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and their high value in ornamental work, their effect, when used as building stones on a large scale, is not pleasing. A house built of a dull purple, a reddish, or a greenish stone, has always a heavy and gloomy aspect, which ought to be chased away as much as possible from the abodes of men. When thus used in large masses, the special beauties of porphyries, which only appear on close scrutiny, are lost; but, on the other hand, w T hen judiciously used for ornamental purposes, or for relieving by contrast large surfaces of light-coloured building material, these stones then assume their true place in archi tecture. It must also be recollected, that porphyries require a polished surface in order to the full display of their colours. This polishing process is necessarily beyond the reach of most individuals, and it is therefore only in their rough state that they can be employed for the walls of houses and larger buildings, a use to which, as already remarked, they are not well adapted. The special use, therefore, of the more beautiful varieties of porphyry is for ornament, or relief in buildings of freestone; while the commoner kinds furnish valuable paving stones, or road-materials, and may be used where resistance to wear and tear are the main purposes for which they are em ployed.
