Vol. 68.] THE GLEN OECHY ANTICLINE. 177 



riddle when it corned to be more fully investigated, especially in its 

 northern continuation. Meanwhile, we take this opportunity of 

 drawing attention to a feature in the tectonics of the range which 

 may in the future prove to be of decisive importance in solving the 

 problem. 



(D) The Beinn Doirean Inversion. 



The massive garnetiferous mica-schist of the Beinn Doirean range 

 is capped by three important outliers, in which the structural 

 sequence, given in descending order, is as follows : — 



Fine-grained quartzite. 



Banded mica-schist and quartzite. 



Thin calcareous tremolite-schist. 



After careful examination, it seems practically certain that we are 

 dealing here with exactly the same sequence as that which underlies 

 the garnetiferous mica-schist, only in inverted order. It is a very 

 small point of difference that quartzite is found farthel' north in 

 the outliers than in the basal outcrop. The broad exposures of the 

 Banded Series in the outliers are due, not to any excessive thickness 

 of the group, but merely to the form of the ground. 



The question immediately arises : what is the relation between 

 these inverted outcrops and the pebbly quartzite of the ' unclassified 

 schists' on the east? Two alternative explanations suggest them- 

 selves (fig. 4, 176), and we have, as yet, not been able to decide 

 between them. Even a preliminary examination makes it clear, 

 however, that the pebbly quartzite rests either directly, or almost 

 directly, upon the great garnetiferous mica-schist. Accordingly, we 

 followed the approximate juhction of the two, in a hurried traverse, 

 in order to see whether we could identify the thin zones of the 

 outliers in this position. In a cliff-exposure in Beinn a' Chaisteil 

 we found what might stand for representatives of this sequence, 

 including a thin tremolite-schist ; but we prefer for the present not 

 to lay much stress on this coincidence, as our search at other points 

 remained unrewarded. 



IV. Difficulties. 



Our main conclusions and suggestions are sufficiently summarized 

 in the maps and sections which accompany this paper. In the 

 following paragraphs a few important difficulties will be dealt with 

 in succession. 



(a) No definite zone of mylonite marks the outcrop of the 

 Ballachulish Slide in this, any more than in the type region. In 

 explanation it is suggested that sliding, when it occurs under con- 

 ditions such as lead to regional metamorphism, does not necessarily 

 result in the production of mylonite.^ 



^ Mr. Clough draws our attention to the fact that the pre-Torridonian 

 thrusts of the North-West Highlands are not invariably accompanied by 

 mylonites. 



