Dr. Mac CuLLOCH ofi t/je Geology of Glen Tilt. 317 



covering of peat. The calcareous matter is commoniy small in pro- 

 portion to the clay, sand, and other foreign substances which they 

 contain, in so much that they are seldom of any value to the agri- 

 culturist. 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 black- 

 ened 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 soutliern 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 distinqj places ; 



