GEOLOGICAL FORMATION 63 



Centre of a synclinal basin. Finally, the northern end 

 of the island has an anticlinal structure; and a mass of 

 gneiss, presumably of Lewisian age, occupies the centre of 

 the fold," e.g. to the north of Balanahard Bay and at 

 Sgeir Nic Fhionnlaidh, etc., "which has a north-easterly 

 trend." * 



Igneous Rocks. Scalasaig granite, already referred to, 

 is the largest mass of igneous rock in the island. It is a 

 diorite, and is described by Professor Geikie as a "coarsely 

 crystalline rock of a very hard, tough, and durable character. 

 It forms a handsomely marked rock the pale and dark- 

 coloured minerals being in about equal proportions and 

 might be advantageously employed as an ornamental 

 building -stone. For structures requiring great strength 

 hardly a better stone could be desired, as its crushing 

 power must be very considerable." It was locally used in 

 the construction of Scalasaig pier. Syenite and kentallenite, 

 other granitic rocks, appear in Balanahard the former 

 above Slochd-a-Chroinn, and the latter in the vicinity of 

 Cnoc Ormadail. Kentallenite is a particularly interesting 

 rock, taking its name from Kentallen, where it has been 

 wrought for years as " the black granite of Ballachulish." 

 There are four smaller plutonic masses two in the 

 northern part of Balanahard, one in Lamalum, and one in 

 Aoineadh-nam-Muc. Lamprophyre dykes of widely different 

 ages are numerous in the north of the island, while basalt 

 dykes (Saor-an-Dao) of Tertiary age are to be met with in 

 the south. 



Glaciation. Viewed some distance off, the hills of the 

 island present certain flowing and undulating features which 

 geologists inform us are characteristic results of glacial 



1 The Two Earth- Movements of Colonsay, by W. B. Wright, B.A., 

 F.G.S. 



