Transactions of the Geological Society, 1st series, vol. 4/Description of the Tunnel of the Tavistock Canal

by John Taylor

VI. Description of the Tunnel of the Tavistock Canal, through Morwel Dow, in the county of Devon.

By JOHN TAYLOR, Esq.

treasurer of the geological society.

[Read 6th March, 1814.]

Morwel Down is a hill, lying between the River Tamar, which divides the counties of Devon and Cornwall, and the River Tavy, which rises in the forest of Dartmoor; and after passing the town of Tavistock, flows on the eastern side of Morwel Down, and falls into the Tamar, a few miles nearer Plymouth.

The neck of high land separating these rivers, extends southwards from Morwel Down, and includes the parish of Beer, in which are situated the Beeralstone Lead and Silver Mines, not far from the point of the peninsula, the lode crossing a part of it in a line from north to south.

In pursuing the Tavy towards its source, the country rises irregularly, and the rocks are found to consist of killas, to the borders of Dartmoor; the same appearances are to be observed by taking a survey of the hills situate between the eastern bank of the river and the range of granite mountains which form the peculiar feature of Dartmoor. True granite has not been found intermixed in the central part of the range of the killas rocks of this neighbourhood, in any instance within my recollection. In the valley through which the little river Walkham flows, and near the point at which it falls into the Tavy, a remarkable change of strata occurs; the side of a very abrupt hill, on the top of which is West Down, in the parish of Whitchurch, is composed of a considerable cluster of detached masses of granitic rocks, which are piled on each other in the most picturesque manner, and form a lofty and steep bank to the river. Killas occurs in the same hill, on each side, and is the only rock observable on that which rises from the opposite edge of the valley.^[1]

If we turn from the country on the east and north of Morwel Down, to that on the west of it, we shall find that the Cornwall side of the river Tamar is more diversified in the rocks that occur; killas generally prevails, but granite crowns the summit of Kithill, which rises gradually from the banks of the river to the height of 1400 feet, and the same rock is to be found near the base of the mountain, at Gunnis Lake Copper Mine, near New Bridge, and again a little higher up the stream, at a place called the Clitter, a provincial word, signifying a collection of loose masses of rock. The killas district is nearly surrounded on the three sides above mentioned by the granite, the line of division of the two rocks describing an irregular horseshoe form, while the southern side of the killas extends to the coast and joins the Plymouth limestone.

The surrounding granite mountains rise to an elevation of from 1400 to 1900 feet above the sea, while the hills of killas keep a much lower range; Morwel Down, through which the tunnel is passing, is one of the highest in the central part of the killas, and is about 700 feet above the tideway in the river Tamar, which washes its base.

The killas district, which is attempted to be here described, is every where intersected with veins, or as they are technically called, Lodes. Those which are worked for copper or tin have universally a direction from north—east to south-west, or nearly so; those which run in other courses have all the appearances of a newer formation, and are generally unproductive of metal, if we except two instances, one of which is the lode on which the Beeralstone mines are working to a considerable extent, and the other the lode in Wheal Betsey Mine, in the parish of Mary Tavy, both of which produce lead and silver.

In the last 20 years this district has been the scene of very active exertion in the pursuit of mining, and the most spirited efforts have been made for tracing the veins, and instituting trials upon them for the discovery of their contents. These effects, as in most similar cases, have been attended with very various success, though on the whole, the result has been a favourable one. On many lodes considerable sums of money have been expended, without discovering sufficient quantities of ore to repay the disbursements, and on many the loss has been heavy; in other instances, though the fewer in number, mines have been established which have produced very large quantities of ore, principally of copper, and have paid the adventurers very handsome profits.

The most important of these mines are Wheal Friendship, in the parish of Mary Tavy; Gunnis Lake and Drake Walls mines, in Calstock, on the Cornish side of the Tamar; Wheal Crowndale, on the banks of the Tavy, below Tavistock; Beeralstone mines, in the parish of Beer; Wheal Betsey, in Mary Tavy; Wheal Crebor, at the foot of Morwel Down, discovered in consequence of the undertaking about to be described, and some others of inferior note.

I am not able to state any account of the produce either of Gunnis Lake copper mine, which has been very considerable, or of the Beeralstone mines, but exclusive of these the others have returned since the year 1805, from 3 to 4000 tons of copper ore annually, and the quantity now raising is at the rate of at least 5000 tons in the year. There has likewise been a considerable quantity of lead raised at Wheal Betsey, and of tin at Drake Walls.

