Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/53

Rh with crystallized calc spar. As the texture becomes more compact it gets harder and of a more intense colour, approaching to black; the fracture at the same time approaching to conchoidal. Iron pyrites dispersed through the mass is not uncommon, and when scratched the smell of sulphuretted hydrogen is rendered sensible: it burns very white, a circumstance which shows that the colouring matter, being volatile, is of an animal or vegetable origin. In diluted muriatic acid it makes a rapid and brisk effervescence, leaving rather a considerable residuum. The mean specific gravity, as resulting from eight specimens, I found 2,704.

I am not aware that the dark marble-limestone of Pool-vash contains organic remains; I have seen stripes or veins of lamellar and greyish limestone full of petrifactions alternating with the darker variety that remained quite freed from those exuviæ.

Thin and crumbling strata intervene between the solid strata of limestone. They are called Soles in the Isle of Man, but generally Partings among the miners.

The magnesian limestone in the places where it occurs, differs enough in itself to induce me to give a particular description of it. At Cass-ny-Hawin it is either finely granular or in lamellar and well determined crystals. In the latter case it abounds with small cavities filled with crystallized rhomb spar, the crystals of which

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