Royal Geological Society of Cornwall

Volume 4 paper 3

III.-A Description of Happy-Union Tin Streamwork at Pentuan.

By JOHN W. COLENSO, Esq.

(Read October, 1829.)

Happy-union tin stream-work lies in a valley near the harbour of Pentuan, in the parish of St. Austell.

Several streams which rise on the southern side of the high hills of Hensburrow, (about eight miles distant from Happy-Union) flow through this valley, and there the high grounds about Polgooth mine (two miles distant) also discharge their waters.

The valley of Pentuan is a continuation of St. Austell moor, where for many ages a great quantity of tin has been gotten by streaming. It is in some parts upwards of six hundred feet in breadth, but in others not more than three hundred, and at one spot little more than one hundred. Its declination from St. Austell bridge to Pentuan is about one hundred and twenty feet; and if to the fall of the valley, we add the depth of the deposit, it will make a total of one hundred and eighty feet, or just forty five feet to a mile.

Above St. Austell bridge there is no alluvial deposit of any kind, the course of the river from its source to that place being confined and rapid; but in the four miles from St. Austell to Pentuan, the deposit gradually increases, so as at the present place of working, to attain the depth of sixty feet.

The floor or rock on which the whole of the deposit rests, is composed of blue killas, of the same kind as that of the adjoining hills and cliffs : in some places it is as hard as the hardest of that kind of stone; in others, so soft as not to support the props which it is found necessary, in driving levels, to place on it. In particular places the rock appears to be worn by friction; but in other places, especially under the tough ground, (described hereafter) there is no appearance of friction.

In describing the different parts of this deposit, I shall refer to the section, in which they are designated by different letters.

a. The tin ground, that is to say, the stratum in which the whole of the stream tin is found. It lies on the solid rock, and is generally from three to six feet, and sometimes even ten feet, in thickness: in width it extends across the valley, except where turned by a projecting hill or rock, when it is found to take the supposed ancient course of the river, which is generally under the steepest land.

The rocks and stones which, with the pulverized matter, compose this stratum, are mostly fragments of granite similar to that of the hills near St. Austell, (exhibiting all the varieties of this rock which usually occur near the base of our moorstone hills) all of which are considerably rounded by attrition, whereas the stones of killas and greenstone which are frequently met with, appear to have suffered but little change. Fragments of elvan, and of the hard black rock called by the miners " irestone," are also occasionally found in the mass, as well as vein-stones, and that kind of iron-ore which is called cal.

Most of the tin lies at the bottom of the stratum: but sometimes it is found in the higher parts, where for two or three inches in thickness the ground is in some parts quite blackened with tin, although perhaps for two or three feet below it, the ground does not repay the expence of washing it: generally speaking, however, the tin ground is best in the bottom, and although the higher part contains tin, and sometimes good pebbles of tin; yet the ground here is generally poor: indeed the smaller the mass of the tin ground, the greater is the quantity of tin contained in it.

The tin is from the size of the finest sand, to pebbles of ten pounds weight; and some rocks richly impregnated with tin weighing two hundred pounds and upwards, have occasionally been found here. The small tin, which is known by the name of grain tin, is of the best quality : the larger stones contain more waste, and sometimes also copper and mundic.

The tin ground is divided by the streamers into what they call " loose ground" and " tough ground:" the loose ground consists of sand, stones, and pebbles; the tough ground has an additional intermixture of yellow clay, which cements the whole so firmly together as to render it difficult to be separated and washed.

The tough ground appears to be formed by the clay rising up from below it; for on removing this ground, we find oozing out of the soft killas under it the same fine yellow clay as is mixed with the tough ground: and sometimes for two or three feet below the regular surface of the soft killas, tin and other pebbles have been deposited in the place of the clay forced up. The tin ground is also of a much greater thickness where the clay appears to have been forced up; from which it may be supposed to have been lodged in its present bed immediately after the removal of the substance which before occupied its place.* We have here found a few grains and small pieces of gold ; they are very scarce, and rarely so large as a pea.

