180 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



are chiefly plagioclase. The dull grey felsitic bands show under the microscope a more 

 thoroughly devitrified ground-mass, with the minutest depolarising microlites, large 

 porphyritic crystals of plagioclase and sanidine, grains of augite, and sometimes 

 exceedingly abundant particles of magnetite. 



That the rocks of the Scuir of Eigg are the products of several eruptions is manifest 

 from their arrangement in distinct beds, and from their variation in petrographical 

 character, and that they were poured into the channel of a stream which had been 

 eroded out of the surface of the plateau-basalts, is proved by the survival of the shingle 

 and drift wood of that channel below the bottom of the pitchstone. The general relations 

 of the rocks are impressively presented on the west side of the island, where the rock of 

 the Scuir is abruptly cut off by the face of the sea-precipice, which is some 500 feet high 

 (fig. 63). 



Fig. 63. — Natural Section of the Sea-cliff at the south-west end of the ridge of the Scuir of Eigg. a, Bedded 

 basalts of the plateau; b, basic dykes; c, gravel of old river-bed; p, Pitchstone of the Scuir. 



At both ends of the pitchstone ridge, we learn that the dykes of fine-grained basalt 

 which traverse the bedded basalts, are older than the ancient Tertiary river-bed of the 

 Scuir, for the river has not only eroded the bedded basalts but has cut down into the dykes. 

 There is thus evidence of enormous denudation of the surface of the basalt-plateau 

 before the final volcanic outbursts, and of the comparatively late date of the pitchstone. 



The sinuous ridge of the Scuir still marks the winding course of the stream whose 

 channel the pitchstone effectually sealed up and preserved. Several minor spurs, which 

 project from the eastern side of the main ridge, show the positions of small tributary 

 rivulets that entered the principal channel from the slopes of the basaltic tableland. 

 One of these, on the south-east side of Corven, must have been a gully in the basalt with 

 a rapid or waterfall. The pitchstone has flowed into it, and some of the rounded pebbles 

 that lay in the channel of this vanished brook may still be gathered where the degradation 

 of the pitchstone has once more exposed them to the light. 



