covering of peat. The calcareous matter is commonly small in proportion to the clay, sand, and other foreign substances which they contain, in so much that they are seldom of any value to the agriculturist. A few which I examined were found to contain from ten to fifty per cent. of carbonat of lime; and independently of the clay and sand which enters into their composition, they are usually blackened by a mixture of half decomposed and carbonized vegetable matter. It is easy to perceive from the flatness of their surfaces and their tolerably uniform thickness, that they have been formed at the bottom of water in lakes of different dimensions, which have been gradually obliterated, partly by the influx of earth, and partly by the growth of those well known vegetables which have covered them with their present stratum of peat. In the instances which I have had an opportunity of examining, the shells from which this calcareous matter has originated have been either so thoroughly decomposed, or from their tender structure so mutilated and broken, that I have never been able to collect a specimen capable of being ascertained. Other mineralogists however have examined the shells found in these beds, which were long since known to the late Dr. Walker, and of which an account has been given in some of his works.
The formation in question is of a nature entirely different, and has never yet been noticed by mineralogists: its novelty at least renders it a matter of some interest.
Where the great limestone bed, which I have described as occupying the southern side of Glen Tilt, is about to meet the quartz rock, it forms a range of small abrupt faces or scarps extending in an interrupted manner for perhaps a mile. These may vary from ten to thirty feet in height. In wet weather small streams fall in cascades over them in two or three distinct places;
