DALY. — ETCH-FIGURES ON AMPHIBOLES. 389 



In these cases, actual separation had taken place along the cleavage 

 planes in question long enough before the specimens were collected in 

 the field to have j)ermitted the physical processes of weathering to de- 

 stroy to some extent the original cohesion of those layers of the crystals, 

 while, for some reason (probably the protection of surrounding minerals), 

 the crystal-face escaped such disintegration. That there may be other 

 and more obscure physical differences which can explain the grouping of 

 etch-figures was suggested by Baumhauer, who attributes the zonal ar- 

 rangement of the pits on etched fluorites to them rather than to chemical 

 variations in the zones.* The same principle is illustrated in the expla- 

 nation of " Aetzgriiben," f linear aggregations of pits following direc- 

 tions of weak cohesional control. Thus, on etched cleavage plates, there 

 are commonly to be observed a regular grouping of figures in directions 

 which are crystallographically fixed on hornblende ; long rows of pits 

 parallel to the trace of the second vertical cleavage, or to the trace of 

 the rudimentary basal cleavage, without, in either case, there being actual 

 cleavage cracks opened which would affect the etching directly. Viola 

 has recently described the zonal distribution of pits on gypsum, the zones 

 being regularly fixed with reference to tlie axis of symmetry.! Whether 

 these zones represent the original stratification of molecular deposition or 

 are the result of secondary physical change, acting on a homogeneous 

 crystalline mass, the behavior of gypsum is another example with those 

 already cited to show the dependence of attackability on physical co- 

 hesion, when the latter varies according to the laws of crystal structure. 

 Incipient weathering and the development of secondary strains will favor 

 an irregular grouping of etch-figures that are structurally accidental. 

 These have been noticed in connection with the amphiboles. These facts 

 lead us to suspect that Ebner's hypothesis of irregularities in the solvent 

 can no better explain the differing attackability of a cleavage piece of 

 calcite in its various parts than the hypothesis of varying physical con- 

 ditions in the crystal itself. § 



* Resultate der Aetzmethode, p. G. Mere heating may render actinolite fibrous. 

 See Doelter and Hussak, Neues Jalirbuch, 1884, Bd. I. p. 24. 



t Ibid., p. 6. 



t Zeit. fur Kryst., 1897, Bd. XXVIII. p. 575. 



§ That no one tlieoretical cause yet adduced in explanations will suffice is clear. 

 Minute fracturings cannot, for example, pave the way for unequal etching in the 

 case of a growing crystal of alum in a saturated alum solution. It is simply neces- 

 sary to dilute the solution slightly in order to etch the newly made surface ; here 

 there could be no reasonable supposition that the crystal had already undergone 

 any disintegration, yet the pits are irregularly dispersed on the surface. See 



