CHAM vii 



MICROSCOPICAL INVESTIGATION 



FREQUENT reference has been made in the foregoing 

 pages to the advantage of studying minerals and rocks 

 oscope. ; i eans we are enabled to 



trace the minuter structures of the earth's crust, and to 

 follow many of the stages in the formation of its rocks. 

 We can tell whirh mineral of a rock crystallised first, and 

 indeed can follow the successive phases of crystallisation, 

 in such a way as to explain many otherwise unknown 

 puts of the history of the rocks. Moreover, by this 

 >d we can trace the subsequent changes which 

 rocks have suffered in the chemical alteration of their 

 minerals by percolating water, with the resulting secondary 

 products. While a chemical analysis informs us of the 

 ultimate chemical constitution of a rock, a microscopic 

 analysis brings before us its mineralogical compos 

 showing in what forms the chemical elements have been 

 combined, and how diverse two rocks may be in struc- 

 ture and texture, though in chemical composition nearly 

 alike. 



The field-geologist, however, besides these inquiries, 

 often needs some ready means of determining the nature 



