70 THE APPLICATION OF THE MICROSCOPE 



England is given by the late Rev. J. Clifton Ward in the "Survey 

 Memoir," as well as in other papers in the Quart. Journal Geol. 

 Soc. 



Professors Hull, P^onney, and others have also contributed to 

 the literature on the subject in the different geological magazines, 

 as well as Mr. Rutley, of the Geological Survey, who has also 

 published a useful treatise on "The Study of Rocks." 



More has been done in Germany than in England in this 

 branch of science, and amongst a number of others I may 

 mention the names of Vom Rath, Zirkel, Vogelsang, and 

 Rosenbusch, who since i860 have puWished numerous works 

 upon the subject. 



Amongst the workers in France we find the names of Descloi- 

 zeaux and Messrs. Levy et Fouquet ; the memoir by the latter 

 gentlemen on " Mine'ralogie Micrographique," descriptive of the 

 eruptive rocks of France, is a splendid work, published by the 

 French Government, accompanied by a set of beautifully coloured 

 plates, illustrating the minerals described. 



The value of the method of examination of rocks by means of 

 thin sections is at once evident, when we have at hand specimens 

 which appear so exactly alike, that it is difficult to distinguish 

 between them, though they may have come from totally different 

 rocks; as, for example, a fine-grained grit and a volcanic ash. 

 The rounded grains of the first show under the microscope its 

 sedimentary origin ; while the unworn, prismatic, crystalline 

 structure of the other proves that it is not a sedimentary rock. 



The discovery by Sorby of the numerous minute fluid cavities 

 in the quartz of granites shows the great value of the microscope 

 in the study of these rocks, as it proved that granites have solidi- 

 fied at a heat far below the fusing points of their constituent 

 minerals, and at such a pressure as to enable them to entangle 

 and retain a small amount of aqueous vapour, which naturally 

 must have been present during liquefaction. The presence of 

 similar fluid cavities, not only in the quartz of volcanic rocks, but 

 also in the felspar and nepheline ejected from the crater of Vesu- 

 vius at the present day, led him to class them as rocks of similar 

 origin. 



When, as is often the case, especially with colourless minerals 



