288 Dr. Mac Culloch on the Geology of Glen Tilt. 



bed of two or three feet in thickness. In its general aspect It so 

 nearly resembles a granular quartz as to be at first sight difficultly- 

 distinguished from it. As it produces no effervescence with acids, 

 it probably contains no great mixture of carbonate of lime, and may 

 therefore be considered as a granular tremolite. It may be distin- 

 guished from quartz by its superior lustre, translucency, and specific 

 gravity, as well as by its inferior hardness. The sp. gr. of the 

 specimens which I tried was 2,840. 



A fifth variety is found in the same place, but without so very 

 marked a character as that of the one preceding. It is foliated, with 

 a slight tendency to a fibrous structure, and forms large concretions 

 in the marble. Its aspect is vitreous, and it is of a greenish grey 

 colour. 



Another modification of this beautiful mineral which occurs here 

 can scarcely be considered of sufficient importance to constitute a 

 mineralogical variety. It splits into thin lami.isB, of which the flat 

 surface has the splendour and almost the uniform smoothness of 

 white satin, but on examination it is seen to consist of very minute 

 fibres of tremolite interfoliated with very white and compact car- 

 bonate of lime. Some specimens are again found which seem to 

 consist of a mixture of tremolite and schiefer spar, a substance which 

 is also found although but rarely, among the calcareous beds. 



The various forms indeed under which the several mixtures of 

 tremolite with the surrounding rocks occur are such as to elude 

 description, and not a little to puzzle a mineralogist. I shall only 

 enumerate two more of the most remarkable. One of these is a 

 compact schistose rock in which the union of the tremolite with the 

 limestone is so intimate that it can only be distinguished in the 

 first instance by its hardness and specific gravity, as the eye cannot 

 detect the mixture. In the other the tremolite is mixed with a kind 



