1889.] on the Highlands of Scotland and the West of Ireland. 531 



One of the most iniin'essive features of our receirt researches 

 among these rocks is the evidence of the magnitude of the interval of 

 time between their original protrusion and the formation of the next 

 group of rocks overlying them. Of the many breaks in the geological 

 record, none is more complete than this. We pass at one step from 

 Archaean rocks, dating no doubt from an early stage in the consoli- 

 dation of the crust of the planet, to the gravelly and sandy deposits 

 of an inland sea, which already present all the familiar characters of 

 tlie sedimentary accumulations of later geological time. 



Some of the more prominent events in this protracted interval 

 may be more or less clearly discerned ; others can only be dimly 

 conjectured. Arranging in chronological order the more important 

 which have lately been recognised by the Geological Survey, I would 

 direct your attention to four main episodes in the Archsean history of 

 our north-western Highlands.* 



In the first place, the crust of the earth over that region was 

 thrown into a series of low arches or folds, the axes of which ran in 

 a general north-east and south-west direction. Its component rocks 

 were crushed and sheared, so as to acquire the banded and crumjDled 

 structure of typical gneiss. Perhaps we may trace to these primeval 

 terrestrial movements the first shaping of the European continent, 

 which certainly has grown from north to south. At all events, it is 

 interesting to note that the undulations into which the rocks were 

 thrown took that north-easterly trend ^'hich is still so marked in the 

 long belt of crystalline schists from the North Cape all the way to 

 the west of Ireland. 



In the second place after these early disturbances, and probably 

 long after them, a remarkable series of manifestations of plutonic 

 energy occurred. The region extending from the north-west of 

 Scotland to the west of Ireland was convulsed by the production of 

 innumerable dislocations in the solid terrestrial crusty having a 

 general west-north-west direction. Up these gaping rents molten 

 basic lava rose from some subterranean reservoir, and solidified 

 in broad dykes of black basalt. Some of these dykes can be traced 

 for ten or twelve miles till they run out to sea at the one end and 

 pass under younger overlying formations at the other. Yet again at 

 a somewhat later period another series of fissures was opened slightly 

 oblique to the direction of the first, and in these still more basic lava 

 formed a second series of dykes trending nearly east and west. Nor 

 was this all, for there followed a third period of convulsion which 

 gave birth to a series of huge dykes of granite. 



Whether or not any of the eruptive material that filled these suc- 

 cessive fissures ever rose to the surface and flowed out there, or gave 

 rise to the explosive phenomena of true volcanic vents cannot be 

 certainly afiirmed. But an interesting piece of evidence points to the 



'^ Those who wish fuller details on this subject will find them in the Survey 

 Report already quoted. 



