part 1] AarOTVEESAUT ADDRESS OF THE PRESIDENT. lxxv 



shapes by growing out chiefly in directions perpendicular to the 

 maximum pressure, as did the alum in the experiment. Moreover, 

 many of the minerals of metamorphism have an advantage not 

 possessed by alum or by garnet, in that their normal habit is tabular 

 or columnar, owing to more rapid growth in certain crvstallo- 

 graphic directions. This habit is exaggerated when the growth 

 takes place in a solid rock ; and, if the rock is under definitely 

 oriented stresses, the habit is further exaggerated in those crystals 

 which lie more or less nearly along the plane perpendicular to 

 the maximum pressure. Any shearing deformation of the rock- 

 mass, besides affording direct relief, rotates existing crystals more 

 nearly into this favourable posture, and — what is of greater im- 

 portance — new crystals, originating under the stress-conditions 

 indicated, set themselves in the same plane. 



The laws governing the growth of crystals in a solid rock have 

 much more than a theoretical interest. In particular, the distinc- 

 tion between strong and weak minerals, as expressed in the crystallo- 

 blastic series, concerns the field-geologist no less than the petrologist; 

 and disregard of it may lead to erroneous conclusions concerning 

 the mutual relations of rocks and the geological history of a 

 district. For example, an eclogite intercalated among well- 

 foliated rocks is not to be set down as an intrusion later than the 

 foliation, merely on the ground that it does not share in that 

 structure. Xor again do Hakes of ilmenite or ottrelite scattered 

 through a mica-schist, without conforming to the parallel arrange- 

 ment, afford evidence of a second epoch of metamorphism. It 

 would be easy to pick out in geological literature passages 

 which call for revision in the light of these and kindred con- 

 siderations. 



The student of metamorphism must realize how radically some 

 simple physical and chemical principles become modified when 

 applied to bodies in a condition of internal stress ; and, moreover, 

 of stress which varies from place to place and from time to time. 

 Our conceptions of the relations between solids and liquids — crystal- 

 lization, fusion, solution, chemical reaction — are naturally based on 

 the behaviour of bodies under uniform pressure, such as that of the 

 atmosphere ; and a considerable readjustment of ideas becomes 

 necessary before we can acquire a clear understanding of what 

 happens in dynamic metamorphism. Petrologists, on their side, 

 have hitherto made but little enquiry into the effects of non-uniform 



