46 



ME. A. HARKER ON THE GEOLOGICAL [Feb. I906, 



pitch stone-complex there is seen in many places a rock which 

 at first sight seems to be quite distinct, though evidently related 

 to the complex above. It shows more or less abundant blocks 

 of the black pitchstone in a soft pale-grey matrix, which is 

 evidently formed by the decay of the pitchstone itself or of 

 its felsitic modification. The weathering-out of this soft basal 

 band has often given rise to a recess at the foot of the pitchstone- 

 escarpment (see PI. Ill & fig. 3, YZ ; also fig. 5, p. 57). It is well 

 seen along the southern face for a distance of 500 yards from the 

 eastward termination of the ridge, its thickness here being usually 

 from 3 to 7 feet, or exceptionally as much as 10 feet. The pitch- 

 stone-blocks embedded in the soft pale-grey matrix range up to a 



Pig. 3. {Key to PI. III.) — The Sgurr of Eigg seen from ilit 

 east-south-east, from near Gahnisdale. 





The lower slopes are made by numerous alternations of basaltic lavas (B) with 

 intrusive sills of dolerite (D). These are truncated obliquely by the thick 

 sbeet of pitchstone (P) which forms the summit-ridge, having a columnar 

 structure in a roughly vertical direction, or locally inclined and divergent. 

 Tbe pitchstone is intersected by thinner sheets of a felsite (F), the devitri- 

 fiecl equivalent of the pitchstone itself, with a generally parallel disposition 

 but deviating in places. 



Along the line YZ is a recess made by the weathering-out of the brec- 

 ciated and decayed basal band of the pitchstone, and below this at Z is the 

 breccia with foreign rock-fragments and fossil wood. 



foot or two in diameter, and exceptionally to 4 feet. Their varying 

 orientation, as indicated by the flow-structure in them, proves that 

 they are not merely relics of an unbroken sheet, but have suffered 

 relative displacement. Sir Archibald Geikie considered this decayed 

 band to represent ' a kind of brecciated base or flow of the main 

 pitchstone-mass,' 1 and this is doubtless its true nature. It is very 

 clear in places where, for a short distance, the base of the pitchstone 



1 Quart. Journ. Geol. Soc. vol. xxvii (1871) p. 307. 



