300 



NATURE 



{July 25, 1889 



best displayed in Finland ; on a small scale, it is repeated 

 all through the chain of the outer Hebrides, as well as on 

 the Archaean areas of the mainland. The most southerly 

 points in Scotland where it can be recognized are the 

 Island of lona and the Ross of Mull. It reappears, how- 

 ever, far to the south in Ireland ; standing out in the bold 

 cliffs from Erris Head to Achill Island in the west of 

 Mayo, and finally covering an area of more than 500 square 

 miles in South-Western Galway. In this last-named 

 district, as Prof. Hull has shown, so completely are the 

 scenic features of the north-west of Scotland reproduced, 

 down even to the minutest details, that the geologist, 

 even before he stands on the rocks, has no difficulty in 

 deciding that they can only be Archaean. 



What, then, are these most ancient rocks of North- 

 western Europe, and what has been their history .^ Un- 

 fortunately, the answer to these questions cannot be 

 succinctly and definitely given. Owing to the antiquity 

 of the masses, and the prolonged series of geological 

 revolutions which they have undergone, their original 

 characters have been somewhat effaced. In those areas 

 where they have been least altered, and where, therefore, 

 they approach nearest to their primitive structure, they 

 have been found by my colleagues of the Geological Sur- 

 vey to be crystalline rocks, such as gabbros, diorites, and 

 other highly basic compounds. These occur in zones 

 or bosses surrounded by and passing into rocks which 

 have acquired the peculiarly banded structure character- 

 istic of gneiss. That these various rocks were eruptive — 

 that is, that they originally formed portions of igneous 

 material that rose in a molten or plastic condition from 

 below — can hardly be doubted. They remind us of the 

 deep-seated portions of some of the eruptive bosses so 

 abundantly intruded into the crust of the earth, and now 

 so plentifully exposed at the surface after prolonged de- 

 nudation. Like these, they show a rudely striped or 

 banded arrangement suggestive of the planes of move- 

 ment or flow-structure seen in consolidated igneous 

 material. They have probably resulted from successive 

 protrusions of eruptive rocks at some depth within the 

 crust of the earth. 



Nowhere, however, in the region to which I am refer- 

 ring, has any trace of superficial eruption yet been de- 

 tected. There are no true volcanic ejections, nor any 

 evidence that the rocks, though certainly of eruptive 

 origin, were ever connected with the ordinary explosive 

 operations of volcanic vents. Not only so, but after the 

 most careful search from Sutherland to Galway not a 

 vestige have we yet found of any unquestionable sedi- 

 mentary material. There are no conglomerates, no sand- 

 stones, no shales ; nor even any materials that might be 

 supposed to represent these in a metamorphosed con- 

 dition. Of the actual surface of the earth these Archaean 

 rocks afford no recognizable trace. They obviously did 

 not form the superficial layer themselves. They must 

 have lain deep under a cover of other material, under 

 which they acquired their crystalline structure, and by 

 the subsequent removal of which they have been exposed 

 to the light. 



One of the most impressive features of our recent re- 

 searches among these rocks is the evidence of the mag- 

 nitude 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 consoUdation of the crust of the planet, 

 to the gravelly and sandy deposits of an inland sea, 

 which already present all the familiar characters of the 

 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 chrono- 

 logical order the more important which have lately been 



recognized by the Geological Survey, I would direct your 

 attention to four main episodes in the Archaean 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 crumpled structure of 

 typical gneiss. Perhaps we may trace to these primaeval 

 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 which is still so marked in the long belt of crystal- 

 line 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 mani- 

 festations of plutonic energy occurred. The region ex- 

 tending from the north-west of Scotland to the west of 

 Ireland was convulsed by the production of innumerable 

 dislocations in the solid terrestrial crust, 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 over- 

 lying 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 successive fissures ever rose to the surface and 

 flowed out there, or gave rise to the explosive phenomena 

 of true volcanic vents, cannot be certainly affirmed. But 

 an interesting piece of evidence points to the probability 

 that such a connection with the surface was really estab- 

 lished. In some of the conglomerates of the next suc- 

 ceeding group (Cambrian or Torridon sandstones), there 

 occur fragments of highly vesicular lavas, which show 

 that at some time previous to the deposit of these coarse 

 sediments, active volcanic vents existed somewhere in the 

 region of the north-west of Scotland. As yet, however, 

 no trace has been discovered of any of the lava streams 

 which flowed out at the surface. 



Although volcanic energy has long been quiescent over 

 the British Isles, probably no area in Europe exhibits 

 within so limited a space so long and varied a record of 

 volcanic eruptions. There is, therefore, a peculiar interest 

 about these traces of the ancient volcanoes which in 

 Archaean time rose along the Atlantic border in the north- 

 west of Scotland, for they stand at the very beginning of 

 that long history. Moreover, so far as we can interpret 

 their remains, they seem in a curious way to have antici- 

 pated the characteristics of the last great volcanic episode 

 in Britain — that to which we owe the Tertiary basaltic 

 plateaux of Antrim and the Inner Hebrides. In both 

 cases the distinguishing feature was the fissuring of the 

 terrestrial crust and the uprise of basic lava in the rents, 

 with the consequent production of innumerable parallel 

 dykes trending in a general north-westerly direction. 



In the third place, after the production of the basic 

 dykes, there came another prolonged interval, during 

 which a series of remarkable terrestrial disturbances 

 affected the north-west of Scotland. The crust of the 

 earth in that part of Europe was once more dislocated by 

 innumerable fissures, produced probably at successive 

 epochs of paroxysm, for they can be grouped into three 

 distinct series. Of these, one runs approximately parallel 



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

 Report already quoted. 



