1889.] on the HigJdands of Scotland and the West of Ireland. 529 



of whicli onr nortli-western mountains are composed.* The story is 

 a somewhat involved and complicated one. But its main points may 

 perhaps be conveniently grasped, if we bear in mind that they 

 naturally group themselves into four sections ; 1st, the Archaean 

 period ; 2nd, the Cambrian period ; 8rd, the Lower Silurian period ; 

 4th, the period of the younger Schists. 



Let me at the outset remark that in the investigation of these 

 early ages of geological history we enjoy in this country a special 

 advantage. The British Isles stand on the oceanic border of a 

 great continental region. They are therefore placed along that 

 critical belt where not only have terrestrial disturbances been 

 especially numerous and violent from the earliest geological times, 

 but where an oscillation U23ward or downward of a few hundred feet 

 has sufficed to make all the difference between land and sea. In the 

 heart of a continent, as, for example, over the vast plains of Russia, 

 long cycles of geological time have passed without serious dis- 

 turbance of any kind. To this day some of the ancient Palaeozoic 

 sediments in that region, for hundreds of square miles in extent, lie 

 as level as when they were deposited on the sea-floor. They have 

 been uplifted bodily into land, but still remain little more than mere 

 hardened mud and sand. In Western Europe, on the other hand, 

 where from the remotest geological antiquity the oscillations and 

 dislocations have been innumerable, every successive continental 

 uplift has recorded itself in some crum'pling or fracture of the rocks. 

 Hence in the geological map of that region the various formations 

 form a pattern of exceeding complexity, while in the maps of Eastern 

 Eurojje each of them covers a broad unbroken expanse. 



I. The Arch^an Period. 



The oldest known rocks of Europe, now generally termed 

 Archaean, are well exposed along the north-western borders of the 

 continental area from the extreme north of Scandinavia, by the west 

 coast of Scotland, to Gal way Bay in the west of Ireland, a total 

 distance of some 1600 miles. They give rise to topographical 

 features which, where fully developed, strongly distinguish them from 

 all younger formations. Nowhere else can such extraordinary un- 

 evenness of surface be found. Knobs, hummocks, and ridges of bare 

 or almost bare rock, separated by narrow gullies or by wider winding 

 valleys, roughen the ground in every direction. In the hollows lie 

 innumerable tarns and lakes, or flat tracts of bog where lakes once 

 were. In some districts, indeed, there is as much water as land in a 

 given number of square miles. On a large scale, this type of scenery 

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



* It would be obviously out of place to iuclude here references to the 

 voluminous literature of the subject. A condensed summary will be found in the 

 Report by the officers of the Geological Survey, ' Quart. Journ. Geol. Soc' vol. 

 xliv. 1888. 



