282 THE REJUVENESCENCE OF CRYSTALS. 



have been slowly elaborated by her during periods which must be 

 measured in millions of years. 



I propose to-night to direct your attention to a very curious case in 

 which a strikingly complicated group of phenomena is presented in a 

 crystalline mass, and these phenomena, which have been revealed to 

 the student of natural crystals, are of such a kind that we can scarcely 

 hope to re -produce them in our test-tubes and crucibles. 



But if we can not expect to imitate all the effects which have in this 

 case been slowly wrought out in Nature's laboratory, we can at least 

 investigate and analyze them, and, in this way, it may be possible to 

 show that phenomena like those in qTiestion must result from the pos- 

 session by crystals of certain deflTute properties. Each of these prop- 

 erties, we shall see, may be severally illustrated and experimentally in- 

 vestigated, not only in natural products, but in the artificially formed 

 crystals of our laboratories. 



In order to lead up to the explanation of the curious phenomena ex- 

 hibited by the rock-mass in question, the first property of crystals to 

 which I have to refer may be enunciated as follows: 



Crystals i)ossess the power of resuming their growth after interrup- 

 tion, and there appears to be no limit to the time after which this re- 

 sumption of growth may take place. 



It is a iamiliar ol)servation that if a crystal be taken from a solution 

 and put aside it will, if restored after a longer or shorter interval to the 

 same or a similar solution, continue to increase as before. But geology 

 affords innumerable instances in which this renewal of growth in crys- 

 tals has taken place after millions of years must have elapsed. Still 

 more curious is the fact, of which abundant proof can be given, that a 

 crystal formed by one method may, after a prolonged interval, continue 

 its growth under totally different conditions and by a very different 

 method. Thus, crystals of (quartz, which have clearly been formed in a 

 molten magma and certain inclosures of glass, may continue their 

 growth when brought in contact with solutions of silica at ordinary 

 temperatures. In the same way, crystals of feldspar, which have been 

 formed in a mass of incandescent lava, may increase in size when sol- 

 vent agents bring to them the necessary materials from an enveloping 

 mass of glass, even after the whole mass has become cold and solid. 



It is this power of resuming growth after interrnption which leads 

 to the formation of zoned crystals, like the fine specimen of amethyst 

 nclosed in colorless quartz, which was presented to the Eoyal Institu- 

 tion seventy years ago by Mr. Snodgrass. 



The groAvth of crystals, like that of plants and animals, is determined 

 by their environment, the chief conditions affecting tlieir development 

 being temperature, rate of growth, the supply of materials (which may 

 vary in quality as well as in quantity), and the presence of certain 

 foreign bodies. 



It is a very curious circumstance that the form assumed by a crystal 



