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 45. — On the Application of the Microscope to the Study 



OF KOCKS. 



By p. W. Eudler, F.G.S. 

 (Bead October 8th, 1884.) 



The subject was introduced by a brief sketch of the history of 

 microscopic petrography, with special reference to the work of 

 Dr. H. C. Sorby in this country, and of Prof. Zirkel and Prof. 

 Eosenbusch in Germany. In order to explain the methods of 

 modern petrography the speaker selected a piece of granite as a 

 well-known rock, and pointed out the characters of the constituent 

 minerals, first as seen by the unaided eye or "macroscopically," 

 and then as viewed under the microscope. 



Granite, in its typical varieties, is a crystallo- granular aggre- 

 gate of felspar, quartz, and mica. These minerals make up the 

 entire rock, without the presence of any amorphous matter 

 representing an original vitreous magma; and hence such a rock 

 is said to be holocrystalline. The felspar is generally the most 

 conspicuous mineral in a granitic rock, and may occur in well- 

 defined crystals, especially in those varieties which are porphi/ritic, 

 or display distinct crystals embedded in a fine-grained base. 

 However fresh the felspar may appear to the naked eye, it 

 usually exhibits in microscopic sections a nebulous appearance, 

 due to incipient alteration into china-clay or kaolin — a process 

 termed kaolinisation. The felspar often encloses microlites, or 

 microscopic crystals of foreign substances. 



In most granites there are representatives of two types of 

 felspar, distinguished from each other by differences in the 

 direction of cleavage. There are in all felspars two well-marked 

 cleavages, and in one type these cleavage-planes are absolutely 

 rectangular to each other, whence this kind is termed orthodastic 

 or orthotovwus ; while in the second type the two cleavages make 

 angles which are never exactly right angles, whence such felspars 

 are termed jjlagioclastic or clinotomous. The plagioclastic felspars 

 are also known, from the system in which they crystallise, as 

 triclinic or anorthic ; whereas the orthoclase is a monoclinic 

 felspar. For the discrimination of these two types of felspar 

 the petrographer relies not so much on the angles of cleavage as 

 on certain structural peculiarities which are easily detected under 

 the microscope. The plagioclase is almost iuvariably twinned 

 in a marked manner, the plane of junction being generally that 

 known to crystallographers as the brachypinacoid. If, therefore, 

 a section be cut parallel to this plane the twinning is not 

 discernible ; but cut in any other direction, the section must 

 needs cross the edges of the component laminae, and consequently, 

 when viewed between crossed Nicols, the mineral exhibits a 



