286 THE REJUVENESCENCE OF CRYSTALS. 



thors. Sorby lias sliown that, iu the so-called crystalline sand gfrains, 

 we have broken and worn crystals of quartz, which, after many vicis- 

 situdes 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 c-ise of 

 augite. In the feldsi)ars of certain rocks it has been proved that crystals 

 that have been rounded, cracked, corroded, and internally 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 tlie biologist. It is only 

 iu the lowest forms of animal life that we find an unlimited jwwer of 

 repairing injuries: in the lihizopods and some other groups a small 

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

 nomenon is exhibited much more commonly, and in forms belonging 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 inter- 

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

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

 further extent. We know in fact no limit to the minuteness of frag- 

 ments which may, under favourable conditions, grow into i^erfect crys- 

 tals, no bounds as to the time during which the crystalline growth 

 may be suspended in the case of any particular individual. 



The next proj)erty of crystals which I must illustrate, in order to 

 explain the i)articular case to which I am calling your attention to 

 night, is the following: 



Two crystals of totally different substances may be developed within 

 the space bounded by certain planes, becoming almost inextricably 

 inter-grow^n, 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 inclosed by its 

 bounding planes. Crystals often exhibit cavities filled with air and 

 other foreign substances. In the calcite crystals found in the Fon- 

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

 calcic carbonate, while more than (>0 per cent is made up of grains of 

 quartz sand, caught uj) during .vystallization. 



In the rock called "graphic granite," we have the minerals orthoclase 

 and quartz intergrown 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 micro-pegmatic intergrowths of the same 

 minerals which are so beautifully exhibited in the rock under our con- 

 sideration this evening. 



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



