part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxiii 



metamorphism, and differentiates it from the crystallization of an 



igneous magma; is that it takes place, not freely in a fluid medium, 

 but in the heart of a solid mass. Hence the nascent and growing 

 crystals, as well as the rest of the rock, are subject to more or 

 less complex stresses. A clear understanding of all that this 

 implies is among comparatively recent developments. It could not* 

 be appreciated while geologists were content to speak vaguely 

 of the effects of ' pressure/ without discriminating between 

 uniform and non-uniform pressure. If, following the general 

 analysis of Thomson and Tait, we resolve any stress at a point 

 within a rock-mass into, first, simple uniform pressure, related to 

 change of volume, and second, shearing stress, related to change of 

 shape, it is not difficult to assess for our purpose the relative signifi- 

 cance of the two elements. In all transformations or reactions 

 involving a gaseous phase simple pressure is important, and may be 

 a determining factor. The thermal metamorphism of the pure 

 carbonate-rocks affords a good illustration. Under different pres- 

 sures dolomite either recrystallizes without change, or is dissociated 

 and its magnesian part decarbonated ; but calcic carbonate, which 

 has at corresponding temperatures a lower dissociation-pressure, 

 is never decomposed. Apart from changes of the class indicated, 

 hydrostatic or equal pressure seems to be in general of small 

 moment. Calculation and experiment alike show that, as regards 

 the displacement caused in a balanced reaction, a very great change 

 of pressure is no more effective than a very few degrees change of 

 temperature. It is only at great depths, therefore, where enormous 

 pressures prevail, that the : volume law ' comes to have any note- 

 worthy consequence in metamorphism ; and certain mineralogical 

 characters of deep-seated metamorphic rocks, such as the increasing 

 prominence of garnets in certain types, may be interpreted in this 

 sense. With this growing importance of pressure goes a decline in the 

 efficacy of shearing stress, since the high temperature which neces- 

 sarily rules diminishes the resistance of the rocks to shearing. In 

 the more general case, however, and certainly for typical crystalline 

 schists, we may without serious error consider metamorphism as. 

 controlled by the two factors temperature and shearing stress, and 

 treat simple pressure as a relatively small correction. 



A true appreciation of the problems of metamorphism was long 

 retarded by the tacit assumption that the rocks play a merely 

 passive part in the operation. Despite such familiar phenomena as. 

 the splitting of rocks by frost, the powerful mechanical force which 



