92 RICHTHOFEN NATURAL SYSTEM 



The entire belt, composed of these three zones, has to be considered as one great 

 area of elevation, characterized, at the same time, over its greater portion, by the rem- 

 nants of the eruptive activity of the volcanic era. No portion of it, however, has, as 

 we mentioned before, experienced an amount of additional elevation during that period 

 equal to that of those mountain ranges which existed before it, and among which the 

 Alps and Himalaya are the most prominent. In these cases there cannot have been any 

 connection between the elevation and the ejection of the rocks to the surface, because 

 these do not occur. But this does not preclude the connection between the elevation 

 and those agencies which are the causes of eruption. It has been often demon- 

 strated that the changes of level must have been accompanied by the formation of 

 such fractures as would be closed next to the surface, and, though allowing of an in- 

 trusion of plastic matter from below, would prevent its extrusion to the surface. It is 

 probable that fissures of this kind may have been chiefly formed where high mountain 

 chains are composed of metamorphic rocks, since they frequently exhibit vertical disloca- 

 tions, partly parallel to their axis, and partly at right angles to it, by which the strata 

 on one side have been moved thousands of feet above the other, and which, notwith- 

 standing, did not give vent to the ejection of rocks. These faulting fissures appear to 

 form frequently the lateral boundaries of mountain ranges and the limits of the mani- 

 festation of vulcanism, and both these circumstances allow us to infer that they extend 

 downward to great depth. Though one of the main features in the geological 

 structure of the Alps, they have hitherto been little examined. But it is probable that 

 their formation was coincident in time with the main phase of elevation, that is, 

 with the volcanic era. If, then, in that part of the crust over which the Alps are ele- 

 vated, such fractures were formed as did not open on the surface, then all the agen- 

 cies below would cooperate towards elevation alone. The expansive force produced 

 by aqueous fusion would find no vent for the discharge of the masses by which the 

 volume had increased, nor would there be any opportunity given for the escape of 

 heat by hot water and other means ; while, on the other hand, the masses in depth 

 would be" relieved from pressure by the formation of this kind of fractures in the same 

 way as by those which would allow the passage of liquid matter to the surface. In the 

 case of the latter, an immense amount of force is spent in other modes of action, while 

 in the first case it could be applied almost exclusively to elevation. Besides this direct 

 action, however, we must also keep in view, that in those regions where fissures would 

 not be open at the surface, the conditions required for metamorphic action would be 

 given on a particularly grand scale. Gases and water would not reach the surface, but 

 be employed in depth in promoting metamorphic action over vast regions, and increas- 

 ing the rate of elevation. It may be by processes of this kind that already in ancient 

 times, contemporaneously with an accelerated elevation, were formed those granitic 

 wedges surrounded by broad belts of foliated metamorphic rocks, which are peculiar 

 to certain mountain ranges, particularly to those which also in the volcanic era have not 

 been the theater of eruptive activity. The intrusion, from below, of heated masses into 

 fissures closed towards the surface, would give all the conditions requiredlbr the exhibi- 

 tion, on a grand scale, of those processes of hydropyric metamorphism which Daubree 

 has rendered probable by his experiments and theoretical deductions. 

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