Ch. VI.] PRIMARY ROCKS. 1 ] 7 



mine), on account of its being metalliferous. And it is, in- 

 deed, worthy of observation, that the contents of the Cornish 

 veins are commonly arranged in a brecciated manner, con- 

 sisting of quartz and of angular pieces of the adjacent rock ; 

 whilst the conglomerated structure is of rare occurrence, but 

 has been observed in veins traversing the granite of St. Just, 

 as well as in those which are situated in the slate. 



A remarkable instance of this kind was seen by Macculloch 

 in the island of Coll, near Ben Feoul. Distinct beds of mica 

 slate alternate, there, with gneiss : one of these abounds in frag- 

 ments of quartz, so as to form a breccia. * Whether a similar 

 kind of rock in Rasay is really a variety of gneiss, or a true 

 fragmentary rock, is not very apparent. " It entirely consists 

 of fragments of gneiss, broken and reunited with very little 

 change of character : it can scarcely be separated from the 

 principal rock." In the same situation a breccia " is formed 

 of dark red felspar and hornblende-schist equally mixed in 

 distinct fragments, and producing, from the contrast of colour, 

 a very peculiar appearance." f 



At Ben-na-chie, in Aberdeenshire, says Dr. Macculloch, 

 the quartz-rock in contact with the granite exhibits an in- 

 teresting appearance. " The regularity of the stratification is 

 disturbed, and, in many places, the strata are fractured and 

 displaced. Where the fractures are considerable, the parts 

 are sometimes reunited by minuter fragments and by crys- 

 talline quartz ; and, in many places, these fragments are so 

 numerous, that the whole mass forms a breccia, or an angular 

 local conglomerate of a peculiar character." J 



The gneiss, near Formo, in Norway, says Von Buch, rises 

 to a great thickness ; and in the Rostenberg, and in the ravine 

 towards Lessbe, it becomes very remarkable in its composition. 

 The gneiss, here, generally abounds in mica, which is not scaly 

 foliated, but in large continuous folia ; and it also contains 



* Western Isles, vol. i. p. 63. t Idem, p. 247. 



f Geol. Trans. New Series, vol. i. p. 58. 



I 3 



