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196 LIMESTONE FAMILY. Observations. , 1. All compact marls fall into powder when exposed to the air, but some more readily than others. 2. The Florence Marble, or Ruin Marble, as it is some times called, appeal’s to be a very compact marl, inclining to compact limestone. It presents angular figures of a yellowish-brown, on a base of a lighter tint, and which passes to greyish-white. Seen at a distance, slabs of this stone resemble drawings done in bister. “ One is amused (says Brard) to observe in it kinds of ruins: there it is a Gothic castle half destroyed, here it presents ruin ed walls; in another place old bastions; and, what still adds to the illusion, is, .that, in these sorts of natural paintings, there exists a kind of aerial perspective, which is very sensibly perceived. The lower part, or what forms the first plane, has a warm and bold tone; the second follows it, and weakens it as it increases the dis tance ; the third becomes still fainter; while the upper part, agreeing with the first, presents in the distance a whitish zone, which terminates the horizon, then blends itself more and more as it rises, and at length reaches the top, where it forms sometimes as it were clouds. But approach close to it, all vanishes immediately, and these pretended figures, which, at a distance, seemed so well drawn, are converted into irregular marks, which pre sent nothing to the eye.” To the sume compact marl, may be referred the variety called Col lam Marble, from being found at Cottam, near Bristol. It resembles in many respects the Landscape Marble. 3. Sartorius describes a mineral under the name Leu- tritc, which he considers to be nearly allied to Marl, $nd of which the following are the characters :—Its co lours are dark greyish-white ; also yellowish-white, and ochrc*
