Vol. 60.] METAMORPHISM IX THE LOCH-LOMOND DISTRICT. 11 



have in all likelihood taken place contemporaneously to some extent, 

 or at least the duration of the latter process has overlapped that of 

 the former : but it is convenient, in dealing with the progressive 

 alteration, to consider each process separately, as they can be dis- 

 tinguished by their effects. 



The dynamic metamorphism is, in effect, chiefly a de- 

 structive process as regards the allothigenic mineral-constituents 

 and their arrangement in a rock : it is an almost purely-mechanical 

 action, which sets up new structures, but may not have had the 

 slightest effect in the development of authigenic minerals. 



The constructive metamorphism, which may be thermal 

 or hydrothermal, is constructive in the sense of developing 

 authigenic minerals; but in its effect upon structures, whether 

 original or produced by dynamic metamorphism, it may tend either 

 to emphasize or to obliterate them. 



The foregoing explanation is necessary, as giving the precise 

 meanings of these terms as I shall use them in this communication, 

 meanings which do not necessarily coincide with those applied to 

 the same terms by other observers. 



II. Dynamic Metamorphism. 



The effects of dynamic metamorphism upon any bed depend on 

 (1) its lithological character, and (2) the nature and position of the 

 folding at the particular place where the bed is observed. If the 

 bed be massive, coarse, and gritty, it will be able to resist the 

 deforming forces more successfully than if fine-grained, less homo- 

 geneous, or of naturally softer material. 



The effect of the nature of the folding is not so apparent, but 

 can be explained readily by a consideration of the physical con- 

 ditions in each case, on the assumption that the motive force is 

 a pressure tangential to the earth's surface. It is evident that a 

 certain amount of shearing-movement must accompany the folding, 

 compression, or stretching affecting the different parts of the fold ; 

 but so long as a bed is free to move as a whole (as in folding), 

 differential movement between the particles cannot take place to 

 any very great extent. The great shearing -movements which 

 have proved so effective in destroying original structures, may be 

 considered quite apart from the folding. They may have, and in 

 many cases certainly have, taken place after all folding-movement 

 in the immediate neighbourhood has ceased. The effect of the 

 nature of the folding is expressed simply in the angle at which the 

 original divisional planes of the rock are presented to the direction 

 of movement, which we assume to be a tangential pressure. The 

 original divisional planes in a sedimentary rock, planes of stratifica- 

 tion, are in fact surfaces between materials differing more or less in 

 physical properties, such as the coefficient of elasticity; and a force 

 applied from opposite sides of these surfaces must be resolved into 

 two components — one normal to the surface, tending to produce 

 compression, and one parallel to the surface, producing what in 



