part 1] AXMVEKSAiir address or the president. lxxix 



of this ordered state of things in some part of the Earth's crust. 

 In such a disturbed tract, and especially when igneous intrusion 

 intervenes, temperature may vary greatly in a manner having no 

 connexion with depth, and the same may be premised of shearing 

 stress. While, however, the assumption is unsound, the classifi- 

 catory scheme erected upon it by Grubenmann does in fact provide 

 places for the great majority of regionally metamorphosed rocks. 

 The reason is not very recondite. Although shearing stress may 

 vary independently of temperature, it has a maximum value which 

 is a function of temperature, since this determines the elastic limit 

 of the material. The maximum stress, and with it the possible 

 range of stress, diminishes with rise of temperature ; and it is 

 accordingly at low temperatures that the inadequacy of Gruben- 

 mann's simplified treatment becomes most apparent. The elastic 

 limit, and consequently the maximum shearing stress, will be very 

 different for different materials ; and it is to be noted that in the 

 empirical classification cited the dividing lines between the three 

 zones of depth are not supposed to be the same in different classes 

 of rocks. 



If now the external forces be everywhere sufficient to maintain 

 the shearing stresses at their maximum, stress does become a func- 

 tion, not indeed of depth, but of temperature, and the main 

 conditions controlling metamorphism in rocks of a given type will 

 therefore be determined by a single variable. Doubtless this case 

 is often closely realized, where metamorphism takes place upon an 

 extensive scale. It is this which renders possible the mapping 

 of successive zones of metamorphism, as worked out by Barrow 

 over a large part of the South-Eastern Highlands : the lines laid 

 down are at once isothermals and isodynamics. To assign the 

 corresponding temperatures in degrees may some day be found 

 possible, but the data will be very scanty. As Johnston & 

 Xiggli have pointed out, the inversion-points of enantiotropic 

 forms seem to be the only points on the ' geological thermometer : 

 which remain true in a tract subjected to shearing stress. 



While metamorphism of the most general type, in which the 

 thermal and dynamic factors have cooperated, is usually developed 

 upon a regional scale, the strict localization of such effects, which 

 may sometimes be observed, presents some points of special interest. 

 It seems clearly attributable to the mechanical generation of heat 

 bv the crushing of rocks. The fact that notable instances are 

 exceptional, proves that crust-movements are in general too slow to 

 bring about a local rise of temperature from this cause. The classical 

 area is that of the Ardenne, where Gosselet has demonstrated belts 



