136 POSITION AND NATURE OF [Ch. VII. 



descend to the junction, with the schistose rocks, in the 

 vicinity of the Tilt. In this place, as in many others, the 

 different kinds of granite are found together, not forming 

 veins nor distinct masses, but graduating into each other by 

 an indistinct transition. The granite, at its junction with 

 the schistose rocks, consists chiefly of red felspar and white 

 quartz, of which the latter is generally in the smallest propor- 

 tion, the compound mass containing obscure crystals of dark 

 green hornblende. These are seldom in large quantity; 

 more commonly they are thinly dispersed through the rock, 

 and not seldom, even in the largest masses of rock, they 

 are altogether absent. Mica has not been observed in the 

 granite at its junction with the slate. 



Amongst the stratified rocks, limestone is the most extensive 

 and the most characteristic of this district. 



The great mass of the limestone, which forms nearly the 

 whole of the left boundary of Glen Tilt, is of a lead blue 

 colour of various intensity, and its texture is almost universally 

 large-grained and highly crystalline : here and there, how- 

 ever, it presents some beds of the most beautiful ornamental 

 marbles which Scotland has yet produced. These marbles 

 are seldom pure, at least to such an extent as to be adapted 

 for the purpose of statuary, but are combined in various pro- 

 portions with a great variety of minerals. Mica is more fre- 

 quently present than any other substance, and, near the 

 junction of the Tilt with the Garry, this mineral is so abundant, 

 that the rock may be easily mistaken for micaceous schist : 

 it has a foliated structure, owing to the mica and calcareous 

 matter being disposed in alternating laminae, the latter rarely 

 exceeding the twentieth of an inch in thickness. Besides 

 mica, this limestone is mixed with steatite, serpentine, talc, 

 asbestus, tremolite, and various other minerals, which are dis- 

 posed in the form of veins or patches ; or, by their intimate 

 union with the limestone, produce mottled, clouded, and 

 otherwise variegated marbles. 



Such is the nature of the limestone at a distance from the 



