MOUNTAIN BUILDING AND ORE DEPOSITION. 



By J. E. \V. RHODES, Assoc. Inst. M.E. 



Of all ph3-sical features, mountains are among the 

 most familiar but yet the least known. Few are 

 ignorant of the romantic legends that cluster round 

 them, of the poets \\ho have sung of them, of the 

 nations who ha\'e defended them ; and to the 

 student of Histor\- the\' have e\-er had a fascination. 



Figure 



FlGUkH 



>Si.-.l- 



as the walls that have kept 

 race from race, and some- 

 times guided the destinies of 

 nations, .\gain, in the hard 

 world of to-da\' the\' are still 

 there, the natural frontiers of 

 the soldier, and the onl\- 

 parth' subdued obstacles of 

 the engineer. Yet after all 

 this is only a superficial \-ie\\ 

 — the\' are something more 

 than view points, and if we 

 enquire into their structure we 

 shall find that every crag has 

 its meaning in the long stor\' 

 that is now revealed to us. 



The first characteristic of 

 a mountain that we notice is 

 its height above its surround- 

 ings, not merely height, but 

 relative height. An uplan<l 

 plateau is not a mountain, 

 however high it be, but all 

 the characteristics of a true 

 mountain range ma\' be shewn 

 by a low ridge. Thus the 

 Malverns rise to little more 



than one thousand feet, yet in structure thev are 

 true mountains. Of almost equal importance is 

 ruggedness, which is intimately associated with the 

 first characteristic ; for it expresses the steepness 

 of the mountain, that is to say, the height divided 

 by the horizontal extent. 



Let us analyse these facts. Elevation requires 

 upheaval, and so a mountain chain must have been 

 at some time upheaved. Impressed by this truth 

 the earlier observers neglected all others and 

 attributed not merely the height but even the 

 details of the form of the mountain to this 



cause. \"on Buch, for instance, easih' explained 

 the perched blocks of the Alps in this way. 

 \\'hen the mountain chain was thrown up, the 

 motion was so violent that rocks were detached, 

 and thrown up into the air, subsequently com- 

 ing to rest on the sides of the mountain. 



But detailed study soon shewed that 

 things were not so simple. No violent 

 catastrophe ever threw up a comiileted 

 mountain chain and left it perfect. The 

 moulding of its form was the work of 

 ages and of more than one cause. And 

 its complicated topography is but the 

 expression of its no more simple archi- 

 Its hard and enduring rocks 

 have been bent, broken, and 

 contorted by great earth move- 

 ments ere the long course 

 of denudation sculptured its 

 present form. 



So the primar\- origin 

 of mountains is in move- 

 ments of the earth's crust, 

 and it becomes of im- 

 portance to us to know 

 in what ways the earth's 

 crust can move, and what 

 effects each kind of mo\e- 

 uu-nt may have. Earth move- 

 ments are of two kinds, vi;^.. 



Figure J. 



Figure 4. 



1. \"ertical. up-and-down, or plateau building. 



2. Horizontal, tangential, or mountain building. 



The plateau building movements are the result of 

 the sinking or rising of one area relati\"ely to another. 

 The\- take place owing to the weight of the strata 

 which tend to sink in blocks into the still fluid 

 portion of the earth's interior, which, being molten, 

 is probably lighter than it, and gives rise to more or 

 less vertical faults or dislocations which bound the 

 blocks. Thus the great basaltic plateaux of the 

 Western Isles of Scotland have been let down b\- 



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