Vol. 65.] THE CAULDRON-SUBSIDENCE OE GLEN COE. 617 



(b) General Distribution and Variation of the 

 Volcanic Series. 



Employing the Coire nam Beith section as a standard, we may 

 now attempt to give in outline an account of the rapid local 

 variations which are characteristic of the volcanic sequence of 

 Glen Coe. To approach this subject in the simplest manner, the 

 usual method of procedure must be reversed and the succession 

 described from above downwards. 



The andesites, rhyolites, and sediments of groups 

 (7) and (6) may be dismissed at once, since, having escaped 

 erosion merely on the southern shoulder of Beinn Ehada, their 

 description has already been included in that of the type section. 



The rhyolite of group (5) occupies much of Beinn Ehada, 

 and also forms an outlier upon the Buachaille Etive Beag. It is 

 likely, too, that it enters into the composition of the contact-altered 

 rhyolitic mass forming the southern end of the Buachaille Etive 

 Mor. It ia everywhere prone to display the brecciated and 

 xenolithic character described in the type section. Its base-line 

 is characterized by an irregular, but moderately high, southerly 

 inclination (PI. XXXIII, Section III). 



The constancy of the hornblende-andesites of group (4) 

 is of the greatest assistance in the task of piecing together the 

 volcanic sequence of Glen Coe. The outcrop of the group is con- 

 tinuous from Stob Coire nan Lochan to the Buachaille Etive Beag, 

 while an outlier forms the median summit of the Buachaille Etive 

 Mor, and flat cakes cap the hill-tops on the two sides of the Cam 

 Ghleann (PL XXXIII, Section I). 



The mass of agglomerate, group (3), coming immediately 

 beneath the hornblende-andesites in Stob Coire nan Lochan, marks 

 an important period during which few, if any, lavaform eruptions 

 occurred. It is possible, indeed, that the deposit is mainly detrital 

 in nature, and not, strictly speaking, a volcanic agglomerate at all. 

 The accumulation is inconstant in character : for, when traced into 

 the eastern slope of Stob Coire nan Lochan, it becomes finer in 

 texture, while hard green siliceous shales make their appearance in 

 the upper portion of the group, and shortly swell to a thickness of 

 100 feet. The shales with bands of grit can be traced along the 

 western slope of Beinn Ehada, but are here much diminished in 

 thickness. 



Eor some distance along the eastern slope of Beinn Ehada, the 

 shales are lost sight of; but, reappearing once more near the head 

 of the Lairig Eilde, they can be followed north-eastwards into a 

 thick mass of agglomerate in the Buachaille Etive Beag similar to 



