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CHAPTER XXIII 



APPLICATION OF THE MICROSCOPE TO GEOLOGICAL 

 INVESTIGATION 



THE utility of the microscope is by no means limited to the deter- 

 mination of the structure and actions of the organised beings at 

 present living on the surface of the earth ; for a vast amount of 

 information is afforded by its means to the geological inquirer, not 

 only with regard to the essential nature and composition of the rock- 

 masses of which its crust is composed, but also with regard to the 

 minute characters of the many vegetable and animal remains that 

 are intombed therein. 



The systematic employment of the instrument in petrographies! 

 research dates from 1858, when Dr. H. C. Sorby, F.R.S., published his 

 classical paper ' On the Microscopical Structure of Crystals, indicating 

 the Origin of Minerals and Rocks.' l The observations in this paper 

 were based upon the microscopical examination of thin sections of 

 rocks and minerals ; still, although Dr. Sorby was the first to apply 

 this manner of investigation to such objects, the first to suggest and 

 arrange the method of preparing thin sections appears to have been 

 William Nicol. A description of his method is given by H. Witham 

 (1831). 2 Previous to 1858 only those minerals could be examined 

 microscopically which possessed the necessary degree of transparency, 

 whilst rocks were largely closed secrets. Nevertheless Cordier (in 

 1815) was able to determine the constituent minerals of many rocks 

 by the study of the powder under the microscope ; a procedure which 

 Fleurian de Bellevue had previously recommended in 1800, and 

 which is still found valuable for certain purposes. Seven years before 

 Dr. Sorby's paper appeared, the German scholar Oschatz exhibited 

 a series of thin sections of minerals and rocks and drew attention 

 to their important bearing upon structural studies, but the collection 

 was regarded more as a curiosity than as a scientific achievement. 3 



That paper, however, gave an enormous impetus to geological 

 research, and this, in the hands of English and German students, 

 led to the growth of a * micro-petrology.' 



In order to examine minerals and rocks, sections must be pre- 

 pared thin enough to permit of the use of transmitted light ; for 



1 Quart. Journ. Geol. Soc. vol. xiv. 1858, pp. 453-500. 



2 Observations on Fossil Vegetables, Edinburgh and London, 1831. 



3 The history of the application of the microscope to geology has been sketched 

 by F. Zirkel in his paper Die Einfiilirung des Mikrosko2)S in das mineralogiscli- 

 geologische Studium, Leipzig, 1881. 



