ST. AUSTELL PARISH
(transcribef by J. Mosman, OPC)
25 JANUARY 1839
THE MINING DISTRICT of ST. AUSTELL comprises parts of the parishes of Saint Ewe, Saint Mewan, Saint Austell, Saint Blazey, Tywardreath, Lanlivery, Luxulion, Roche, and Saint Stephens [in Brannel].
The northern portions consist of high granitic hills, between which and the sea are several varieties of the slate formation; and the southern parts contain some organic remains. Elvan-courses are numerous in the western part, and there is a small patch of serpentine in the cliff near Duporth.
The granitic formation here is of much greater extent than in any of the western districts. In general this rock is rather coarse-grained, is composed of feldspar, quartz, and mica, and frequently much schorl also. In the northern, central, and western parts, however, it is often very talcose and decomposing; and the sides of Hensbarrow, the highest land in the district, supply nearly all the china-clay and china-stone sent from this county to the Potteries; indeed, all, except very small quantities from Breage and Towednack, which we have already noticed in former publication.
The micaceous and talcose varieties of granite are not mingled together, but seem to affect distinct banding; still there is, not unfrequently, a sort of transition... The granite throughout the district, but particularly that of the high-grounds immediately north of St. Austell, which is very talcose, is traversed by innumerable subcutaceous veins; the composition of those, and their relations to the containing rocks, are well known. These veins are generally of small size; sometimes they consist of schorl and quartz; at others, of schorl and feldspar; frequently of schorl alone; and occasionally portions of them are composed of quartz and feldspar only; usually, however, they are a mixture of all three substances, and most commonly abound in tin-ore. That ore is, however, seldom confined to the veins alone, for it is generally also dispersed through the substance of the adjoining rock; into which, though the line of separation is mostly distinguishable, the veins frequently pass by imperceptible gradations. On the whole they preserve a parallelism; but there are many exceptions, and in such cases they often exhibit the ordinary phenomena of heaves and throws; when, however, they unite, they are frequently enlarged and become proportionally productive. They are generally so small, numerous, and closely connected to the rock that in several places they are "worked open to the day" as the tin ore contained in both rock and vein repays the expense of stamping and dressing. Carclaze is the best known locality, but the Bunny and Bean are equally remarkable. Mr. Thomas has obligingly furnished us with a statement of the dimensions of the excavation at Carclaze. "It occupies an area of five acres statute, and its solid content is about 63,000 cubic fathoms, or about one million of tons, and its depth is 136 feet." Whilst operations were conducted with activity, a steam stamping-mill was set up at the very bottom of the cavity; this, although long-disused, still remains. At present several small water-wheels in different parts of the opening are worked by a small stream collected from the neighbouring commons, the water escaping through the adit; and on the lower ground working many other similar machines in its course to the sea.
[Note: Carclaze mine was first worked for
tin, and was very productive. It was then mined for china-clay. Eventually,
the mine was more than 1 mile across, and was the richest mine in all of Cornwall.]
The small tin-veins in these spots are exactly similar to those of the same ore at Bullerwidden, in St. Just, and in the slate at Polberou, and also to the little strings of copper ore in slate at Wheal Music.
On the outskirts of the granite are many interesting compounds of schorl with quartz and feldspar. A fine-grained and crystalline mixture of schorl and quartz forms the much-admired Roche Rock, and a very beautiful variety, consisting almost wholly of quartz, with radiating groups of crystals of schorl, occupies the brow of Saint Mewan Beacon.
The magnificent architectural decorations introduced by J. T. Treffry, Esq., into his residence at Place, are for the most part varieties of three rocks, and of the granites of this district, which have been polished by a machine erected for the purpose at the Fowey Consuls Mines. No description can do justice to their variety and beauty.
As the granite nowhere reaches the coast and as the mining operations on its junction with the slate are not very extensive, the contact of the two rocks is not visible in many places.
At Trenance-bridge, near St. Austell, the granite and slate meet in the bed of the rivulet; but as the stream is muddy with china-clay, their junction can be observed only when the water is low. The granite is fine.-grained and felspathic, and the slate deep blue, thick lamellar,, and in some places micaceous, with an irregular cleavage which dips S.E. whilst the line of junction bears about E. and W. The joints generally traverse both without interruption.
In the cutting by which Carclaze is entered from the south, the granite and slate join, or rather pass into each other, through the medium of a compact rock, mostly composed of schorl and feldspar, which are sometimes mixed; at others, in alternating laminum of felspar and schorl; in some places, however, this rock is quartzose; whilst in others, the quartz resembles a veined structure. The lamination of the rock inclines southward, but at a very high angle; and this must continue to a great depth, as several shafts have been sunk to the level of the adit, through which the water flows from the bottom of the mine in that direction, without touching the granite. The granite at Saint Blazey comes nearly as far east as the church, but its actual contact with the slate is uneven. The general character of the slate is a fine grain with a silky luster, and schisto.... structure, and the ar.. dip of the cleavage is towards the S.E.; the prevailing colour is a deep blue, varying greatly, however, in intensity. In many spots it is quartacous, and often abounds in quartz veins.
