1891.] on the Bejuvenescence of Crystals. 257 



We are now in a position to explain the particular case whicli I 

 have thought of sufficient importance to claim your attention 

 to-night. 



In the Island of Mull, in the Inner Hebrides, there exist masses 

 of granite of Tertiary age which are of very great interest to the 

 geologist and mineralogist. In many places this granite exhibits 

 beautiful illustrations of the curious intergrowths of quartz and 

 felspar of which I have already spoken. Such parts of the rock 

 often abound with cavities (druses), which I 'believe are not of 

 original, but of secondary origin. At all events, it can be shown 

 that these cavities have been localities in which crystal growth has 

 gone on — they constitute indeed veritable laboratories of synthetic 

 mineralogy. 



Now in such cavities, the interpenetrant crystals of quartz and 

 felspar in the rock have found a space where they may grow and com- 

 plete their outward form ; and it is curious to see how sometimes the 

 quartz has prevailed over the felspar, and a pure quartz-crystal has 

 been produced ; while at other times the opposite effect has resulted, 

 and a pure felspar individual has grown up. In these last cases, 

 however much the original felspar may have been altered (kaolinised 

 and rendered opaque), it is found to be completed by a zone of abso- 

 lutely clear and unaltered felspar-substance. The result is that the 

 cavities of the granite are lined with a series of projecting crystals of 

 quartz and clear felspar, the relations of which to the similar 

 materials in an altered condition composing the substance of the 

 solid rock are worthy of the most careful observation and reflection. 

 These relations can be fully made out when thin sections of the 

 rock are examined under the microscope by the aid of polarised 

 light, and they speak eloquently of the possession by the crystals 

 in question of all those curious peculiarities of which I have reminded 

 you this evening. 



By problems such as those which we have endeavoured to solve 

 to-night, the geologist is beset at every step. The crust of our globe 

 is built up of crystals and crystal-fragments — of crystals in every 

 stage of development, of growth, and of variation — of crystals, under- 

 going change, decay, and dissolution. Hence the study of the natural 

 history of crystals must always constitute one of the main founda- 

 tions of geological science ; and the future progress of that science 

 must depend on how far the experiments carried on in laboratories 

 can be made to illustrate and explain our observations in the field. 



[J. W. J.] 



