Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/416

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Dr. Mac Culloch on the Geology of

are purely accidental. An examination of many specimens of the latter in my possession, has proved the existence in it of variations in regard to lustre, and other characters, precisely similar to those I have noticed in the stone of Rum. In these too, the fracture is now and then waxy, flat, and dull; occasionally it is conchoidal, shining, and highly translucent, but it never approaches to the earthy aspect of true jasper. The irregular diffusion of the green earth is also equally remarkable in some of the oriental specimens, since among those which have fallen under my examination the green colour is often diffused in partial stains through a pure chalcedony, so that the distinct grains of green earth are discernible by the lens. These also being accompanied by the blood marks esteemed characteristic of this stone, leave the determination of its name free of doubt.

We may therefore conclude, that the green stone of Rum is the true heliotrope of lapidaries, and that this stone is merely a mixture of green earth with chalcedony, its external characters varying either from the variations in the quality of its base, or from the quantity of the green matter with which it is combined.

I did not perceive among the green chalcedonies of Rum any specimen resembling the plasma of the Italians, unless perhaps that in some of them small parts of a more remarkable transparency might occasionally be traced. Yet, as among specimens of the real plasma which I received from Egypt, there are green spots in the stone, resembling minute crystals of chlorite, independent of the general green stain which pervades the whole, I think it not unlikely that this substance is also the colouring matter of plasma, and that the only difference between that mineral and heliotrope, will be found to consist in the different transparency of the siliceous stone which receives the colour. It is not therefore