GEOLOGY. 279 



investigation, afford to the geologist information regarding the 

 origin and history of the earth's crust. Leaving their optical 

 properties, crystalline forms, and other characters as the proper 

 subject of mineralogy, he notes their composition as made known 

 by chemical analysis, the positions in which they occur, whether 

 they appear to have been formed contemporaneously with the 

 rocks on which they lie, or to have been subsequently introduced, 

 whether they indicate that water percolating deep within the 

 earth's crust has played a part in their origin, or that they have 

 crystallized from fusion, anti how far their substance has been 

 affected by later changes. In the elucidation of these and other 

 questions which minerals suggest to him, he obtains much invalu- 

 able assistance from the co-operation of the chemist. But although 

 chemical analysis will carry him a long way in such researches, it 

 cannot reveal to him the inner structure of the crystals, their 

 mode of growth, and the stages of their subsequent alteration 

 points which, when known, may perhaps help him to explain the 

 history of vast mountain masses of rock. This additional and 

 important knowledge is supplied by microscopical investigation. 



The Microscope has now become an essential part of geological 

 apparatus. It appears to have been first used for the examination 

 of thin slices of stone by transmitted light in the year 1829, when 

 W. Nicol, of Edinburgh, cut sections of various fossil woods, and 

 subjected them to scrutiny under the microscope. For this pur- 

 pose he cemented the polished surface with Canada balsam to 

 glass, and ground the section down until it showed the required 

 degree of transparency. By this method he obtained sections 

 which revealed the minute structure of plants that had been 

 converted into hard siliceous stone, and he showed how the 

 structure of fossil and living plants might be brought into com- 

 parison. Little further development of this interesting and 

 important means of research was made until, between 1856 and 

 1858, Mr. H. C. Sorby, F.R.S., published the results of his 

 application of it to the structure and history of minerals and 



