370 PROF. T. G. BONNET ON THE GARNET-ACTINOLITE [Aug. 1 898, 



we can infer from the other constituents, the amount of shearing 

 has not been great. Hence it seems more probable that, when a 

 crystal has begun to form, it can bring together its constituents from 

 some distance, even when the rock is in a powdered condition. Such 

 an occurrence is familiar enough in a molten rock, but in the cases 

 before us the constituents mast travel rather as particles in suspen- 

 sion than as molecules in solution.^ The force of crystallization, if 

 the term be permissible, must act, like gravitation, across space ; 

 it may be more aptly compared with magnetism, for it is selective 

 in action, and perhaps capable of being excited by certain conditions. 

 I mean that it is active when crystal-building is going on, but is at 

 other times, as it were, dormant. This seems to me the ' how'; as 

 to the ' why' it is better, I think, to confess ignorance than to trj^ to 

 mask this under long words. How, then, shall we account for the 

 divergent habits of the actinolites ? I have elsewhere shown ■^ that 

 hornblende under severe pressure tends to assume an acicular form, 

 and we can understand the needles developing in the planes of 

 cleav,age-foliation as those of least resistance, but this is not all. 

 The stouter crystals, as I have already said, have a tendency to 

 run into slightly diverging needles at their ends, while the longer 

 resemble the sticks of a half-opened fan. I infer from what I have 

 seen in the field and under the microscope that when the forces 

 which operate in crystal-building are unequal, when ' growth ' is 

 more easy in the direction of any axis, the tendency to unequal 

 development is exaggerated by obstruction. Now a crystal of 

 ordinary hornblende is commonly rather elongated in the direction 

 of the vertical axis ; hence, when it has to encounter opposition in 

 growing, as, for instance, to force its way through powdered rock, 

 the crystal will lengthen much more rapidly than it will thicken, 

 and this tendency will be exaggerated if the vertical axis happen 

 to lie in the plane of easiest development — namely, that of cleavage- 

 foliation. Should the crystal encounter obstacles as it enlarges, these 

 may be pushed aside if s nail enough, or incorporated with it, as we 

 so often see, or they may cause it to branch into either divergent 

 needles or root-like processes. The results will be either tufted 

 forms, or some kind of skeletal crystal, or a micrographic structure. 

 To an influence of this kind I attribute the frost-flowers on glass 

 and some of the mineral structures called dendritic. 



The effects of crystallization in the presence of obstacles of slightly 

 larger size are often exhibited when a film of mud covering a sand- 

 stone-flagged pavement is frozen. If the film be thin, it produces 

 coarse and rather large frost-flowers ; if thick, groups of tufted 

 ^nd branching crystals, with a general resemblance in size and 

 shape to the actinolites in the above-described rocks, but, as might 

 be expected under the circumstances, more closely connected 

 together. In order to test this idea, Miss Haisin, who has been 



^ The segregation of powdered flint from china-clay (Sedgwick, Trans. Geol. 

 Soc. ser. 2, vol. iii, pt. iii, 1835, p. 461) and the 'kernel-roasting' of copper-ore 

 to some extent illustrate the same propensity. 



2 Quart. Journ. Geol. Soc. vol. xlix (1893) p. 94. 



