164 HARRIS. GEOLOGY. 



require the aid of the tool to render them fit for masonry. 

 They are all so loose as to be readily moved by the 

 lever, and would form an admirable quarry for archi- 

 tectural purposes where large masses were wanted, should 

 any demand for sucl\ works occur in a situation where 

 they could be conveniently obtained. In Roneval* also 

 the gneiss, as in many places already described, loses, in 

 part or entirely, the laminar character, becoming thus 

 scarcely distinguishable from granite. Hornblende is 

 here abundant, and in the same specimens are numerous 

 large garnets often possessing an imperfect degree of 

 transparency. In some instances the garnet is not se- 

 parately crystallized, but so equally diffused through the 

 rock as to give the whole a dark crimson blush. Among 

 the veins of granite are found beautiful examples of the 

 graphic variety, of which the felspar is white, translu- 

 cent, and nacreous, acquiring after exposure an argen- 

 tine brilliancy. 



Trap veins, or indications of them, occur in Harris, 

 but they are not abundant. The most remarkable which 

 I observed is on the top of Roneval, being 20 feet wide, 

 vertical, and lying in a north westerly direction. 



At the foot of Roneval a low irregular ridge of 

 limestone is found extending in an interrupted manner 

 for a mile or more towards Loch Rowdill. A church 

 of ancient and tolerable architecture, formerly the church 

 of the monastery of Rowdill, and one of the very few 

 buildings which time and the Scottish reformation have 

 spared, will guide the mineralogist to the spot where 

 it is most easily observed.f It lies among the gneiss, 



* The level surface of this mountain presents a singular instance 

 of the force of the wind ; the fragments that cover it, often of con- 

 siderable size, being disposed in prolonged ridges divided by distinct 

 furrows in the direction of the prevalent western gales ; the beginning 

 of each ridge being determined by the shelter of some fixed rock. 



I The church of Rowdill presents some peculiarities in sculpture 

 well worth the notice of an antiquary, and from their analogy to 



