lOO PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



these rocks just north of Stonehaven ; the most gritlike may possihly 

 be a crushed-np granite, but some of them, I feel sure, have been 

 fine-grained quartzose gneisses very similar to those at Muchalls. 



We are thus led up to understand the peculiar structure of the 

 rocks forming the northern escarpment of the " newer gneiss," which 

 I have already described macroscopically. My remarks, as heretofore, 

 applj^to those near Loch Maree ; but I may add that through the kind- 

 ness of Professor Lapworth and Mr. Teall, I have had the oppor- 

 tunity of examining collections from other districts in the north-west 

 Highlands, to which my remarks, mutatis mutandis^ applj- When 

 examined microscopically, these newer gneisses, as I have said, 

 perplex us by the evidence on the one hand of a fragmental struc- 

 ture, on the other of crystallization in situ. The former structure 

 is most conspicuous in the immediate vicinity of the great overfold 

 fault. A few miles back, the rocks often scarcely differ from a 

 normal mica-schist or quartzose gneiss. Careful examination of a 

 large number of specimens collected expressly for the purpose, illus- 

 trated by materials obtained in the Alps and compared with gneisses 

 and schists collected from the admitted Hebridean series, have con- 

 vinced me that the apparently fragmental structure of the '^ newer 

 gneisses " is due to the crushing in situ and partial recrystallization 

 of a varied series of fine-grained gneisses, mica-schists, &c. not 

 unlike those at Gairloch, and not very different from those already 

 mentioned from the Aberdeenshire coast. The peculiar '''undulate 

 structure," if I may so call it, of the constituents — often indicated 

 especially by the mica, which differences these rocks from those 

 already described — is due to the exceptional character of the pressure 

 to which they have been exposed in the overthrust, which has caused 

 a certain squeezing or dragging-out and lateral shearing of the con- 

 stituents in the direction of apparent bedding. Thus the felspar 

 grains, so marked a feature in some of the varieties, are simply 

 remnants of original felspars ; and this accounts for their similarities 

 to the felspars of the Hebridean series. Some of the mica, but, I 

 believe, by no means all, is of secondary origin ; so also is some of the 

 quartz ; and probably any previous fragments of that mineral have re- 

 ceived additions which are in crystalline continuity with them. The 

 squeezing or rolling-out is especially conspicuous in one or two 

 specimens taken from low down in the series in Glen Logan, the 

 quartz granules having a peculiar dragged-out, clotted aspect, and 

 having their optic axes approximately parallel, so as to produce a 

 marked uniformity of tint when examined with the two Nicol's 

 prisms. 



The correctness of this explanation is supported by the exami- 

 nation of rocks formed by local crushing in the Hebridean series 

 itself; one, discovered by Professor Lapworth and lent to me by 

 Mr. Teall, is so similar, both in the hand-specimen and under the 

 microscope, to some of my specimens from the neighbourhood of 

 Glen Lo^an that, had it been sent to me as coming from the newer 

 series in that locality, I should never have questioned it, and should 

 only have remarked that "it had had a very hard pinch." This 