At Polgooth the joints are filled with felspar-clay; and this occurs at Pembroke, East Crinnis, and in some parts of Fowey Consols also. At Pembroke the slate is of dirty white colour passing into a reddish brown.
A greenstone rock appears in irregular patches among the slate, in many parts of the district; for example to the road-side near Saint Mewan Church; and a somewhat similar rock, but decomposing and exhibiting traces of a slaty cleavage, appearing to consist of feldspar and hornblende, occurs between Saint Austell and Saint Mewan-Beacon. It has been already stated that there is a laminated rock of schorl and feldspar with traces of quartz, near Carclaze; on the hillside towards Mount Charles, the common is covered with the debris of that rock, though the subjacent rock is there reddish brown slate alternating with beds of a pale buff colour, and contains large quantities of mica. At Apple-tree and Crinnis beaches we have found large quantities of encrinal slate, and in a coarse-grained arensceous slate at Porthpean are some portions which may perhaps be obscure. At Par, Polkerris, and Fowey, Mr. Peach (of the Preventative Service at Gorran Haven) has discovered many more. In the cliffs of Duporth, a vein of serpentine runs through the slate; it is variegated, and contains a hornblende, or some similar dark coloured mineral, which are often decomposed, and apparently cemented by extremely thin layers of a very pale green stratice. Some portions are much disintegrated, whilst others which tend to decomposition are still coherent.
Elvan-courses may be studied with great advantage near Polgooth. The largest of these, Reskilling .. is extensively quarried for building; it breaks S.E. and S.W., dips N.W., and is from three to eight fathoms wide. ... Most probably this is contiguous of the Polgooth elvan. The composition of all of these is much the same, viz. a basis of feldspar, quartz, and in some ... schorl,with large porphyritic crystals of pale buff and yellow coloured feldspar. Some parts of the Polgooth are traversed by minute veins of quartz, with certain tin-ore.
The directions of the lodes in the greater part of the district are a few degrees S. of W; this is the case in several of the lodes at Beans, Polgooth, Pembroke, East Crinnis, and Fowey Consuls; there are, however, some in Polgooth, and Fowey Consols, and also the lode of Charlestown Mines, which bear nearly S.E. and N.W., and thus approximate the directions of the Counter lodes in the western districts.
There are not many cross-courses in this neighbourhood. At Polgooth a [flces?] heaves the lodes and elvan thirty fathoms; a similar one heaves the lode at Pembroke about 7 fathoms; there are some cross-courses at Fowey Consols, but they are all inconsiderable.
Some very unusual phenomena attend the intersections of the lodes and elvan at Polgooth. Saint Martin's and Screed's lodes are both heaved by Reskilling-great-elvan; an occurrence, we believe, without a parallel in the mining districts.
At Restormel, a large iron vein or cross course bears a few degrees W. of N., and dips E., it is from 2 to 4 fathoms wide, and is generally in two branches, which includes a mass (heave) of slate between them. The working on it have been extended for more than two miles in length, but no more than ten fathoms in depth, and mostly not more than 3 or 4 fathoms. Its chief produce is brown earth, red, and hemastitic iron ore much mixed with quartz, but in some places it yields large quantities of oxide of manganese. It abounds in drosy cavities (..) which are lined with crystals of quartz, and lydrous oxide of iron. Almost the whole of the northern and western parts of this district are granitic, yield only tin-ore; but in the eastern (slate) portions of it copper ore prevails, which is generally in the state of copper pyrites (yellow ore) but native copper, red, oxide of copper, felderz, b.. (purple copper), vitreous copper ore (gray copper ore), and some of the still rarer compounds of that substance, often with fine crystals, have also occurred in Crinnis, East Crinnis, Pembroke, and Fowey Consuls. In the last-named mine, sulphuret of bismuth occurs in abundance; usually crystallized in the cavities of the (yellow copper) copper pyrites.
The shoots or bunches of ore all slip from the granite, and as the rock lies mostly to the westward, the east parts of the mines are commonly the deepest.
More Stream-Tin ore has been found in this district than in all the country besides. It always occurs in a rounded form, the masses varying in size from the finest sand to several inches in diameter. At Pentuan the tin-ground was below sea-level, and was covered by large quantities of wood, nuts, sand, shells, silt and gravel. In the higher grounds, it rests on the granite, and there is not unfrequently a second layer of tin-ore separated from the first by a bed of distintegrated granite (false-shift). The whole is usually reversed by two distinct beds of peat, divided by granite provel.., and the whole by sand, silt, and gravel, the wash of the present streams. This formation is so interesting and peculiar, that we purpose returning to its consideration at some future opportunity. East Crinnis, Pembroke, and Beam have been stopped after having been sunk to the depth of above 100 fathoms. Polgooth is now worked to about 40 fathoms under the adit, Charlestown Mines to about 80, and Fowey Consols to 180 or 230 fathoms from the surface.
As printed in the West Briton and Cornwall Advertiser, Friday, 25 January, 1839 - Truro, Cornwall
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