60 SECTIONAL ADDRESSES. 



Two factors are involved in the geological classification of folded 

 mountains, namely date and position. One half of the surface of Europe 

 has escaped mountain deformation since the dawn of the Cambrian. 

 This stable area, which we may call Baltica, has roughly the form of an 

 equilateral triangle. Two of its boundaries diverge from South Wales : 

 the one follows approximately the Norwegian-Swedish frontier ; the 

 other, highly complex in its development, passes south of London and 

 Berlin and north of the Crimea and Caucasus. The third side of Baltica 

 is furnished by the Urals, but of this I do not propose to speak. 



Let us look a little more closely at Baltica, because it will repay us 

 when presently we cross the Atlantic. On the north and west sides of the 

 Baltic Sea the prevalent rocks exposed at the surface are Precambrian 

 and most of them are crystalline. This part of Baltica, Suess has called 

 the Baltic Shield, to convey the idea of a gently convex surface. Its 

 immunity from Cambrian and later folding movement is inferred from 

 the uniform testimony of its girdle of almost undisturbed Palaeozoic 

 outcrops. The rest of Baltica lies cloaked in sediments ranging from 

 Cambrian to Tertiary. It has been named the Russian Platform, and its 

 western continuation probably extends through Denmark into the English 

 Midlands. 



Two Palaeozoic mountain chains meet in South Wales about the western 

 angle of Baltica. In 1887 Suess named the older of them Caledonian, 

 out of compliment to Scotland. It runs north-east and its folded, cleaved 

 and broken rocks appear at the surface in many parts of the British Isles, 

 in most of Norway and along much of the Swedish frontier. They 

 frequently include marine representatives of the Cambrian, Ordovician 

 and Silurian ; but the Devonian, where developed within the Caledonian 

 belt of Britain and Scandinavia, and often in adjacent districts, is of 

 continental or, in other words, of Old Red Sandstone facies ; and is later 

 than the more violent of the mountain disturbances. 



Great Britain is unique in being crossed by both margins of this 

 Caledonian Chain. Under the North Sea the old mountains are com- 

 pletely submerged, and where they reappear in Scandinavia it is with their 

 north-western edge still hidden off the coast of Norway. 



In Shropshire and Radnor, where England and Wales meet, Lower Old 

 Red Sandstone follows conformably on Downtonian that forms the top 

 of the Silurian ; and the important unconformity of the district s between 

 Silurian and Ordovician. It is an unconformity that is rather more 

 striking upon a map than in field exposure, for here we stand at the south- 

 east margin of the Caledonian Chain, and there has been comparatively 

 little folding of Palaeozoic rocks. 



Proceeding north-westwards, we soon enter a mountain element 

 characterised by intense post-Silurian unconformity. On the far side 

 this element is bounded by an ill-determined north-east line that passes 

 close to Girvan and Edinburgh, so that its cross-strike measurement is 

 about 180 miles. Eastwards its rocks are hidden beneath comparatively 

 undisturbed Carboniferous and later formations that occupy the surface 

 from Shropshire to Northumberland. Westwards they delight our eyes in 

 Wales, the English Lake District and the Southern Uplands of Scotland. 



