Vol. 62. ~] GEOLOGICAL STRUCTURE OF THE SGURR OF EIGG. 



45 





in the face of the wall. 

 This rock differs from 

 the pitchstone chiefly in 

 having a compact stony 

 groundmass instead of 

 one largely made up of 

 glass, and the two rocks 

 are beyond doubt closely 

 related to one another. In 

 places there is indeed, as 

 we shall notice, a gradual 

 transition between them. 

 The felsitic sheets come 

 in at various horizons in 

 the complex, but are 

 more abundant in the 

 lower parts. They are 

 best seen in the higher 

 eastern portion of the 

 ridge, and especially in 

 r _ i its southerly face. They 

 are not continued inde- 

 finitely, but die out in 

 the pitchstone. The 

 a £ latter rock, so far as 

 I© any visible evidence goes, 

 a S may be a single mass 

 © -| from top to bottom. 

 §■"§, The pitchstone and 

 g its felsitic modification, 

 taken together, are 

 clearly the youngest 

 igneous rocks in this 

 neighbourhood. Not 



only are the subjacent 

 basalts and dolerites 

 obliquely cut off at the 

 face of junction, but also, 

 as Sir Archibald Geikie 

 remarked, certain basalt- 

 dykes, which intersect 

 these lower rocks, ter- 

 minate abruptly at the 

 same surface. Further, 

 the ' plateau '-faults, 

 which displace the basalts 

 and dolerites, do not 

 affect the pitchstone. 



At the base of the 



