THE MICROSCOPE IN GEOLOGY. 117 



mens are best preserved in glycerine, in cells ; we find 

 that spirits render them opaque, and even Canada 

 balsam has the same effect." 



Mr. David Forbes, in a very interesting paper 

 "The Microscope in Geology" in the Popular Science 

 Review for October, 1867, has shown how much may 

 be learnt of the mineral composition of rocks by a 

 careful and patient use of the microscope. Previous 

 to Mr. David Forbes' application of the microscope 

 to determine the composition of rocks, very little 

 appears to have been done, with the exception of Mr. 

 Sorby's memoirs on such special points of inquiry. 

 Mr. David Forbes' collection of sections of rocks and 

 their constituent minerals were, for the most part, 

 made by himself; it amounted, in 1867, to upwards of 

 2,000, and represents a wide geographical distribution. 

 " As long as the geologist encounters in the field any 

 rocks of so coarse or simple a structure as to admit 

 of their being resolved by the naked eye into their 

 constituent mineral species, or of distinguishing the 

 fragments of previously existing rocks, of which they 

 may have been built up, he may speculate with a fair 

 chance of success as to their probable origin or mode 

 of formation. When, however, as is often more the 

 rule than the exception, rocks are everywhere met 

 with presenting so fine-grained and apparently homo- 

 geneous a texture as to defy such attempts at ocular 

 analysis, all speculations as to their nature and for- 

 mation, based merely upon observation in the field, 

 can but be compared to groping in the dark, with the 

 faint hope of stumbling upon the truth. 



" In these cases the geologist must call in the aid of 

 chemistry and the microscope ; by chemical analysis 

 he learns the per-centage composition of the rock in 

 question, and the microscopic examination informs 

 him how the chemical elements are mineralogically 



