Plumbago in the Rock

This substance is found in the form of pebbles in rocks, making Pudding-Stones, Amygdaloids, and a sort of Traps, and seems to have been but little understood by mineralogical writers.

The rock is seldom schistose or slaty. It is sometimes veined with red Carbonate of Lime; sometimes it consists of a grayish or greenish compact steatitic or magnesian mixture, including the Plumbago, very intimately mixed, or in powdery-looking particles, minute pebbles, and other forms, some resembling fragments, in common with particles of yellow Pyrites, pebbles of Quartz, &c., tightly or loosely included^*.

The upper figure represents an amorphous spotted mass with the Plumbago, Pyrites, and Quartz pebbles.

The lower figure has much the resemblance of common dark Limestone, and is much impregnated with Plumbago, a small mass of which lies on the surface.

I have a specimen of somewhat largely foliated Plumbago holding small cubic yellow Pyrites, varying from a bright colour to different states of decomposition, but not passing towards Plumbago. I have some Plumbago in a stratified piece of Quartz, coloured by red Oxide of Iron: indeed the rock it is found in varies much.