!268 Dr. Mac Culloch on the Geology of Glen Tilt, 



of those laminated limestone^ which contain alternate layers of mi- 

 caceous schistus or clay slate. 



There is yet one other disposition of these two classes of rock ; 

 this consists in minute points or fragments, if they may be so called, 

 of the same siliceous matter, inhering in the limestone, and which 

 from their smallness are scarcely to be detected, unless where 

 from having been exposed to the action of water they are 

 found to give a rough and echinated surface to the calcareous rock. 

 This appearance is very widely diffused through the whole extent 

 of the body of limestone hereafter to be described, as forming a 

 great portion of this district, wherever it is found in the vicinity 

 of the great mass of granite. 



The singularity of these appearances renders it proper to dwell a 

 little on them, and to enquire into their connections and probable 

 origin. Saussure indeed has mentioned a transition firom granite 

 to limestone, and, as there are some situations in Glen Tilt where 

 the limestone in contact with granite becomes so siliceous and in-, 

 durated as by decrees entirely to lose its mineralogical, and pretty 

 nearly its chemical character, so it is possible that he may have met 

 with some similar fact, although no very accurate notion can be 

 derived from his account of it and consequently no assistance ob- 

 tained from his observation. 



As I shall have occasion to enter fully hereafter into the minera- 

 logical description of this and of the other rocks which occur in 

 Glen Tilt, and shall then vindicate the term granite, which as a 

 general term I have applied to it, I think it only necessary to say at 

 present that the mass of rock above described as traversing the 

 stratified rocks, is a portion of a more continuous one which may be 

 traced to the hills constituting the right hand or northern boundary 



