Vol. 60.] MOIXE GXEISSES OF THE EAST-CENTRAL HIGHLANDS. 445 



thickness of at least 100 feet. Over a very large area, however, 

 this little sill rarely exceeds 3 feet in original thickness, and it 

 must often have been less, while its outcrop can in many cases be 

 crossed in a single stride. It is, of course, still repeated by folding, 

 but now it and the associated beds fold together as one little group, 

 or ' entity in the folding,' together building up a sheet, and thus at 

 each complete fold of the group both the top and the base of the 

 little sill are exposed in the outcrops. 



Xow, just as this sill, owing to its original hardness, folds on 

 itself, and forms a homogeneous sheet when it thickens, so the 

 Honestones on the margin of the Quartzite form a similar but 

 larger sheet when they not only thicken to the north-west but were 

 composed of harder material originally. Here, however, the change 

 is no longer local, but is maintained over a very large area. 



Later Structures. 



In a typical quartzite-mountain the original isoclinal folding is 

 left, and this structure only is shown on the south-east side of Glen 

 Tilt : but as the line along which the Moine Gneisses set on is 

 approached a remarkable buckling structure is set up in the rocks, 

 conveniently known as ; Moine-structure,' shown on the left of 

 the section. It is obviously impossible to say exactly where this 

 structure ends off underground. 



Considerable light is again thrown on these points by the little 

 sill where folded on itself. Some little distance north-east of the 

 line of section there is a scar of hornblende-schist, aud in this the 

 stages in the formation of the mass can be made out as follows :— 



I. The sill was folded on itself to form a large mass free from 

 infolds of the other material (concertina-structure). 



II. A fine buckling-structure, reproducing in miniature that of 

 the Moine Gneiss, has been superinduced on the older folding. 

 Specimens showing this can be easily found. 



III. A powerful strain-cleavage was set up in the mass, and 

 the cleavage-planes intersect the convex faces of the minute 

 buckles that face the south-east. They never cut those that 

 face the north-west : an important fact, as showing that the 

 crushing movements came from the south-east. This cleavage 

 imparts to the rocks, at first sight, the aspect of a well-bedded 

 mass, with a steady south-easterly dip of some 10 D to 20° ; but a 

 careful inspection of the scar-face already referred to soon shows 

 how complex the structure and history of the rock- mass really is. 

 Thus study of this sill throws great light on the history of the 

 Moine Greisses, which cover so large an area to the north-west. 



EXPLANATION OF PLATES XXX1II-XXXYII. 

 Plate XXXIII. 



Map of the Gilberts-Bridge area, Glen Tilt. In tins the principal small 

 outcrops of the Main Limestone are shown ahont the bed of the Tilt. On the 

 hillside above is a large mass of the same limestone, in a much purer phase, 