All the lodes that have been worked, are in killas, excepting that at Gunnis Lake, where the copper is found in granite. The ores of this mine differ very much from those of the other mines; those of the latter are almost entirely copper pyrites or yellow copper ore, varying in their proportions of metal from 5 to 15 per cent. while in the former mine are found besides the yellow copper ore, carbonates of copper, grey copper ore, arseniates, &cc. This fact is the more striking, as the vein is certainly the same as that worked at Wheal Crowndale and Wheal Crebor, where it traverses the killas, and at Gunnis Lake passes into granite.

About the year 1802, when the mines of this district were assuming an importance they had never before attained, and their prospects were such as to encourage fresh adventures, the proprietors of the principal ones were led to think of the scheme of driving a tunnel through the hill, which is the subject of the present remarks. The chief inducements were, that Morwel Down was known to be traversed by numerous lodes, which might be discovered and worked by such an undertaking, and that while a tunnel should be carried in a direction to cross them all, it might make a navigation practicable from the vicinity of Tavistock and the adjacent mines, to the river Tamar where the produce of the neighbourhood is shipped.

In 1803 an Act of Parliament for cutting a canal from the town of Tavistock to Morwelbam, a quay on the river Tamar, was obtained, and the driving the tunnel was immediately begun.

A canal from the north end of it to the town of Tavistock was soon after cut, by which means a copious stream of water was obtained from the Tavy, which was carried across a valley upon an embankment 50 feet high, and afforded the means of working an overshot water-wheel of immense power, which was required for sinking the requisite shafts on the hill through which the tunnel was to pass.

It is unnecessary here to enter into further detail of the nature of the works, as they may be understood from a collection of reports on the subject, which I have formerly laid on the table of the Society; it is sufficient to remark that this tunnel, which was to pass through hard rock for a length of nearly a mile and three quarters, and for the principal part at a depth of about 130 yards from the surface of the hill, was an undertaking of no small enterprize, and that difficulties of various kinds presented themselves in its progress.

The tunnel, as may be seen by the section, is not yet complete,^[2] but the obstacles are all surmounted, and nothing now remains to be done but the simple operation of driving. The draining the deep shaft in the centre of the hill, and the ventilation of the tunnel, having been some time since provided for.

It does not often happen that the processes of the miner lead to so much geological discovery as might be expected; the works he undertakes follow the course of the vein he is exploring, or are confined within a small space bordering upon it. As the veins are sought after in but very few varieties of rock, so the number that are laid open to view is generally limited compared with those that exist in mining districts.

A tunnel of such an extent as the one now to be described, in such a district, crossing the direction of the metalliferous veins, and passing at such a depth under the surface, could hardly fail of proving an interesting object to the geologist as well as the miner.

Two facts have been ascertained by its progress:

1st, Relative to the rocks, that the killas of which the hill is mainly formed, is traversed by beds of other rock, whose direction is inclined to that of the metalliferous veins, and which have a pretty uniform dip or underlay to the north.

2d, Relative to the metallic veins or lodes, that they traverse all the strata, and that they have a remarkable difference in their dip or underlay on the two sides of the hill. Those on the north side dipping to the north, and those on the south side to the south.^[3]

Commencing at the north end of the tunnel, I shall proceed to detail the strata that have been passed through, referring to the section accompanying this paper to shew their position, and to the specimens of the rocks themselves which I have selected to exhibit their character.

I give the provincial names of the rocks as they are in general use among the Cornish miners.

A		—		311	fathoms		Killas
B		—		11	—		Elvane		Chlorite and Quartz
C		—		23	—		Killas
D		—		6	—		Grouan		Clay Porphyry
E		—		12	—		Killas
F		—		26	—		Grouan		Clay Porphyry
G		—	$\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}$				Killas
H	$\left.{\begin{matrix}\ \ \end{matrix}}\right\}$	—		436	—	$\left\{{\begin{matrix}\ \ \end{matrix}}\right.$	Ditto
I		—					Ditto,		with veins of Quartz
K		—		15	—		Elvane
L		—		3	—		Killas
M		—		7	—		Grouan		Porphyry
N		—		12	—		Elvane		Quartz, granular and crystalline
O		—		408	—		Killas
				———
				1270	Whole length of the tunnel.
				———

The direction of all these beds seems to be parallel, and to range nearly east and west.

All the veins that at present are known in the part of the hill which the tunnel will intersect are shewn in the drawing, by lines, which describe their dip as nearly as is ascertained from the little that has as yet been seen of most of them.

