170 MK. E. B. BAILEY AND MR. M. MACGREGOE Olf [June I912, 



The flag group consists of well-bedded quarfczo-felspathio 

 gneisses, in which there is a constant alternation of more and less 

 micaceous layers. In places small pebbles of quartz or felspar 

 have been recognized, but it is fairly certain that the original 

 impure sandy sediments, from which the gneissose flags have 

 been derived, were essentially fine-grained. The flags occurring in 

 Beinn Udlaidh resemble exactly the Eilde Flags of the Loch Eilde 

 district east of Loch Leven, and we have accordingly placed them 

 in this group. Eocks of like character occur, almost uninter- 

 ruptedly, between Beinn Udlaidh and Loch Eilde ; although, since 

 they are crossed by the Glen Strae Eault and also by the Glen Coe 

 Eault with its accompanying Eault-Intrusion [4, p. 611], their 

 outcrop cannot properly be described as continuous. Over 500 feet 

 of flags are exposed in the slopes of Beinn Udlaidh, and the group 

 is certainly very thick indeed. 



The succeeding division consists of fine-grained, white, highly 

 siliceous, thinly-bedded quartzite. In Beinn Udlaidh it varies 

 in thickness from about 50 to 100 feet ; but it is much thinner 

 south of Glen Lochy, where it averages about 15 feet — and for 

 a short distance north of Lochan Bhe (pronounced Yay) it is 

 entirely absent. Apart from its small thickness and its thinly- 

 bedded character, this quartzite agrees very closely with the 

 Glen Coe Quartzite of the type-locality. Since, moreover, the 

 sediments associated with it on either side are precisely such as 

 accompany the Glen Coe Quartzite, we feel great confidence in 

 referring it to this well-known horizon. 



In certain outcrops previously described in connexion with the 

 Glen Coe district, among them that situated near Loch Dochard, 

 which is included in the map (PI. X) accompanying the present 

 paper, the Glen Coe Quartzite is very thin, and in some cases 

 altogether absent. Such occurrences, up to the present, have all 

 been explained as a result of sliding^; but we hesitate, without 

 local evidence, to extend this interpretation to account for the 

 thinness of the Glen Coe Quartzite in Beinn Udlaidh. It seems 

 equally probable here that the feature may be an original sedi- 

 mentary character. 



A banded passage-zone links together the quartzite and 

 mica-schist groups of Beinn Udlaidh. It is very well exposed 

 in the northern spur of the mountain crossed by Section C (figs. 2 

 & 3, pp. 168 & 169), and also at the side of the road which runs along^ 

 Glen Orchy (Section H). In the latter locality the Banded Series, 

 as we may call it, consists of intercalated quartzite and grey mica- 

 schist, with thin black seams of politic material. An 

 exactly similar banded series links the Glen Coe Quartzite of the 

 Loch Leven and Glen ISTevis district with the Leven Schists. 



The main mass of the mica-schist group of Beinn Udlaidh 

 consists of well-crystallized, homogeneous, grey, politic mica- 

 schist, often studded with conspicuous garnets. Garnets are well 



^ Slide is used throughout this paper as a synonym of fold-fault 

 [1, p. 593]. 



