THE MINERAL RESOURCES OF DEVON AND CORNWALL. 487 
for, with the greatest probability of success. The most recent 
mining operations everywhere tend to support the theory that the 
richest deposits of metalliferous minerals are to be found at the 
greatest depths. 
At the celebrated Dolooath Mine, in Cornwall, the deeper the 
working the richer the ore. The silver mines of Mexico, and the 
still more wonderfully rich silver with gold mines of Nevada, 
teach the same lesson. Confirmation of this theory comes in 
another form. The specific gravity of the earth in its entirety is 
about 6*5. A great portion of the surface of the earth is covered 
with water. 
To the greatest depths that mining operations have yet reached, 
the average specific gravity of the products obtained will scarcely 
equal 2 5. The average of the whole of the earth's crust will 
probably be less than 1*5. 
The constituents of the inner crust must be of much greater 
and of rapidly increasing specific gravity in order to make up the 
average of 6*5. No other substances than metalliferous minerals 
are known which possess such high gravities, and those too of the 
more valuable and precious metals ; therefore the probabilities at 
least are greatly in favour of the expectation, that researches 
greatly extended in depth beyond those surface scratchings of the 
globe that have hitherto been undertaken, may lead to the most 
important developments. 
The limits of this paper render it necessary to concentrate 
attention on the principal minerals obtained in Devon and Corn- 
wall ; viz., those of tin and copper. Tin was obtained by the 
Phoenicians from Cornwall long anterior to the Christian era ; but 
I have not met with any statistical returns of earlier date than 
1750, in which year the production of metallic tin in Cornwall 
amounted to 1,600 tons. In 1790 the returns had increased to 
2,000 tons; but they fell off to 1,500 tons in 1800; 1,400 tons 
in 1810; increasing again to 1,700 tons in 1820 ; to 3,500 tons 
in 1830; 5,000 tons in 1840; 10,353 tons in 1850. Up to the 
present time the average may probably be taken at 10,000 tons. 
Over this period prices of metallic tin have varied, from as low as 
£60 per ton in 1843, to as high as £165 in 1872. 
From a rough estimate that I have made, it is probable that 
the production of Cornwall, if taken at £50,000,000, will be 
considerably within the truth. 
