Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/292

 white marble which I found in Glen Fernat, not far from the junction of the Brerachan with the Airdle. The well known ancient green marbles owe that colour to the same admixture, which indeed in the Egyptian green and the Verde antico, is such that the calcareous matter is overpowered by the serpentine. The aspect of the green marble of Glen Tilt is however perfectly different from that of any marbles ancient or modern which have yet been wrought, and it offers therefore a valuable addition to the arts as well as a new commodity to the list of our mineral productions. I may add that, with a similarity of composition, many specimens bear a considerable resemblance both in character and colour to some varieties of the Cipolino of the Italians. Two other distinct varieties of marble occur where the calcareous beds terminate. The one is of an uniform ochre yellow, but of a much paler tint than the giallo antico; the other is of a flesh colour graduating into dark blue, but neither of these beds is of great magnitude.

The extent of the limestone which I have now been describing is considerable. The mass is of great thickness, and from the obliquity of its section a very large horizontal surface is exposed. As the beds run towards the north they may be traced to a considerable distance up the hill on the right of the river, but dipping to the south they plunge into the opposite hill and disappear. Independently of the steatitical matter which is mixed with this rock, some interesting minerals are found imbedded in it. The beds where they come in contact with micaceous schistus contain so much mica that this latter becomes at length the predominant substance, and there is then a perfect transition from micaceous schist to marble. In these cases it is raised in thin flags of great extent; between these flags there are often seen beds of steatitical clay, of a pure white or greenish hue, and often of considerable thickness. In the