The Solfataras.—Sulphur is formed by certain gases generated underground by volcanic action, and in solfataras these gases find their way to the surface of the earth through sand, ashes, or other volcanic substances, and in their passage sublime and deposit a certain portion of their sulphur, a certain amount escaping into the air.
This formation of sulphur is continuous and increasing, and in proportion to the strength of the volcanic influences so is the rapidity with which the sulphur is formed and the amount taken from the solfatara replaced. For this reason they are called “living.”
The solfataras of Italy require a period of twenty-five or thirty years to renew the sulphur in sufficient quantities to pay for extraction, whilst these are said to require only three years to produce the same result, the same area of solfataras in Iceland being consequently ten times as valuable as an equal area in Italy.
The methods of extracting the sulphur from these are most inexpensive, and the plant required of the simplest description.
The gases at present escaping into the air can be condensed and the sulphur obtained in a pure crystallised state, without any expenses for refining, by collecting the gases in clay vessels.
2d. The Sulphur Banks, or Fields.—The gases before mentioned escaping into the air condense and deposit sulphur, which, were the atmosphere always calm, would be precipitated in regular banks, but owing to the constant shifting of the wind it is blown in all directions, forming layers varying from a few inches to several feet in thickness, and extending over vast areas of the surface of the surrounding ground.
3d. Sulphur Quarries.—In these localities the accumulation of sulphur has ceased, and when once extracted is not replaced; they are therefore called “dead.” The sulphur is found imbedded in, and mixed with, lime, clay, etc., and nearly all the sulphur exported from Sicily is obtained from this description of sulphur-bearing strata.
The same kind of strata exists in the Romagna in Italy, and in some districts of Spain, but in the Romagna the deposit is 390 feet below the surface, and only yields, in the furnaces, 15 per cent. of sulphur, while the best of those in Spain are from forty to sixty feet below the surface, and contain a varying quantity of sulphur of from 21 to 36 per cent.—the poorest strata being nearest the surface—whilst these (in Iceland) are upon the surface; and Henderson, the missionary, a most trustworthy authority, describes a valley one mile wide and five miles long in the neighbourhood of Krabla, the surface of which is very uneven, and consists of immense banks of red bolus and sulphur, with mixtures of yellow, light-blue, and white coloured earth.
Forbes found similar clays to contain, the white from 30 to 40 per cent., and the red and blue clays about 16 per cent. of sulphur.
The plans made by J. F. Johnstrüp, Professor of Mineralogy at the University of Copenhagen, by order of the Danish Government, and attached to the leasing contract, a copy of which will be found in the Appendix, show the solfataras, or living sulphur-fields, to extend over a district of more than six square miles, viz.: