- Of an uniform dove colour, very compact and translucent, assuming an excellent polish.
- Pure white, of a large platy fracture, and translucent appearance, capable of being used in plain ornaments, but too transparent for sculpture.
- White, with irregular yellow stains, from serpentine intermixed; extremely fine, compact, and fine grained splintery.
- Intersected by thin laminæ of schiefer spar, of an exceedingly splendent appearance.
My object in laying before the Society the series of specimens collected in Isla, is the correction of some mistakes which have been entertained about the nature of its limestone district; a district of some importance in more than a speculative view, as it forms a mining field in which lead ores have been wrought to a considerable extent. My researches in Isla having been limited to this particular part of it, I shall confine myself to that which I have seen, referring to Mr. Jameson's book for such further information as may be found in it. I may, however, venture to point out this island as a spot in which the very important circumstance alluded to in another place, of the connection of the quartz rock with the mica slate, and clay slate, might be studied to great advantage. This deduction is not drawn from my own observations, which were not sufficiently numerous to enable me to decide on a question so important, but from the narrative of Mr. Jameson, who appears to describe frequent alternations of those different classes of rocks. Extensive and correct observations on Isla, would probably determine the true relative æra of the quartz rock, and