PRIMARY LIMESTONE. 365 



In Scotland, it occurs in contact with granite, 

 in which case it is often indurated, and when very 

 impure, sometimes converted into a cherty sub- 

 stance hereafter described. In gneiss, it is found 

 either in strata or nodules ; and these are some- 

 times penetrated by the same granite veins that 

 pervade the gneiss. Where it occurs interstratified 

 with micaceous schist, as well as with gneiss, it 

 is sometimes so intermixed with mica as to be 

 difficultly distinguishable from these rocks, more 

 particularly in the laminar fracture. In the same 

 manner, where it is associated with argillaceous 

 schist, the limestone is often invisible except in 

 the transverse fracture ; the rock yielding in a 

 parallel direction to the stratification, only in 

 those parts where a lamina of schist is interposed. 



Beds of limestone are often bent, and even 

 considerably contorted, as are those of the rocks 

 which it accompanies ; and these flexures are 

 most visible in the varieties which contain mica. 

 They seem to present no variations of structure, 

 but the laminar, which are deserving of notice. 



The texture varies from the highly crystalline, 

 of a larger or finer grain, to the uniformly com- 



