72 



PBOCEEDLNGS OF THE GEOLOGICAL SOCIETY. 



to the action of solvents, they are attacked in such a manner as to 

 give rise to peculiar geometrical forms. The subject has been 

 followed up by Baumhauer, Leydolt, Becke, and others, who have 

 shown what a wonderful variety of " etched-flgures " may be 

 produced by operating upon the various faces and cleavage-surfaces 

 of different crystals. 



Quite recently, however, it has been shown by Yon Ebner, as the 

 result of his studies of calcite and aragonite, that all the complicated 

 phenomena of the etched^figures arise from the existence of planes 

 along which solvent or chemical action takes place most readily 

 within a crystal. It thus appears that these complicated etched 

 figures, with their curved and striated surfaces, are indications of 

 the combination or oscillation of tendencies to chemical action along 

 the different solvent-planes of the crystal. 



My own experiments have enabled me to show that the chemical 

 action taking place along the solution-p]anes of crystals leads to the 

 development of cavities, often assuming the forms of negative crystals, 

 which may become wholly or partially filled with the products of 

 the chemical action. 



Although the solution-planes are quite distinct, both from the 

 gliding-planes and the cleavage-planes of crystals, I have been able 

 to show that some curious ^and interesting relations exist between 

 them. If lamellar twinning has been already developed in a 

 crystal, then chemical action takes place along the gliding-planes in 

 preference to the normal solution-planes. 



It is only when we study the minerals building up the rock-masses 

 of the globe that we fully^realize the importance of these molecular 

 structures, and the wonderful changes which crystals are capable of 

 undergoing, as a consequence of their internal " organization.'' 

 Then, and then only, do we begin to understand the significance 

 and the far-reaching consequences of the physiological changes of 

 which minerals are susceptible. 



The crystals forming the rock-masses of the globe have been 

 subjected to every variety of mechanical force — violent fracture, 

 long-continued strain, steady but enormous pressure — prolonged 

 over vast intervals of time, to which must be added the potent effects 

 of alternate heating andlcooling. Such crystals, moreover, are trans- 

 fused through their whole substance by various liquids and gases 

 acting under tremendous, and sometimes varying, pressures. 



Under such circumstances it is not surprising to find that the 

 crystals have often yielded along their cleavage-planes, and that 



