Dr. Mac Culloch on the Geology of Gkti Tilt. 289 



of steatltic slaty marie, very friable, of a snow white colour and 

 great specific gravity. 



Associated with this limestone and in a similar position, I also 

 discovered beds of sahlite, a mineral of considerable rarity in thiS 

 country. These vary in thickness from two inches to a foot 

 and more, and are sometimes intimately mixed with thin laminse 

 of the fibrous variety of tremolite. This mineral, from its superior 

 hardness, resists the action of the water of the river better than 

 the limestone does with which it is interstratified, and may there- 

 fore be distinctly traced projecting from those beds of which the 

 calcareous portions have been more or less worn away. 



The beds of sahlite may be seen extending for a space of from 

 fifty to a hundred yards along the bank of the river at the same 

 place. Considerable variety of aspect occurs throughout their 

 course, from a greater or less intermixture of calcareous matter with 

 the pure sahlite,. Thus, like the more common specimens of this 

 mineral it sometimes effervesces with acids, while in other cases 

 it appears to contain no uncombined calcareous matter. When 

 it is unmixed it resembles at first sight the whiter varieties of 

 felspar, exhibiting a large platy fracture, and a similar lustre. 

 In the angle of its fragment as well as in its specific gravity, 

 hardness, and habitudes with the blow-pipe, it perfectly resembles 

 the better known varieties of this mineral. As it becomes more 

 and more mixed with the calcareous matter its platy fracture first 

 becomes less regular and gradually disappears, the mixed mineral 

 however still retaining a specific gravity greater than that of the 

 neighbouring limestone. Its sp. gr. varies from 3,153 to 3,246, the 

 most purely lamellar being the lightest. It is invariably white with 

 a slight tinge of grey.* 



* I. may here add that I. have foundih'is mineral in Glen Elg, and in a similar situation, 

 but exhibiting difierent colours, the predominant variety being snow-white, and the more 

 rare one green : I have also found it in great variety in Tiree and Harris. 



Vol. hi. 2 o 



