406 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 diflTusion 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 