* The quantity of tin ground opened at Pentuan has been seven hundred fathoms in length, averaging about twenty six fathoms in breadth, making a total of eighteen thousand and two hundred square fathoms. The average quantity of black tin gotten per square fathom has been one hundred and eighty pounds : the quantity of overburthen removed has been upwards of two hundred thousand tons.

In this stratum I have never met with any animal remains; but roots of trees are seen, and lately has been opened what was once an oyster bed, on the top of the tin ground, where the shells still remain fastened to some of the large stones, and the stumps of trees. The roots of the oak are in their natural position, and may be traced to their smallest fibres even so deep as two feet: from the manner in which they spread, there can be no doubt but that the trees have grown and fallen on the spot where their roots are found. I understand the first appearance of decay in the oak, is its becoming stag-headed: the trunk fails next, and then the root; but at Pentuan the timber is often found quite sound, as a proof of which, I have recently applied one of the trees to make the axle of a water wheel. It appears to me likely that at this period, the rising of the sea had so far checked the current of the river as to prevent its discharging the mud and sand brought down with it; thus the roots were buried to a considerable depth, and the trees killed, before the timber underwent its natural process of decay.*

*The specimens from the stratum of tin ground are, No 1, serge or grain tin. No. 2,-5, varieties of wood and other tins. No. 6, common pebbles of tin. No. 7,-9, specimens of the tin ground. No. 10, part of a large pebble containing mundic, broken to shew the effect produced by the action of the water on it.

b. A stratum of dark silt, about twelve inches thick, apparently mixed with decomposed vegetable matter, and on the top of this is a layer of leaves of trees, hazel nuts, sticks, and moss, from six to twelve inches more. The moss appears in a perfect state, retaining almost its natural colour, and seems to have grown where it is now found. This layer of vegetable matter is about thirty feet below the level of the sea at low water, and about forty eight feet at spring tides. It is not found in particular spots only, but extends, with some interruption, across the valley.

c. Above the vegetable matter just described is a stratum of sludge or silt, of ten feet in thickness: in this there is little variation, except by its changing from a brownish to a lead colour, in particular places. The whole is sprinkled with recent shells, together with wood, hazel nuts, and sometimes the bones and horns of deer, oxen, &c. The shells, particularly the flat ones, are frequently found in rows or layers; they are often double and closed, with their opening part upwards, so as to render it likely that the fish lived and died where their remains are now found.

Shells exposed to the action of the sea are seldom found united; and it is well known that shell fish are only discovered at or near the surface of the sand or mud; this stratum, therefore, as it contains the remains of shell fish throughout, must have increased gradually, and formed layer after layer, as the fish became possessed of the surface, according to the gradual rising and influx of the sea,

The silt at Happy-Union is of so fine a texture, that it will scarcely admit air or water to pass through it; insomuch that after being once dried, it may be placed under water for a considerable time without being affected by it. It is therefore often applied by the streamers to resist water, and if formed into a basin, will hold it like earthenware. By the weight of the sand-banks on it, it is so compressed, that when taken up it is a comparatively dry mass. It is in and by this silt that animal and vegetable substances are preserved, and it is to the greater or less admixture of these, that we must ascribe the variations of colour in this stratum.

There has been recently found imbedded in the silt, about two feet from the top, a piece of oak, that has been brought into form by the hand of man; it is about six feet long, one inch and half broad, and less than a half an inch thick; this is the greatest depth at which I have ever seen any converted substance. It appears to have floated in the sea, as at one end, which is much decayed, a small barnacle has fixed its habitation.

d. A stratum of sea-sand, about four inches in thickness : this is easily distinguished from the river- sand, being much finer, and having always more or less shells mixed with it. It is remarkable that the water which drains from this sand is nearly as salt as the sea, whilst all the water above and below it is fresh.

e. Above the sea-sand there are two feet more of silt. About the middle of this stratum there is imbedded in it a layer of stones of various sizes and forms, and of different degrees of hardness; they appear to be conglomerations of sand and silt, with sometimes wood and bones : and are found only at this level, all over the stream-work.

