IN THE NORTH-WEST HIGHLANDS OF SCOTLAND. 



389 



foliation must have been developed after the consolidation of the 

 igneous rocks and after the segregation of the pegmatites and 

 other veins. 



Of equal moment is the evidence supplied by the existence of 

 thrusts-planes, oblique foliation, and overfolding in these original 

 gneisses. When the thrust-planes are traced along the face of a 

 cliff, it is observable that they truncate folia lying at an oblique 

 angle to them, thus producing a phenomenon resembling current- 

 bedding. On the surfaces of these movement-planes there is no 

 trace of brecciation, the line being sharply defined after the manner 

 of the Post-Lower-Silurian thrusts in the Moine schists. The 

 oblique foliation viewed in connexion with the prevalent over- 

 folding indicates a gradual movement and piling up of the materials 

 as the Plutonic rocks underwent enormous pressure. Further, it is 

 possible to detect on the foliation-surfaces subparallel lines in- 



Fig. 1. — Section of origincd Arclicean Gneiss^ sliowing Thrust-planes, 

 oblique Foliation^ and Overfolding. 



t, t. Thrusts. 



The figure represents an area of several hundred square yards, the plane 

 being vertical, and the observer facing the south. 



dicating the direction of the movement of the particles over each 

 other. The latter, however, are by no means so common in the 

 Original gneisses as in those which have been affected by the 

 later Pre-Cambrian movements to be described presently. From 

 these data it would appear that even the first or original foliation 

 of the Plutonic rocks was produced by mechanical movement. 



4. Igneous Rocks injected into the Archcean Gneiss after the first 

 Foliation and 'prior to the later Pre- Cambrian Movements. 



After the development of the first foliation, the original 

 gneisses were pierced by a remarkable series of igneous intrusions. 



