THE GROWTH OF BRITAIN 211 



second followed the Ordovician, but was not completed till the 

 middle of the Devonian Period ; the third was late Carboniferous, 

 and lingered till the Trias ; and the last began after the deposition 

 of the Chalk, and was not completed till the beginning of the 

 Pliocene, if indeed it is yet complete. 



Each period of movement is likely to have resulted in the 

 uplift of continental masses and mountain chains, accompanied 

 by folding and faulting of the strata, the metamorphism of the 

 rocks, and the outburst of volcanoes. Each intervening period 

 of rest or subsidence is likely to have given rise to incursions of 

 the sea and the formation of marine deposits. This sequence of 

 events is to some extent traceable in British geological history. 



The earliest movement began well before Cambrian times, 

 but the land and mountains of the period can at present only be 

 doubtfully traced. The pre-Cambrian volcanoes were an outcome 

 of the movement in many parts of Wales and the Midlands, and 

 possibly the Longmyndian and certainly the Torridonian strata 

 were made of materials rapidly denuded from newly elevated 

 lofty land. Later came the marine deposition of the Cambrian 

 strata. 



The next period of movement began during the deposit of 

 the Ordovician rocks, and the site of the volcanoes of this period 

 was determined by it. There is striking unconformity between 

 Ordovician and Silurian rocks, but the land seems to have attained 

 its most remarkable relief during Old Red Sandstone times. 



The continental conditions and the great lakes of this Period 

 were the most important results of this movement, to which we 

 owe the Scandinavian chain of Europe, of which the Highlands 

 and Uplands of Scotland, the mountains of Lakeland and Wales, 

 and those of northern and western Ireland are a part. The nature 

 of the Old Red Sandstone material, the relation of its strata to 

 the older rocks on which they rest, and the presence of massive 

 conglomerates and breccias at the base of the Formation, resting 

 unconformably on Archaean, Cambrian, and Ordovician rocks, 

 are the evidences by which this movement and its effects are dated. 



The period of rest was comparatively short, but in it the 

 lower rocks of the Carboniferous System were deposited in a sea 

 which was shallowed later by the beginning of the next earth- 



