1891.] on the Bejuvenescence of Crystals. 255 



have broken and worn crystals of quartz, whicli, after many vicissi- 

 tudes and the lapse of millions of years, have grown again, and been 

 enveloped in a newly formed quartz-crystal. Bonney has shown 

 how the same phenomena are exhibited in the case of mica, Becke 

 and Whitman Cross in the case of hornblende, and Merrill in the 

 case of angite. In the felspars of certain rocks, it has been proved 

 that crystals that have been rounded, cracked, corroded, and inter- 

 nally altered — which have, in short, suffered both mechanical and 

 chemical injuries — may be repaired and enlarged with material 

 that differs considerably in chemical composition from the original 

 crystal. 



It is impossible to avoid a comparison between these phenomena 

 of the inorganic world and those so familiar to the biologist. It is 

 only in the lowest forms of animal life that we find an unlimited 

 power of repairing injuries ; in the rhizopods and some other groups, a 

 small fragment may grow into a perfect organism. In plants the same 

 phenomenon is exhibited much more commonly, and in forms belongs 

 ing to groups high up in the vegetable series. Thus parts of a plant, 

 such as buds, bulbs, slips, and grafts, may — sometimes after a long 

 interval — be made to grow up into new and perfect individuals. But 

 in the mineral kingdom we find the same principle carried to a much 

 farther extent. We know in fact no limit to the minuteness of fragments 

 which may, under favourable conditions, grow into perfect crystals ; 

 no bounds as to the time during which the crystalline growth may 

 be suspended in the case of any particular individual. 



The next property of crystals which I must illustrate, in order 

 to explain the particular case to which I am calling your attention 

 to-night, is the following : — 



Two crystals of totally different substances may he developed within 

 the space hounded hy certain planes^ hecoming almost inextricably inter- 

 grown, though each retains its distinct individuality ^ 



This property is a consequence of the fact that the substance of 

 a crystal is not necessarily continuous within the space enclosed by 

 its bounding planes. Crystals often exhibit cavities filled with air 

 and other foreign substances. In the calcite crystals found in the 

 Fontainbleau sandstone, less than 40 per cent, of their mass consists 

 of calcic carbonate, while more than 60 per centw is made up of grains 

 of quartz-sand caught up during crystallisation. In the rock called 

 " graphic granite " we have the minerals orthoclase and quartz inter- 

 grown in such a way that the more or less isolated parts of each can 

 be shown, by their optical characters, to be parts of great mutually 

 interpenetrant crystals. Similar relations are shown in the so-called 

 micrographic or micropegmatitic intergrowth of the same minerals 

 which are so beautifully exhibited in the rock under our consideration 

 this evening. 



There is still another property of crystals that must be kept in 



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