THE REJUVENESCENCE OF CRYSTALS. 283 



may be eoinplctely altered by tlie ]>reseiicc of inluiitesinuil traces of 

 certain foreign substances — foreign sitbstauces, be it remarked, wliicli 

 do not enter in any way into tlie conipositioii of the crystallizing mass. 

 Thus there are certain crystals which can only be formed in the pres- 

 ence of water, fluorides, or other salts. Such foreign bodies, which 

 exercise an influence on a crystallizing substance without entering 

 into its composition, have been called by the French geologists "miner- 

 alizers." 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 fornmti<m, zoned crystals fall 

 naturally into several difl'erent classes. 



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

 or zones ditter only in color or some other accidental character. Some- 

 tinu's such ditterently colored shells of the crystal are sharply cut off 

 from one another, while in other instances they graduate impercei)tibly 

 one into the other. 



A secoml 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 nuiy be indi- 

 cated in several difterent 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 volatilization, 

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

 lapid or irregular growth. ISTot unfrequently is it clear that the 

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

 resorbed in the mass in whi(;h 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 depo- 

 sition of uncrystallized material along certain zonal planes in the 

 crystal. 



Some very intei-esting varieties of minerals, like the C'otterite of 

 Ireland, the red <|uartz of Cumberland, and the spotted amethyst 

 of Lake Sui)erior, can be shown to owe their i)eculiarities to thin bands 

 of foreign matter zonally included in tliem during theii- giowth. 



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

 the habit of the crystal during its growth. Thus, as Levalle showed 

 in IS'A {Bull. Gt-ol. Hoc. Paris, 2""\ ser., vol. Yiii, pp. G10-i;>), if an 

 octahedron of alum be allowed to grow to a certain size in a solution 

 of that substance, and then a (|nantity of alkaline carbonate be added 

 to the liquid, the octaliedral crystal, without change in the length of 

 its axes, will be gradually transformed into a cube. In the same way, 

 a sealenohedrou of calcitc maybi' found inclosed in a prismatic crystal 

 of the same mineral, the lengths of the vertical axes being the same in 

 both crystals. 



By far the most numerous and iiiq)i)rtant class of zoned crystals is 



