LITHOLOGY. 115 



Microscope. " It often happens that neither the naked 

 eye nor a good lens will help us to get at the composi- 

 tion and textural arrangement of fine-grained rocks, 

 while the rough forms of analysis mentioned in the 

 foregoing paragraphs are equally unavailing. In such 

 cases much may be learnt by examining the rocks under 

 a microscope. For this purpose a thin slice of any rock 

 which it is proposed to examine is taken, ground smooth, 

 and polished on one side."^ The polished surface is then 

 securely fastened with Canada balsam to a piece of plate- 

 glass, and the other side is ground down until the section 

 is of the required thinness and transparency. The pre- 

 paration may be covered with a plate of very thin glass 

 mounted with balsam on the slide, care being taken to 

 exclude all air-bells, and to remove all traces of the 

 emery powder and other substances used in the grinding 

 and polishing process. 



" A rock-section prepared in this way enables us to 

 ascertain with precision the manner in which the dif- 

 ferent minerals are built into each other, and often 

 throws a flood of light on the origin of a rock, and on. 

 the subsequent changes which it has undergone. It 

 furnishes an opportunity of applying the delicate analysis 

 of polarised light, and thus reveals points of structure 

 in the composition of a rock which could not be ascer- 

 tained in any other way."^ 



" When prepared slices are examined under the mi- 

 croscope, it is often surprising to see how minerals which 



* Mr. Jordan (of the Mining Eecord Office) has invented an 

 ingenious rock-slicing machine, which may be obtained from 

 Messrs. Cotton and Co., Graf ton Street, Soho, London. 



t " Students' Manual of Geology." Jukes and Geikie. 1872. 



82 



