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THICKNESS OF VEINS.

copper ore in the lowest part, silver ore above>and over that iron ore.

The thickness of veins and the quantityand quality of the ore they contain vary inevery mine. Some veins are only a few incheswide; others are several feet, and sometimesseveral yards, in width. Veins are often nar-row in one part, and swell out in another.The vein at the Dolcoath mine in Cornwall varies from two or three feet to forty feet, andin some places it contracts to little more thansix inches. The vein stone is quartz, in which areimbedded masses called bunches of copper py-rites, consisting of copper united with sulphur.

Veins sometimes descend in a directionparallel with the beds of rock in which theyoccur, and swell into large cavities which con-tract again to a narrow string, often so smallthat the ore appears lost; but by pursuing theindications of the vein it is a2;ain found to en-large : these are called pipe veins.

The blue john or fluor spar mine nearCastleton is of this kind. The vein whichcontains this spar is separated from the lime-stone rock by a lining of cawk or sulphat of' . . • barytes,