bestowed on Mr. Henwood, through the medium of Her Majesty’s Duchy Officers, and principally of Sir George Harrison, in consideration of his scientific attainments, and of his desire to render these attainments available to the development of further inductions.
I have endeavoured to render the work cheap by adopting the octavo form, and by abstaining from all decoration, except a slight sketch of the Pitt Diamond, which by raising that family into an influential situation, has modified the fate of Europe in a degree impossible to have been contemplated
by the Regent Duke of Orleans, when he purchased that bauble at the expense of an hundred and thirty-five thousand pounds; and excepting also the Tomb of Archbishop Tregury, a view of Cotehele house, and the Seal of a Corporation, all of which had been previously cut on wood. I have abstained from further expense with the view of placing my work in the hands of as many persons in Cornwall as I possibly could, thereby diffusing the entertainment likely to arise from Local Anecdotes, from Provincial Occurrences, and from Historical Events, not of sufficient general importance for securing to themselves a place in national records. I have caused an ample Index to be prepared for the whole work; and among the Appendixes will be found an Index to the Survey of our most respected historian Mr. Carew; and I will add as a proof of my own disinterestedness, that I have engaged to leave with the Publishers all the profits, if any should arise, reserving to myself the much more probable alternative of sustaining the loss.
The concluding paragraph proves the least agreeable of my work. I am sorry to say, that the Typographical errors far exceed my expectation. I must entreat of all my readers to excuse them, and to correct the Text from the too extensive Tables of Errata. The want of early habit, dimness of sight, and absence from the Press, must be alleged on my behalf; perhaps the compositor may plead unusual names or terms, and subjects not rendered familiar by his ordinary practice.
Davies Gilbert, 1837.
[1] While this sheet is passing through the press, we have to lament the premature decease of Dr. Hingston, at Falmouth, on the 15th July 1837.
AN INTRODUCTION
TO
THE GEOLOGY OF CORNWALL.
By Dr. BOASE.
The geological notices inserted in this Work, are necessarily so brief and so unconnected, on account of the alphabetical arrangement of the parishes, that it is desirable that a few introductory remarks should be made that this defect may be in some measure obviated.
Cornwall does not possess a great variety of rock formations, being composed of primary and transition or intermediate groups, covered here and there with deposits of gravel, sand, and clay, which belong to the modern epoch. But this limited range of formations is more than compensated for by the great facilities which the geologist here enjoys in his investigations; the very extended line of cliffs which deeply indent the Cornish shores proffer numerous and instructive sections; and the vast mining operations have brought to light most important phenomena. Indeed, no country of equal extent can, in these combined advantages, be compared with Cornwall.
On a general view of this country, the surface exhibits two systems of valleys; one running parallel
with the central ridge, which is highest near Launceston, and gradually, but irregularly, declines till it terminates at the Land’s End, in cliffs about a hundred feet in height; the other intersecting the longitudinal valleys at right angles; and as all the intermediate hills are more or less rounded, the country has an undulating appearance in both directions, not unaptly compared to the waves of the sea. Through these two systems of valleys, the rivers flow, seeking outlets into the sea by the nearest continuous descent; sometimes they effect this along the longitudinal, at others through the transverse valleys, receiving tributary streams on either hand from the lateral valleys, or curved hollows which they intersect. But sometimes the river is diverted in its course more than once, in consequence of the concavity of a cross curve rising above its level—or by that of the other system descending below this point—by which irregularity the stream is compelled to flow along the more favourable drainage; thus the Camel rises about four miles from the sea, in the moors near Davidstow, and flows more than twenty-five before it empties itself into the sea at Padstow. In the hollows of these valleys the drainage is often sufficiently obstructed to produce marshy ground; but seldom of such extent as to deserve the name of a lake.
As regards the soil, on the high grounds it is frequently very shallow and barren; but in the valleys it is very productive, and here and there is well wooded, more particularly on the southern coast. On the northern coast, in the vicinity of the sea,
it is very sandy, owing to the light testaceous sand of the shore being carried inland by high winds; in this manner no inconsiderable tracts of fertile land have been devastated.
Let us now turn our attention to the internal or geological structure of Cornwall. The highest parts of the central ridge, already alluded to, are composed of granite, which occurs in the form of four large insulated patches, so disposed at nearly equal distances from each other as to resemble a chain of islands extending from Launceston to the Land’s End, that is in the direction of N.E. and S.W. On the same ridge, but rather parallel than continuous thereto, is the great granitic group of Dartmoor in Devon, the whole of which is sometimes called the Ocrynian Range.
The granitic patch of Dartmoor is by far the most extensive, being nearly twenty miles in diameter; that of Launceston is ten miles in length by six or seven in breadth at its widest part; and its most elevated hills, Rough-tor and Brown-Willy, do not much exceed 1,300 feet in height. The granitic rocks of this patch, like that of Dartmoor, are not much exposed by artificial excavations, so that their varieties cannot be easily examined; the weathered blocks, which on the summit and sides of the hills form tors and detached masses, consist of the hard or siliceous varieties of the common and fine-grained granites, such as have withstood the action of the elements. One of the most curious of these tors is the Cheese Wring, near Liskeard, a pile of single blocks, each being larger than the one immediately beneath. Proceeding
along the ridge towards the Land’s End, the next, or St. Austel patch of granite will be found, much less than the last, but more interesting both in a scientific and a commercial point of view. In addition to the kinds of granite already noticed, it contains beds of talcose granite, or protogine, which by its decomposition furnishes that valuable substance china clay or kaolin, many thousand tons of which are annually exported for the potteries. The third, or Redruth patch of granite, affords many varieties of this rock, and has been well explored by numerous mines which have been productive in both tin and copper ores, affording also to the mineralogist a great variety of rare specimens. But the fourth, or Land’s End granite, is by far the most important to the geologist, for the land becoming here very narrow, the sea has produced cliff-sections, both in the granite, and also at its point of junction with the slate, exhibiting many interesting phenomena. Among these, the veins of granite in the slate are beautifully displayed, and have long been a great attraction to geologists; but the modes in which these rocks meet and unite, are not less deserving of notice. But for a detailed description of these curious facts, the reader may be referred to my “Treatise on Primary Geology.”