f. Another stratum of sea-sand, twenty feet in thickness, rests on the silt. In all parts of this sand there are timber trees, chiefly oaks, lying in all directions ; and also the remains of animals, such as parts of the red deer; heads of oxen of a different description from any now known in Britain, the horns of which all turn downwards; the scapula and humerus of an animal supposed to be of the ox tribe, measuring round the largest part twenty inches: these are supposed to be the bones of an ox such as is now in existence near the Cape of Good Hope. Human skulls have also been found in this sand, one of which, supposed to be either African or Asiatic, I beg to present to the society, with other animal remains, &c.* These lay near the bottom of the sand. In the upper part of this stratum, and about two hundred fathoms nearer the mouth of the harbour, the bones of a large whale were found, which shew that at the time they were conveyed here, there must have been a depth of water
in the harbour, sufficient to have floated such an animal into it.
*The specimens from this stratum of sea-sand are,-No. 1, the humerus, and No. 2, the scapula abovementioned. No. 3, jaw of a hog. No. 4, the skull abovementioned. No. 5, radius of a whale (imperfect). No. 6, humerus of a whale. Nos. 7-8, phalanges of a whale. Nos. 9-10, large bones, supposed of a whale. Nos. 11-12, stag's horns.

The sea at this time appears to have extended nearly a mile up the valley, as is evident from the same kind of sand being found at that distance ; but further up it lessens in depth from the acclivity of the land.

After the sea had occupied the valley long enough to raise the sea-sand to twenty feet in height, the sand and wash from the hills and tin stream-works, near Hensburrow and St. Austell moors, appear to have forced the sea back to its present boundary, from which it is still retreating, and must continue to retreat, whilst the quantity of stones, sand, and gravel, amounting annually to many thousands of tons, is carried down by the St. Austell river to the sea.

g. A bed of rough river-sand and gravel, here and there mixed with sea-sand and silt. This bed is also about twenty feet in depth, and brings us to the surface. The workings being now at the mouth of the valley, and only about half a mile from the present ordinary flow of the tide; there is here more sand and less silt than further up. There have been recently found in this sand the remains of a row of wooden piles, sharpened for the purpose of driving, which appear to have been used for forming a wooden bridge for foot passengers: they crossed the valley, and were about six feet long, their tops being about twenty four feet from the present surface-just on a level with the present low water at spring tides. Had the sea level been then as now, such a bridge would have been nearly useless.

How far the valley of Pentuan once extended towards the sea is a mere matter of conjecture: but supposing it to continue at the same declivity as the ground between St. Austell and Pentuan, viz. forty five feet to a mile, it must have continued at least a mile further than it now does, to allow the trees to have grown, where we now find the roots, and still further, if we suppose the tin ground to have been lodged in its present bed either by the deluge or some other violent current. The sea now flows forty eight feet higher than the top of the tin ground.

Happy-Union stream-work was first opened in the year 1780. In the course of a short time it was found necessary to work in both directions, that is to say, up the moor towards St. Austell, and down, towards the sea; the first was called Wheal Virgin, and the latter Happy-Union. The two works moving in opposite directions, their distance gradually increased, and they are now about a mile asunder. In Wheal Virgin the tin ground is only thirty two feet from the surface. The tin is of nearly the same quality as at Happy-Union; but both the pebbles and grains of tin are more thinly strewed, nor is the tin ground of so great an average thickness. The overburthen contains no sea-sand, but is composed of silt and river gravel, with a considerable quantity of wood, chiefly oak and willow, with the roots of the same and other trees in all places where there is any soil. Very recently I have seen on the surface of the tin ground two small pieces of oak, with artificial holes in them: and there were near them several oak stakes, sharpened and driven into the ground, and supported by large stones. Near the same spot has been found a substance resembling the ashes of charcoal. Here it is evident (to use a streamer's phrase) the old men have been; but whether the tin ground was then the surface; or whether they went down by the aid of machinery, (it being under the present sea level), must, until we see more of it, remain undetermined.

In closing this paper I beg to observe, that where the valley is narrowest the rock on each side is steepest; and, both on the sides and in the bottom, harder than in other places. Where I have noticed the tin ground, in such situations, I have observed it to be less in depth, but richer in quality. The rocks and vein-stones of which the tin ground is chiefly composed, being also larger than where the valley is wider.