part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxvii 



ideally such as are stable under the immediate conditions. That 

 the stress-condition is indeed often a determining factor is suffi- 

 ciently clear from petrographical evidence, but a systematic inter- 

 pretation of the observed mineral-associations remains a task for 

 the future. Johnston & Xiggli have set forth in convenient 

 terms some general principles relative to the influence of unequal 

 pressure, but the application of those principles demands data which 

 are yet to seek. Upon all reactions involving a liquid or gaseous 

 phase the influence is paramount, always favouring the production 

 of the liquid or gas. To transformations and reactions between 

 solid bodies alone the conception of unequal pressure is manifestly 

 inapplicable, and it seems to be assumed that shearing stress in 

 itself is of no direct effect. It is admitted, however, that reaction 

 between solid bodies, which is negligible in other circumstances, 

 may become effective with shearing movement, which has the 

 result of bringing fresh particles continually into contact with one 

 another. 



Even on so brief a presentation of the case, it appears that the 

 chemistry of bodies under unequal stress is in some important 

 degree a new chemistry, different from that of the laboratory. 

 One class of transformation-points may be greatly displaced, while 

 another class — such as the inversion-point between tw r o dimorphous 

 forms — is unaffected. Consequently the range of stability of a 

 particular compound, or a particular form of a dimorphous com- 

 pound, may be either extended or contracted. A place may be 

 made for some form which has no stable existence under any 

 conditions of temperature and uniform pressure, or again another 

 form may have its range of stable existence wholly cut out. 



The facts of observation which await elucidation on these lines 

 are sufficiently familiar to geologists, Shearing stress manifestly 

 favours the production of sericite and the chlorites, of albite among 

 the felspars, of the epidote-zoisite group, of amphiboles as opposed 

 to pyroxenes, of cyanite and staurolite, chloritoid and talc, haematite 

 and rutile. These may conveniently be styled stress-minerals. 

 Some of them — cyanite, anthophyllite, gastaldite, jadeite, lawsonite, 

 antigorite — are so exclusively associated with crystalline schists 

 that we may reasonably infer shearing stress to be necessary for 

 their production. On the other hand numerous minerals, such as 

 anorthite and the potash-felspars, augite, olivine, and andalusite, 

 known as common products of simple thermal metamorphism, are 

 rare or absent in proportion as the dynamic element has entered, 



