310 SECTIONAL TRANSACTIONS.—C. 
Cornwall and south-west Devon consist of a mass of Paleozoic sedimentary 
rocks, and associated dykes and sills of basic igneous rocks, which has been folded 
and, about the close of the Carboniferous period, intruded by acid magma, giving 
rise to a huge granite batholith with minor bosses or ‘ cupolas’ on its surface. The 
main intrusion of acid magma was followed by minor intrusions of similar composition 
giving rise to dykes and veins of pegmatite, aplite and quartz porphyry or ‘ elvan.’ 
The last are very numerous with a strongly marked parallelism in their strike. 
After both these intrusions had taken place the volatile constituents of the residual 
acid magma percolated the rocks, effecting chemical changes in them and depositing 
metallic minerals in fissures to form the lodes. 
The metallic minerals of the lodes were deposited in a definite sequence from below 
upwards so that the lodes possess a series of primary zones as follows :— 
5. Iron Manganese Antimony zone - Most distant from the granite, always 
in the sedimentary rock and outside 
the metamorphic aureole. 
4, Lead, Zinc, Silver, Uranium zone . In, and just outside, the metamorphic 
aureole. 
3. Copper, Zinc, Arsenic zone . . In the metamorphic aureole just above 
the granite. 
2. Copper, Wolfram, Arsenic, Tin zone . Just above and inside the granite. 
1. Tin zone ; : Ne . At depth in the granite. 
The geological history of Cornwall from the close of the Carboniferous period to 
Pliocene times is almost a blank, no Secondary or early Tertiary rocks being found 
in the area. During this long period the area must have been subjected to great 
denudation, and during the latter part of the period the area was partly submerged. 
As a result, the sedimentary cover of the granite was to a great extent removed and the 
minor bosses exposed. They rise out of the sedimentary peneplain as granite hills. 
An elevation of about four hundred feet took place after Pliocene times, patches 
of the Pliocene sands and gravels being left at several localities such as Crousa 
Common, St. Agnes, &c. 
The lodes were thus truncated and exposed at surface, the zone occurring at the 
outcrop depending on the position of the outcrop relative to the granite. 
After the Pliocene upheaval there was a depression of the area followed by a series of 
elevations, so that the streams cut in the peneplain had their flow stimulated at each 
elevation, and there alluvial gravels were sluiced, resulting in the formation of 
tin-bearing gravels rich enough to work. 
Thus the ore bodies of Cornwall consist of alluvial gravels and deep lodes. 
In the beginning, mining in Cornwall was restricted to the working of alluvial 
gravels and of lodesat theiroutcrops. Theimpossibility of raising water from any depth 
prevented deep mining all through the Middle Ages, The use of adits to drain away the 
water was, however, developed along the coast in the sixteenth and seventeenth 
centuries, and to some extent made use of in the inland mines. Till the middle of the 
eighteenth century mines seldom worked below 100 feet. The invention of pumping 
engines by Savory, Newcomen, Watt, Trevithick and others during the eighteenth 
century rapidly extended the depth to which mines could be worked. After that the 
chief factor which determined the type of ore deposit worked was the value of the 
metal. At first, copper was the more valuable metal, and the lodes were worked in the 
copper zones, tin being considered a by-product ; then, when foreign copper ores reduced 
the price of that metal, the copper mines were either abandoned or carried down into 
the tin zone, thus becoming tin mines. 
Of late years, the older mines in the Camborne-Redruth area have become more or 
less exhausted, and deep development to the north has been carried out on the basis 
of geological forecast. This development is now in process, but has been successful 
already in one instance. 
The available mineral wealth of Cornwall, so far as tin is concerned, is of two 
types :— 
1. The unworked tin zones below abandoned copper mines. 
2. Lodes some distance from the granite outcrop, which carry tin at depths which 
were too great for profitable mining in the past. 
Pe 
