Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/66

 54 raised, on an annual average, fifty or sixty thousand tons of rock-salt. The greater part of this quantity is exported to Ireland and the Baltic: the remainder is employed in the Cheshire district in the manufacture of white salt by solution and subsequent evaporation.

It is very doubtful whether in any instance the body of rock-salt can be considered as stratified, or disposed in distinct layers. A perpendicular section does sometimes indeed present irregular appearances of this kind, and more especially in the purer part of the lower bed, but the great body of the rock offers to the eye merely a confused red mass, varied here and there by the occurrence of the crystalline portions of salt.

One of the most striking facts connected with the internal structure of the Northwich rock-salt, is the appearance observable on the surface of an horizontal section of the rock, as viewed in any of the mines. On this surface may be traced various figures, more or less distinctly marked, and differing considerably in the forms which they assume; some appearing nearly circular, others perfectly pentagonal, and others again having an irregular polyhedral form. The lines which form the boundary of these figures are composed of extremely pure white salt, forming a division between the coarse red rock exterior to the figure, and the equally coarse rock included within its area. These bordering lines or rims vary from two to six inches in width. The figures themselves differ greatly in size; some of them being less than a yard in diameter, others as much as three or four yards ; and they very frequently are observed, one within another, gradually diminishing in size to a centre. Professor Playfair, in his Illustrations of the Huttonian Theory, has stated, that the compression of these figures is always mutual; the flat side of one being turned to the flat side of another, and never an angle to an angle, nor an angle to a side. This remark, as far