252 Professor John W. Judd [Jan. 30, 



which exercise an influence on a crystallising substance without 

 entering into its composition, have been called by the French geolo- 

 gists " mineralisers." Their action seems to curiously resemble that 

 of diastase, and of the bodies known to chemists as "ferments," so 

 many of which are now proved to be of organic origin. 



Studied according to their mode of formation, zoned crystals fall 

 naturally into several different classes. 



In the first place, we have the cases in which the successive shells 

 or zones diifer only in colour or some other accidental character. 

 Sometimes such differently coloured shells of the crystal are sharply 

 cut off from one another ; while, in other instances, they graduate 

 impercej)tibly one into the other. 



A second class of zoned crystals includes those in which we find 

 clear evidence that there have been pauses, or at all events changes in 

 the rate of their growth. The interruption in growth may be indi- 

 cated in several different ways. One of the commonest of these is the 

 formation of cavities filled with gaseous, liquid, or vitreous material, — 

 according to the way the crystal has been formed, by volatilisation, 

 by solution, or by fusion ; the production of these cavities indicating 

 rapid or irregular growth. Not unfrequently, it is clear that the 

 crystal, after growing to a certain size, has been corroded or partially 

 resorbed in the mass in which it is being formed, before its increase 

 was resumed. In other cases, a pause in the growth of the crystal is 

 indicated by the formation of minute foreign crystals, or the deposi- 

 tion of uncrystallised material along certain zonal planes in the 

 crystal. 



Some very interesting varieties of minerals — like the Cotterite of 

 Ireland, the red quartz of Cumberland, and the spotted amethyst of 

 Lake Superior — can be shown to owe their peculiarities to thin bands 

 of foreign matter zonally included in them during their growth. 



A curious class of zoned crystals arises when there is a change in 

 the hahit of a crystal during its growth. Thus, as Lavalle showed in 

 1851,* if an octahedron of alum be allowed to grow to a certain size 

 in a solution of that substance, and then a quantity of alkaline car- 

 bonate be added to the liquid, the octahedral crystal, without change 

 in the length of its axes, will be gradually transformed into a cube. 

 In the same way, a scalenohedron of calcite may be found enclosed in 

 a prismatic crystal of the same mineral, the length of the vertical axis 

 being the same in both crystals. 



By far the most numerous and important class of zoned crystals 

 is that which includes the forms where the successive zones are of 

 different, though analogous, chemical composition. In the case of the 

 alums and garnets, we may have various isomorphous compounds 

 forming the successive zones in the same crystal ; while in substances 

 crystallising in other systems than the cubic, wc find iilesiomorphous 

 compounds forming the different enclosing shells. Such cases are 



* Bull. Ge'ol. Soc. Paris, 2nd ser. vol. viii. pp. 610-13. 



