ST. KILDA. GEOLOGY. 55 



Mull, and in Rum. It is composed of compact felspar, 

 varying in colour between yellowish, reddish, and brownish 

 grey; with a very slight intermixture of hornblende, 

 and, occasionally, of quartz. At times it assumes a 

 vague porphyritic character, and on other occasions 

 undergoes further variations of aspect which it is un- 

 necessary to describe, as similar circumstances have 

 been sufficiently noticed in treating of the preceding 

 islands. Extending from the summit of Conochan, it 

 includes the whole of the hill which constitutes the 

 northern boundary of the harbour, descending to the 

 shore in those flat beds already described as the landing- 

 place for boats. It has here, indeed, as is not unusual 

 elsewhere, a tendency to the bedded disposition; the 

 beds being commonly straight and, like granite, divided 

 into huge rectangular masses. A few caves occur in 

 it on the shore, and it is traversed near the bay by 

 two long and nearly horizontal basaltic veins at no great 

 distance from each other, the fragment of a third being 

 also seen near the village : these appear to be about 

 ten feet in thickness. The cliffs formed by this rock 

 are in most places mural in the strict sense of the word ; 

 defying the climbing powers of the natives as well as 

 the lodgment of the sea fowl. I may add that Levenish 

 is also composed of this substance. 



This syenite occasionally presents cavities containing 

 regular crystals of transparent smoky quartz, as well as 

 obscure and imperfect crystallizations of felspar. I per- 

 ceived none of a large size; but Martin, whom, on subjects 

 of this nature, there is no reason to doubt, describes 

 crystals of brown quartz two inches in length as found 

 in it. This is the only instance in which I have observed 

 the constituents of syenite thus crystallized, and like 

 the micaceous variety formerly described in Sky, it 

 presents an example of the near affinity between this* 

 substance and granite. Its resemblance indeed to certain 

 specimens of the granite of Arran is very remarkable ; 



