Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/414
The channelled surface is evidently produced by the action of the winter rains, which have given the whole a striking resemblance to the surface of a mass of ice, which had been partially thawed and again frozen.
The general colour of the exposed surface is greyish white, with here and there a tinge of pale reddish brown, from the colouring matter of the superincumbent bed of clay. Towards the extremities of the mass of salt, extremely thin layers of a pure and plastic clay are insinuated between layers of salt, so as to give it the waved delineations which often occur in some species of calcsinter. The general mass of salt is however of the greatest purity; and in order to be converted into snow white culinary salt requires no other process but grinding. The greyish hue of the external surface is owing to the rain penetrating a portion of the salt, and by diminishing its opacity, depriving it of the whiteness which the fresh fracture generally presents. At the period of my visit the surface of this immense mass was perfectly dry, and in some places, where water had most recently flowed, was covered with a snow white efflorescence. This circumstance, as well as the sharpness of the edges above mentioned, shew the little hygrometric water in the atmosphere of that country, and the general purity of the salt from earthy muriates.
The fracture of the salt is highly crystalline, and usually exhibits large granular distinct concretions, which give it sometimes the appearance of a breccia, or of containing imbedded crystals
A perennial brine spring flows at the foot of the great precipice, and affords a strong proof of the little effect of water on this very compact salt. The aperture through which the stream has issued for many years, is not wider externally than two feet, and suddenly contracts to a few inches; while the channel worn in a solid floor of salt, through which the stream has long flowed, is not a foot in