400 EECEXT WOEK OF THE GEOLOGICAL SUEVEY 



less basic type (gabbros, peridotites, palaeopicrites, qnartz-diorites, 

 &c.), in which pegmatites and segregation-veins were formed. 



(2) The development of foliation in these eruptive rocks and 

 pegmatites by mechanical movement, and their conversion into 

 " gneisses," the axes of the folds running generally N.E. and S.W. 



(3) The intrusion of a great series of igneous rocks, mainly in the 

 form of dykes, in the gneisses, consisting of (a) basalt-rocks, 

 (h) peridotites and palaeopicrites, (c) microcline-mica rocks, 

 (d) granites, &c. 



(4) The subjection of the original gneisses to enormous mechanical 

 movemeuts, giving rise to disruption-lines or thrust-planes trending, 

 (a) N.W. and S.E., or W.N.W. and E.S.E., (6) E. and W., (c) N.E. 

 and S.W. 



(5) The alteration of the dykes consequent upon these move- 

 ments : — (a) the dolerites being changed into diorites and horn- 

 blende-schists, (6) the peridotites into talcose schists, (c) the micro- 

 cline-mica rocks into mica-schists, (d) the granites into granitoid 

 gneiss. Pegmatites were also formed in the hornblende-schists and 

 granitoid gneiss. 



The plication of the gneiss in sharp folds trending E. and W. or 

 !N".W. and S.E., the development of secondary foliation running 

 generally N.W. and S.E., or W.IS'.W. and E.S^E., or E. and W., 

 and the partial or complete reconstruction of the gneiss along the 

 old planes of schistosity. 



6. The foliation of the granitoid gneiss and associated pegmatites 

 by a still later series of movements. 



II. Cambel^n Eoejiatiom". (Toeeidon Sai^dstoke.) 



1. Denudation of the Pre-Cambrian Land-surface. 

 Between the formation of the Archaean rocks in the west of 

 Sutherland and the deposition of the overlying Cambrian conglom- 

 erates and sandstones an enormous interval of time must have 

 elapsed, during which the primeval land-surface was subjected to 

 extensive denudation. In order to form an accurate idea of the 

 outline of this ancient land, the observations must be confined to the 

 area bordering the line of junction between the gneiss and the 

 sedimentary deposits. It would obviously be unsafe to accept the 

 present contour as any indication of the primeval one, in tracts where 

 the Archaean rocks have been stripped of the red sandstones and 

 quartzites, and exposed for long ages to denudation. Indeed, there 

 is clear evidence for maintaining that where such has been the case 

 the agents of waste have been to a large extent guided in their 

 operations by the trend of the basic dykes, the various Pre-Cambrian 

 disra^Dtion-lines, and the great series of normal faults that followed 

 the terrestrial displacements of Post-Lower-Siluiian date. Erom 

 the evidence obtained in the district of the Parph, west of the Kyle 

 of Durness, it is apparent that the old land-surface must have been 

 worn down to a comparatively level plane ; while in Assynt it must 

 have been carved into a series of dome-shaped eminences, sometimes 



