Br. C. Callaway— Kotes on Metamoiyhism. 221 



the elaboration of every point. In some localities, where the 

 Archaean gneiss has been brought (by folding or thrust) over the 

 Assynt Group, the latter grows progressively metamorphic towards 

 its junction with the gneiss. 



Some of the most interesting examples are found in Glen Coul,' in the 

 Assynt district. This wild valley, walled in on both sides by vertical 

 precipices, and accessible only by dangerous mountain paths, or from 

 the sea, contains some of the most remarkable and sublime geological 

 phenomena in the British Isles. In the perpendicular cliifs of the 

 fiord of which the glen is a continuation, the Hebridean gneiss lies 

 in almost horizontal masses upon the crushed and contorted frag- 

 ments of Ordovician limestone and quartzite. Here, however, 

 although the actual contact between the older and newer groups can 

 be seen at several points, the alteration of the Ordovician is very 

 slight. The probable cause of this I will notice further on. The 

 phenomena to which I call special attention occur about a mile up 

 the glen, just under the escarpment of Caledonian gneiss, where it 

 rises into the peak called the Stack of Glen Coul. 



About fifty yards from this gneiss, the quartzite, dipping easterly, 

 begins to lose its distinctive character. It grows flaky, as if 

 affected by a rough cleavage ; the grains of quartz become indistinct, 

 but on careful examination they can be seen to be flattened out ; 

 and the rock acquires a glazed appearance, as if by the introduction 

 of a new mineral. The alteration, however, has not obliterated the 

 " worm-holes," which are quite distinct, though greatly distorted 

 and lengthened out, as if by a force pushing from the east. The 

 following is Professor Bonney's account^ of the rock under the 

 mici'oscope : "Chiefly quartz (fragmental) and sericite (?), evidently 

 much compressed, as shown by the flattening-out of the grains." 

 A second specimen ^ is taken from about the same distance from the 

 Caledonian, but perhaps a little further off. The rock is from the 

 lower part of the quartzite, and displays clearly the characteristic 

 grains of felspar. The alteration does not appear to be so far 

 advanced as in the first example. The microscopic structure is thus 

 described : — " It consists of quartz and felspar, .... separated by 

 thin films of a micaceous mineral, moi'e or less dotted with opacite. 

 The structure is undoubtedly fragmental ; the rock has undergone 

 great compression, the fragments being crushed, flattened out, and 

 ' packed ' together, as one sees in slates." 



Towards the contact with the Eastern Gneiss, the alteration 

 increases, and, at the contact, nearly every trace of a clastic structure 

 is obliterated, and the rock assumes the appearance of a quartz-schist. 

 Indeed, when I first saw it, I supposed it was a band at the base of 

 the Caledonian, and so described it in my note-book. A similar 

 change is to be seen also in the Erriboll district. We will take one 

 example. In Ben Arnaboll, the quartzite is folded in with the 

 Arnaboll gneiss, and the syncline is bent back to the west, so that the 



1 Ibid. pp. 373, 378, 384, 390. 



^ See his appendix to the above paper for this and the following descriptions. 



3 In Prof. Bonney's Notes, p. 418, jSTos. 97 and 102 should be transposed. 



