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.