256 Professor John W. Judd [Jan. 30, 



mind if we would explain the phenomena exhibited by this inte- 

 resting rock. 



A crystal may undergo the most profound internal changes, and these 

 may lead to great modifications of the optical and other physical pro~ 

 perties of the mineral : yet so long as a small — often a very small — 

 proportion of its molecules remain intact, the crystal may retain, not only 

 its outward form, hut its capacity for growing and repairing injuries. 



Crystals, like ourselves, grow old. Not only do they suffer from 

 external injuries, mechanical fractures, and chemical corrosion, but 

 from actions which affect the whole of their internal structure. Under 

 the action of the great pressures in the earth's crust the minerals of 

 deep-seated rocks are completely permeated by fluids which chemically 

 react upon them. In this way negative crystals are formed in their 

 substance (similar to the beautiful "ice-flowers" which are formed 

 when a block of ice is traversed by a beam from the sun or an electric 

 lamp), and these become filled with secondary products. As the 

 result of this action, crystals, once perfectly clear and translucent, 

 have acquired cloudy, opalescent, iridescent, avanturine, and " schiller '* 

 characters, and minerals thus modified abound in the rocks that have 

 at any period of their history been deep-seated. As the destruction of 

 their internal structure goes on, the crystals gradually lose more and 

 more of their distinctive optical and their physical properties, retain- 

 ing:, however, their external form : till at last — when the last of the 

 original molecules has been transformed or replaced by others — they 

 pass into those mineral corpses known to us as " pseudomorphs." 



But while crystals resemble ourselves in " growing old," and at 

 last undergoing dissolution, they exhibit the remarkable power of 

 growing young again, which we, alas ! never do. This is in consequence 

 of the following remarkable attribute of crystalline structures : — 



It does not matter how far internal change and disintegration may have 

 gone on in a crystal ; if only a certain small proportion of the unaltered 

 molecules remain, the crystal may renew its youth and resume its growth. 



When old and much-altered crystals begin to grow again, the 

 newly-formed material exhibits none of those marks of "senility" to 

 which I have referred. The sand-grains that have been battered and 

 worn into microscopic pebbles, and have been rendered cloudy by the 

 development of millions of secondary fluid-cavities, may have clear 

 and fresh quartz deposited upon them to form crystals, with ex- 

 quisitely perfect faces and angles. The white, clouded, and altered 

 felspar-crystals may, in the same way, be enveloped by a zone of 

 clear and trans;j3arent material, which has been added millions of 

 years after the first formation and the subsequent alteration of the 

 original crystals. In these, and many similar examples which might 

 be cited, the kernel, representing the original crystal, may exhibit 

 evidence of having undergone the most profound chemical and 

 physical modification, while the outer shell displays all the normal 

 characteristics of the mineral. 



