ANNIVERSARY ADDRESS OF THE PRESIDENT . 



73 



cleavage- cracks have been produced. These, by affording a ready- 

 channel for the passage of solvents, not unfrequently determine 

 the course of various chemical operations going on within the 

 crystal. 



Not unfrequently, too, the rock-forming minerals have yielded 

 along their gliding-planes, and the development in them of twin- 

 lamellaB is the result. Every crystal of calcite in an ordinary meta- 

 morphic limestone, and many of the plagioclase felspars in igneous 

 rocks, exhibit the secondary lamellar twinniug which has arisen 

 from the action of mechanical forces upon the mass *. The 

 microcline structure in orthoclases, with many other similar struc- 

 tures in other minerals, must almost certainly be ascribed to the 

 same cause. 



Still more remarkable are the consequences which follow from 

 the existence of the solution-planes in crystals. By the action of 

 various solvents under pressure, augite is made to assume the forms 

 known as diallage and pseudo-hypersthene, and ferriferous enstatite 

 of bronzite or hypersthene, while the felspars acquire their avanturine, 

 schiller, and chatoyant phenomena. "When, in addition to the statical 

 pressures due to thousands of feet of superincumbent rocks, these 

 solvent agencies work with those tremendous dynamical aids afforded 

 by deforming stresses, such as make the rocks to flow during 

 mountain-making, it is not surprising to find the molecules of the 

 original crystals breaking from their old allegiances, and the 

 liberated atoms uniting to form new minerals, the position of which 

 is determined by the lines of flow in the mass. 



Not a few of our gems owe their exquisite beauty to these 



* It has often been asserted that the "striation " on the faces or cleavage- 

 surfaces of crystals is an indication of the existence of polysynthetic twinning. 

 In the oligoclase of Ytterby and other localities, however, I have found that 

 many crystals which exhibit striation do not affect polarized light differently 

 in the alternate striae ; but on submitting the crystals to alternate heating and 

 cooling, and sometimes by percussive force, the twinning may be easily developed 

 in them. It appears from these observations that the crystals are built up of 

 lamellae, in which the molecules are alternately in stable and unstable equili- 

 brium. I have in some cases found that the stresses upon a slice of felspar 

 which is being heated and cooled and then ground into a thin section, while 

 cemented to a glass plate during the preparation of a microscopic slide, are 

 sufficient to cause the rotation of the molecules in the alternate lamellae. In 

 some cases, I have no doubt that twin-lamellation, like cleavage-cracks, may be 

 induced in the crystals of our rock-sections during the processes to which they 

 are submitted in their preparation. 



