go PROCEEDINGS OV THE GEOLOGICAL SOCIETY. 



acted roughly at right angles to the planes of original stratification ; 

 (8) that when the basement-breccia of the Torridon Sandstone was 

 formed, these rocks had arrived substantially at their present 

 mineral and structural condition. 



Proceeding now to the upper end of Loch Maree, I shall facilitate 

 my description by referring to my paper read in 1880. In this I 

 contended that the so-called Logan Eock was not an intrusive mass, 

 but a portion of the Hebridean gneissic series brought up by 

 faults. I also maintained that the alleged unconformity in Glen 

 Docherty between the older and newer gneisses did not exist, and 

 that the latter series, though exhibiting in places a fragmental 

 structure, was rightly called metamorphic. All this my recent 

 examination has confirmed. But in one theoretical conclusion I 

 went completely astray. As the evidence for separating the " newer 

 gneiss " between Glen Logan and Glen Docherty had broken down, 

 and as the fragmental structure which I observed in it seemed to 

 disappear as the rocks were followed southward, I concluded that 

 Sir Roderick Murchison was right in maintaining a progressive 

 metamorphism in this direction. At that time, however, I thought 

 that possibly the whole series might turn out to be Archaean ; but 

 this solution of the difiiculty was soon demonstrated to be impossible, 

 and further study of specimens from this and other districts, viewed 

 in the light of my alpine work of 1883, caused me to suspect what 

 would be the explanation of the puzzle *, namely, that the frag- 

 mental structure was not a record of original deposition, but of a 

 subsequent crushing in situ of crystalline rocks. In the hope of 

 clearing up this point, I worked (last autumn) up the eastern arm 

 of Glen Logan to the back of Ben Fyn, and on another day, from 

 near the little gorge where the stream cuts through the so-called 

 syenite, I ascended the cliffs to the upper moorland and summit of 

 Craig Boy, whence I descended to the high-road in Grlen Docherty, 

 near the seventh milestone from Achnasheen. I thus examined as 

 continuous a series of outcrops of the " newer gneiss " as was 

 possible, and collected a number of specimens which I have since 

 studied with the microscope. I have now no doubt that here, as in 

 the districts further north, the apparently bedded structure cannot 

 be relied upon as the result of original deposition, but is to a 

 great extent, if not wholly, a record of the efi'ects of earth-move- 

 ments upon rocks originally crystalline. These have been crushed 

 and, as it were, rolled out, as the whole mass has been thrust 

 forward and upwards over the broken surfaces both of older 

 crystalline and newer sedimentary rocks, which once formed parts 

 of a great and perhaps complicated overfold. At the same time, 



* When I wrote the lithological notes to Dr. Hicks' s and Dr. Callaway's 

 papers on N. W. Scotland in the winter of 1882-3, I was still, as may be seen, 

 is a fog about the significance of the fragmental structure in this peculiar 

 " upper " or " slabby" gneiss, though I was becoming almost daily more 

 impressed with the importance of the effects of subsequent crushing. If, for the 

 grovip numbered I., and there supposed to be the newest chronologically of the 

 three, as retaining traces of an original clastic structure, we read " No. II. or III. 

 crushed in situ,'' all will be clear. 