Some of these lodes have been discovered by the tunnel, and some are known by old works upon them near the surface.

It was not to be expected that any great proportion of the number would turn out productive of ore, or at least that they should be so at the exact point where the tunnel cut them. One or two, if rich in ore, might render the speculation a profitable one, and it is rather extraordinary that the first which was discovered, at the commencement of the work at the north end, should be one of that description. It is called Wheal Crebor lode, and has already been worked about 60 fathoms deep under the level of the tunnel, and has produced between 8 and 9000 tons of copper ore; its direction is as usual from north-east to south-west, and it has been traced to be the same vein that is worked at Wheal Crowndale mine to the east, in killas; and at Gunnis Lake mine to the west in granite; at both of which concerns very large quantities of ore have been raised. The lode at Wheal Crebor is in some places fourteen feet wide, though in others not as many inches. It is traversed by cross veins which heave the lode, as the miners call it, a few feet.

The mine is now producing near 4000 tons of ore in the year: a specimen will be found with the others.

The next lode found in following the course of the tunnel southwards contained tin, but not in any great quantity, and very little work has been done upon it in the way of trial in consequence.

Further south is a lode called Wheal Georgiana, which has produced some rich copper ore in the porphyry, where the tunnel discovered it. It has been pursued into the killas, but in this rock it appears to be less productive of metal.

At the present end of this part of the work which is approaching the centre of the hill, a vein has just been met with holding copper, but too little is yet known of it to afford any description.^[4]

In the space yet unopened between Bray's shaft and the end approaching it from the south, is a lode called Holming Bram, which was formerly worked for tin, and on which considerable expectation is grounded. Having simply stated the facts as far as my knowledge of them goes, I abstain from speculating on the support they may afford to any hypothesis on the formation of the rocks or the veins, though they may offer some hints on the subject.

Imperfect as this sketch is, it may serve to lead the attention of some more able member of the Society to the consideration of the appearances of this district, which I thought sufficiently curious to encourage an attempt at their description.

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POSTSCRIPT

December, 1816.

I have lately visited the tunnel in consequence of its completion, and therefore am enabled to complete the section of the hill, shewing that no new strata have been discovered since the preceding paper was written.

I have likewise ascertained more satisfactorily the dip or underlay of the lodes near the centre of the hill, and inserted them in the section with two cross lodes or cross courses, which traverse the lodes near that place. A remarkable alteration in the texture of the killas occurs on each side of one of these cross courses, it is found in such a decomposed state that it is converted into a soft clayey matter, so as to be very difficult to preserve a passage through until it can be securely arched.

A period of thirteen years has been occupied in bringing this great work to a conclusion, and it has not been done without the anxieties consequent on such an undertaking.

Two things of great importance in the practice of mining may be remarked of this work. First, the extreme accuracy of the line of direction which has been preserved in so long a drill, although the junctions were made from several different points.

Second, the small number of shafts, and consequently the length of tunnel between each, which was ventilated during the progress of the work. I am inclined to believe that it exceeds in this respect all other attempts of a similar kind, and the section may therefore, in the hands of the Geological Society, be a useful document for future engineers. The means which I adopted for obtaining perfect ventilation will be found to be described in the Transactions of the Society for the Encouragement of Arts, Manufactures, and Commerce, for the year 1810.

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1. The situation of these rocks would point out a connexion between them and some of the beds or veins of porphyry which are to be described as occurring in the tunnel through Morwel Down; the line of their direction would lead us al this point, and the inference is strong that this is s part of one of them. It is however rather extraordinary that it should have escaped notice in the deep valley of the Tavy, where it must pass, and where I have little doubt it will be found from a recollection of the general features, though unfortunately it did not occur to me to look for it at the time when I could have done so. It may likewise probably be traced through Morwel Down to Gunnis Lake Mine, which is mentioned in the following paragraph.

   I do not venture to decide on what this rock should be called; in describing the strata of the tunnel I have assumed that it is porphyry. That which occurs in the Walkham valley has much more the character of granite, and so I should incline to call it.

   The specimens will enable more competent judges to decide.

2. See the Postscript to this Paper.
3. Since the paper was written it has been ascertained, as I have been informed, that some veins lately discovered in the space between Renfrews shaft and Brays shaft underlie to the south, which is an exception to the preceding observation. But it may be observed that this deviation takes place near the centre of the hill.
4. This vein underlies to the south, and is mentioned in a preceding note as an exception to the usual dip of the lodes on this side of the hill.