7© PROCEEDINGS OF THE GEOLOGTCA.L SOCIETY. 



volcanic ejections associated with the most ancient parts of the 

 fundamental gneiss, we may be reasonably certain that the rocks 

 there visible consolidated from igneous fusion at some depth, and we 

 may plausibly infer that they may have been actually connected with 

 the discharge of volcanic materials at the surface. 



But the progress of investigation in the North-west Highlands 

 has brought to light the remarkable fact that the ancient gneiss of 

 that region is a much more complex formation than had been sup- 

 posed. By a series of terrestrial stresses that came as precursors of 

 those which in later geological times worked such great changes among 

 the rocks of the Scottish Highlands, the original igneous bosses and 

 sheets were compressed, plicated, fractured, and rolled out, acquiring 

 in this process a crumpled, foliated structure. Whether or not 

 these disturbances were accompanied by any manifestations of super- 

 ficial volcanic action has not yet been determined. But we know 

 that they were followed by a succession of dyke-eruptions, to which, 

 for extent and variety, there is no parallel in the geological structure 

 of this country, save in the remarkable assemblage of dykes belong- 

 ing to the Tertiary volcanic period. 



Por the production of these dykes a series of fissures was first 

 opened through the gneiss, having a general trend from E.S.E. to 

 W.i^.W., running in parallel lines for many miles, and so close 

 together in some places that fifteen or twenty of them occurred 

 within a horizontal space of one mile. The fissures were probably 

 not all formed at the same time ; at all events, the molten materials 

 that rose in them exhibit distinct evidence of a succession of up- 

 weUings from the igneous magma below. By far the largest pro- 

 portion of the dykes consists of basic materials. The oldest and 

 most abundant of them are of plagioclase-augite rocks, which diff'er 

 in no essential feature of structure or composition from the dolerites 

 and basalts of more modern periods. They present, too, most of the 

 broad features that characterize the dykes of later times — the central 

 more coarsely-crystalline portion, the marginal band of finer grain, 

 passing occasionally into what was probably a variety of tachylyte, 

 and the transverse jointing. They belong to more than one period 

 of emission, for they cross each other. They vary in width up to 

 nearly 200 feet, and sometimes run with singular persistence com- 

 pletely across the whole breadth of the strip of gneiss in the west of 

 Sutherland and Eoss. 



Later in time, and much less abundant, are certain highly basic 

 dylies — peridotites and picrites — which cut across the dolerites in a 



