Twenty years after Brenton Symons published his map of the Liskeard mining district he wrote this description of the Caradon District within his sketch of the geology of Cornwall.
A sketch of the Geology of Cornwall
By Brenton Symons, F.C.S., Assoc. Mem.Inst. C.E. Mining engineer and metallurgist
The barren aspect of the Bodmin Moor is reflected its rocks, which are very destitute of metallic ores, and it is only on the southern fringe of the granite that copper and tin ores abound. At Roughtor, east of Camelford a large sum was expended to discover whether the tin veins in granite improved with depth, but the failure was complete. At Blisland where there is a well marked, though very granitic group of elvans, no lodes of any promise have yet been noticed, but no exploration of importance have been made.
The mines around Caradon Hill- 1208 feet high-were originated by some miners driving an adit in 1836, but though comparatively modern, after a brilliant existence the first fruits of the district have been gathered, and the mines once so numerous and prosperous are now mostly stopped. South Caradon, the first mine opened, yielded 9% ore, and gave for many years handsome dividends, the total profit having being £380,000. The copper group extends eastward through East Caradon to Glasgow Caradon both very profitable mines. To the north is the Phoenix group of tin veins, where owing to the projections of granite ridges, and the faulting of the lodes, the hanging wall is slate, whilst the foot wall is often granite. The matrix of the tin ore is composed of quartz, chlorite and earthy iron ore. Adjacent the surface, copper pyrites and malachite are found. Nearly all the lodes dip steeply towards the granite, and have average width of rather more than three feet. At Gonemena tin ore is found in a manner somewhat resembling Carclaze, the excavation is a third of a mile long, and occupies a dozen acres, but the depth is only fifty feet.
To the west, the lodes are principally tin producing, and continue with a group of elvans through St. Neot to Warleggan. Though the mines have only been worked in a partial and desultory way, there is ample evidence that good tin lodes, which merit exploration, exist. At a mine called Tin Hill a large quantity of stream tin was obtained from a remarkable deposit of gravel and boulders beneath cliffy granite. Some elvan courses have been worked for tin with moderate success in this district.
This post is the fourth in the series looking at the geology of the Caradon Hill area in Cornwall through the information shown on a amazing Victorian map. So far the posts have covered the granite, cross-courses and elvans; and now it looks at the most important feature- the lodes.
For the majority, if not all, of the buyers of Brenton Symons’ map the lodes are the features they would have had the most interest in. For lodes are where the potential of wealth lay, their fickle nature driving the fortunes and losses of the industry.
Lodes are cracks or pressure filled with minerals, from which the miners extracted the ore. They were normally vertical, or near-vertical; often extending for considerable distances. In other parts of Britain they are known as a veins or seams.
Brenton Symon’s displayed lodes as red lines, dotted when their existence had not been confirmed. This post summarises the lodes in the immediate area around Caradon Hill, near Liskeard.
The number of lodes
The modern British Geological map denotes the location of 9 lodes whilst Brenton Symon showed 89 in the same area. This massive discrepancy is a clear example of how every map reflects the priorities of its maker, sponsors or potential customers. The Victorian customers were focused on the red lines cluttering the maps, but modern geologist have little use for the information.
The direction of the lodes ( the Course)
In this part of Cornwall tin and copper lodes broadly run east to west and lead north to south. In general tin lodes are in or near the granite, copper near the granite contact, and lead in the killas. This pattern is reflected in Symons’ map which has all bar three of the lodes in that Caradon area running east to west.
Not all the lodes run purely east to west; some trend slightly to the north, and some to the south. The distribution according to Symons is:
East to west 20 percent
East by North 2 percent
East by south 78 percent
However, a transcription of the course of the lodes onto a modern 1:25000 map reveals a different picture:
East to west 21 percent
East by North 58 percent
East by south 5 percent
This is a discrepancy highlights an error in datum between the two maps, an error
that must be considered when using the older publication.
Dip or underlie is the angle of slope of a lode.
The underlie is the angle measured from the vertical, whilst the dip is the angle measured from the surface.
Dip is shown on the Victorian map by arrows on the side of the lode. A summary of the dip shown in the mine setts around Caradon Hill is:
None shown 32 percent
Southerly 14 percent
Northerly 53 percent
Length of the lodes
By transposing the lodes onto a modern Ordnance Survey map it is possible to determine the length of the lodes in the area. The average length is .3km, with the longest shown being 2.5km. The majority of the lodes ony have short runs:
Less then .5k 79 percent
.5 to 1km 13 percent
1km plus 8 percent
The average lode in the Caradon area therefore runs East by North, is .3km long and dips to the north.
Brenton Symon’s map is far superior to the modern map as a source of information on lodes, as long as the datum discrepancy is allowed for- the next post will look at the map information displayed on a modern map.