ANNIVERSARY ADDRESS OF THE PRESIDENT. 7 I 



more nearl}^ east-and-wcst direction. Last of all conies a group of 

 thoroughly acid rocks — varieties of granite and " syenite," — which 

 form intrusive sheets and dykes. These dykes coincide in direction 

 •svith the basalts and dolerites, but they are apt to run together into 

 belts of granite and pegmatite 1500 feet broad. 



Up to the present time no evidence has been found of any super- 

 ficial outpouring of material in connexion with this remarkable series 

 of dykes in the Lewisian gneiss. That they may have been con- 

 comitant with true volcanic eruptions may be plausibly inferred from 

 the close analogy which, in spite of their antiquity and the raeta- 

 morphism they have undergone, they still present to the system of 

 dykes that forms a part of the great Tertiary volcanic series of Antrim 

 and the Inner Hebrides. The close-set fissures running in a W.N.W. 

 direction, the abundant uprise into these fissures of basic igneous 

 rocks, followed by a later and more feeble extravasation of acid 

 material, are features which in a singular manner anticipate the 

 volcanic phenomena of Tertiary time. 



There can be no question as to the high antiquity of these dykes. 

 They were already in place before the advent of those extraordinary 

 vertical lines of shearing which have so greatly affected both the 

 gneiss and the dykes ; and these movements, in turn, had long been 

 accomplished before that ancient member of our stratified forma- 

 tions — the Torridon sandstone — was laid down. Though later than 

 the original igneous material out of which the gneiss was formed, 

 they have become so integral and essential a part of the gneiss as 

 it now exists that they must be unhesitatingly grouped as Archaean. 

 1 may add that this interposition of dark basic dykes appears to be 

 characteristic of the oldest gneiss far beyond the limited area of the 

 North-west Highlands, for it may be noticed in the Archajan tracts of 

 Donegal and Galway. With so wide an extension of the subter- 

 ranean relics of volcanic energy, it is surely not too much to hope 

 that somewhere there may have been preserved, and may still be 

 discovered, vestiges of the superficial products of the Archaean vol- 

 canoes. Among the pebbles in the conglomerates of the Torridon 

 sandstone there occur fragments of highly-cellular basic lavas, and 

 of felsites, some of which are beautifully spherulitic and somewhat 

 granophyric. These fragments may point to the existence of volcanic 

 materials at the surface when the Torridon sandstone was deposited. 

 Possibly they may represent some of the vanished Archaean lavas. 

 But the time between the uprise of the ArchcTan dykes and the 



formation of that sandstone was vast enough for the advent of 



O' 



many 



TOL. XLVn. / 



