288 THE REJUVENESCENCE OF CRYSTALS. 



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

 granite of Tertiary age, whick are of very great interest to the geol- 

 ogist and mineralogist. In many places this granite exhibits beau- 

 tiful illustrations of the curious inter- growths of quartz and feldspar, 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 sec 

 ondary origin. At all events, it can be shown that these cavities have 

 been localities in which crystal growth has gone on; they constitute in- 

 deed veritable laboratories of synthetic mineralogy. 



Now, in such cavities the inter-penetrantcrystals of quartz and feldspar 

 in this rock have found a space where they may grow and complete their 

 outward form; and it is curious to see how sometimes the quartz has 

 prevailed over the feldspar and a pure quartz crystal has been produced, 

 while at other times the opposite effect has resulted and a pure feldspar 

 individual has grown up. In these last cases, however much the orig- 

 inal feldspar may have been altered (kaolinized and reiulered opaque), 

 it is found to be completed by a zone of absolutely clear and unaltered 

 feldspar substance. The result is that the cavities of the granite are 

 lined with a series of projecting crystals of fresh quartz and clear feld- 

 spar, the relations of which to the older materials in an altered condition, 

 composing the substance of the solid rock, are worthy of the most care- 

 ful observation and reflection. 



Those relations can be fully made out when thin sections of the rock 

 are examined under the microscope by the aid of polarized light, and 

 they speak elociuently of the possession by the crystals of all those 

 curious peculiarities of which I have reminded you tins 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 undergoing change, 

 decay, and dissolution. Hence the study of the natural history of crys- 

 tals must always constitute one of the main foundations 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. 



