Professor Geikie on the " Fitchstone " of Eshdale. 243 



lustre of the central portions, has a dull aspect, with numerous 

 minute glimmering points, and many dispersed crystals of 

 triclinic felspar. A minutely crystalline structure is indicated 

 by the abundant kaolinised felspars on its weathered surface. 

 Yet, different though they are in outward aspect, there can 

 be no doubt that these ribs, or rather sheaths, and the in- 

 ternal vitreous cores which they surround, were originally 

 parts of the same molten mass, and that the present contrast 

 of texture must be referred to some subsequent alteration. 

 The real nature of the difference of texture, however, could 

 only be satisfactorily examined by the aid of the microscope. 

 In nothing does the petrography of to-day stand out in 

 greater contrast to that of Jameson's time, than in the employ- 

 ment of the microscope as one of the great instruments of 

 research. In this connection it is interesting to remember 

 that it was here, in Edinburgh, that William Mcol first 

 devised the method of making sections of mineral substances, 

 which, cemented with Canada balsam on glass, could be 

 reduced to any desired degree of thinness and transparency. 

 The applicability of this method to the study of rocks, was 

 not perceived for many years. The late Mr Alexander 

 Bryson, our fellow-townsman, had for some time employed 

 it in the preparation of slices of quartz and other minerals in 

 granite, showing cavities partially filled with fluid. It was 

 the examination of his collection that suggested to Mr Sorby 

 the important aid likely to be afforded by the microscope 

 to some branches of geological inquuy.* Mr Sorby soon 

 set to work on the subject, and produced in 1857 his great 

 paper on the '' Microscopical Structures of Crystals, indicating 

 the origin of minerals and rocks." -|- With the appearance of 

 this memoir a new era began in the study of rocks. Not long 

 after its publication, the subject was taken up by Zirkel in 

 Germany, where it has been since pursued with ever increas- 

 ing interest and enthusiasm by a continually augmenting 

 band of observers. The literature of microscopic petro- 

 graphy is already voluminous, and bids fair to go on in rapid 

 multiplication, for geologists have now very generally recog- 



* See Mr Sorby 's paper, Quart. Jour. Geol. Soc, xiv., p. 454. 

 t 0}} ciL, p. 453. 