Besides these four principal patches of granite, there are four others: 1st. that of Kitt Hill, near Callington; 2nd. that of Tregonning and Godolphin, near Helston; 3d. that of Cligga Point, near St. Agnes; and lastly, that of the celebrated St. Michael’s Mount. The two first are of some size,
and at the surface are both subdivided near the middle, by the overlapping of the slate: the other two are small, but very interesting to the geologist.
It is worthy of remark that the fertility of these granitic groups gradually increases as they diminish in elevation; and it is a curious but not surprising coincidence, that the number of parish churches thereon follows the same order:—thus on the eastern and most extensive tract of granite, near Launceston, there is only one church; on the next there are three; on the Redruth patch six; and on the Land’s End granite no less than nine, within a space considerably less than that of the eastern tract.
Each of these insulated groups of granite is surrounded by schistose rocks, the layers of which, on all sides, incline from the granite at various angles, from 20° to 40°. Although these groups are thus separated from each other by the slate at the surface, yet it is the general opinion that they gradually approach beneath, until they are all united into one and the same mass—the intermediate hollow spaces (the valleys, as it were, between the granitic mountains) being occupied by the slate. It might, however, be maintained that the granite is imbedded in the slate, in large rounded masses, which would also account for the former rock underlying the other, within the limits of mining operations: and such an opinion would derive some support from the fact, that small insulated masses of granite in the slate are not of unfrequent occurrence.
The slate formation consists of two very distinct groups, when the most characteristic rocks are alone regarded; but it is not easy to trace the boundary between them, as the contiguous rocks appear to pass gradually into each other. These groups have one character in common: viz. that they consist of several distinct kinds of rocks, each genus of which respectively may be subdivided into schistose and compact rocks; the latter are imbedded in the former—are more crystalline—and often contain clusters of their constituent minerals: so that the complicated composition of these rocks is made known by Nature’s own analysis.
The group next the granite is primary, the more remote one is transition, or as they are termed in the following pages the porphyritic and calcareous series of the slate formation. The series next the granite is characterised by its beds of porphyry, and by its abounding in veins of tin and copper ores; the other series by the frequent occurrence of calcareous spar and strata of limestone, with occasional organic remains, and by its being sparingly metalliferous, containing no tin-ore, but being productive of lead and antimony. It is within the last series that the magnesian or serpentine rocks occur, and which in the Lizard district are developed to a large extent, forming one of the most interesting geological features of Cornwall. Those who are desirous of more information concerning this slate formation, will find a copious account in the fourth volume of the Transactions of the Geological Society of Cornwall.
It is there proposed to give these rocks, whenever they possess well-marked characters, distinct names, and not to refer a great variety of rocks to the vague and indefinite genera of clay-slate and greywacké. An accurate and more extended nomenclature would have rendered the following notices much more intelligible.
In describing the general features of Cornwall, we must not omit to mention the metalliferous veins—the great source of its commercial prosperity—and the channel through which much curious information has been obtained, concerning the structure of the earth. These veins traverse indifferently both the granite and the slate, but are most abundant in the latter rock, in the vicinity of the granite. The general direction of the tin and copper veins (or lodes, as they are provincially called) is nearly E.N.E. and W.S.W., and they are crossed by another system of veins, nearly at right angles, which are not commonly metalliferous, and when they do contain ores these are often of lead, antimony, silver, and other metals. This is not however a general rule, for in the parish of St. Just, Penwith, the tin and copper ores occur in the cross veins. The course of the veins is not straight, but they are always more or less undulating, both in their direction and in their dip or underlie. Various interesting phenomena result from their meeting with and intersecting each other, known to the miner under the names of heaves, throws, and slides. This subject is replete with curious facts, but would require considerable space for their enumeration; the reader must
therefore be again referred to the work already alluded to on “Primary Geology.”
Before concluding this brief sketch, a few words must be said on the beds of clay, sand, and gravel which cover the low grounds, both on the granite and on the slate. These may be resolved into two kinds: 1st. those resulting from the decomposition of the rocks, and which are in their original position; and 2d. those which are not in situ, affording by their appearance, composition, and position, evidence of their having been transported. These deposits belong to three distinct periods, and alternate with terrestrial and marine remains, according to their situation in respect to the sea-level at their respective epochs. Each of these transports was accompanied, or followed by, a change in the sea-level: the last corresponds with that now existing; the previous one, by the elevated ancient beaches, indicates that the sea was about fifty feet above its present level; and the earliest transport, being covered with sub-marine forests, shows that the sea was at that epoch at least fifty feet lower than it now is. (See 4th vol. Geol. Trans. of Cornwall, pp. 466, et seq.) In the lowest or oldest deposit the tin-ore (stream-tin) is found in the form of sand and gravel mixed with earthy substances: it affords the purest kind of tin, known in the market by the name of grain-tin; and appears to be the source from which the ancients derived all their tin. The notices under each parish will furnish more particulars concerning these interesting deposits.