338 



KNOWLEDGE. 



September. 1911. 



crumples up on account of the extreme friction, or is 

 crushed into powder. This major thrust, as it is 

 called, allows the movement to affect the hitherto 

 undisturbed rocks below, which in their turn become 

 thrust. This is illustrated in Figure 6 and Figure 7. 

 the latter shewing how the most ancient rocks in the 

 Scottish Highlands have yielded to these movements. 

 The chief earth mo\ements that ha\-e affected great 

 Britain and \\'estern Europe ma\' be gathered into 

 four great periods : — 



1. The Pre-Cambrian. which for our present pur- 

 pose is unimportant and will not be dealt with. 



Figure 8. 



2. The Post-Silurian or Caledonian, which folded 

 the ancient rocks of North Wales, the Lake District, 

 Southern Uplands and Highlands of Scotland, and 

 whose folds generally strike W.S.W. to E.N.E.. so 

 that the movement must have come from the S.S.E. 



3. The Post-Carboniferous, or Hercynian. which 

 folded the Coal Measures and included two separate 

 movements, one of which produced most marked 

 effects in the South and the other in the North of 

 England. The Armorican movement came from the 

 South and crumpled the strata in the South and 

 West of England into a range of mountains forming 

 part of the great Armorican-\'ariscan chain, which 

 then almost ranged across Europe. Though its 

 effects are concealed from view in Eastern England 

 by a mantle of newer rocks, it is well shewn in 

 Devon, Cornwall and S. \\'ales. 



The other Post-Carboniferous movement is called 

 the Hercynian movement, and uplifted the Pennine 

 Chain, forming great folds running North and 

 South in the North of England. 



4. The fourth great series of movements raised the 

 chalk and converted it into land. These movements 

 continued until the middle of the Tertiary period, 

 and in Britain, especially in the Western Isles of 

 Scotland, were of a plateau-building t)-pe. In 

 Switzerland, however, great mountain building 

 stresses were set up and resulted in the formation 

 of the Alps. 



It must not be imagined, how e\'er, that the highest 

 peaks of any mountain range have received most 

 uplift, and that the valleys necessarily lie in the 

 troughs of the folds. Such is far from the case, for 

 the uplift has always been so slow that the wasting 

 action of air and water has had enormous effects, 

 and the effects of the earth movements are only 

 indirectly responsible for the scenery. The eleva- 

 ticm of the .\lps as a whole is due to earth move- 

 ment ; its individual peaks and vallej'S are due to the 

 relative hardnesses of the diverse rocks of which 

 they are composed. 



In the stud\' of mountain chains two things have 

 long been noticed and as will now be seen have more 

 than a practical interest. They are the abundance 

 of mineral veins and of granitic and other igneous 

 rocks. .\s has been mentioned before, igneous rocks 

 also characterise plateaux, but they occur in a 

 markedh- different manner. If we examine the 

 British metalliferous areas we shall find that they 

 have all suffered from the mountain-building move- 

 ments of the Caledonian and Hercynian, the great 

 plateau regions of N.E. Ireland and the ^^'estern 

 Isles of Scotland being devoid of mineral veins. It 

 is also remarkable that most of the veins occupy 

 fissures which appear to be due to the Hercynian and 

 Armorican movements : onlv in a few parts do they 

 appear to be Caledonian. To explain these 

 phenomena it will be necessary to take a glance at 

 the modern theories of the relationship of igneous 

 rocks and mineral veins. 



According to modern theories, beneath the solid 

 crust of the earth lies a magma composed of fused 

 rocks. Chemically it is composed, in the neighbour- 

 hood of the crust at anv rate, of silicates and 

 sulphides, which at the enormous temperature and 

 pressure which there obtain are probably mixed 

 together, although anv lowering of temperature and 

 pressure would cause them to separate into two 

 layers. 



The enormous i)ressure of the overlving rock tends 

 to make this magma rise up through any cracks that 

 may be present in the crust and dissolve large 

 cavities in it. B\- so doing it draws nearer the 

 surface and the sulphides and silicates separate into 

 la}-ers, the silicates at the top and the sulphides at 

 the bottom. The fractures of the rocks caused by 

 the great earth movements afford a ready means of 

 escape, and in man\' cases the powerful uplifting 



>'\ 



Figure 9. 



action of the ascending rock has important effects. 

 Thus a body of molten rock ascending a fissure may 

 meet an impervious bed of rock and spread out as a 

 sheet or sill, or arch up the strata to form a laccolite 

 or occupy the core of a fold to form a phacolite. 

 (See Figures 8 and 9.) 



Since the silicate or rock-forming magma floats on 

 the top we naturalK' expect that the formation of 

 igneous rocks will precede the production of mineral 

 veins, and when we come to examine the actual 

 distribution of the ores we find that this is always 

 the case. The tin veins of Cornwall are later than 

 the granite, although certainly originally derived 

 from the same magma, whilst the sulphide ores of 

 copper in the same district are later still. In the 

 same way the phosphate (apatite)) deposits of 

 Norwa)' represent the last stages in the consolida- 



