Campbell. — On Crystalline Rocks. 



453 



" One of the arguments against the igneous origin of granite is that its 

 quartz has a s.g. of 2-6, identical with that of siHca, derived from aqueous 

 sokition, while the s.g. of fused silica is only 2-2" (Eutley's "Petrology," 

 p. 207). 



Professor Haughton, in his annual address to the Geological Society of 

 Dublin in 1862, in alluding to a table of the specific gravities of natural and 

 artificially fused rocks, remarks : — "It appears to me that the column of 

 differences greatly strengthens the arguments of those chemists and geolo- 

 gists who believed that water played a much more important part in the 

 formation of granites and trap rocks than it has done in the production of 

 trachytes, basalts, and lavas, and that they owe their relatively high s.g. to 

 its agency." 



The accompanying table from Dr. Page's " Geology" shows admirably 

 the component parts of granite, the felspar occm-ring in two varieties, 

 " orthoclase " and " oligoclase," the former being associated with white 

 and black mica (uniaxial and biaxial). 



A table of felspars is also shown for the sake of reference. It shows the 

 crystallographic relations with the chemical. 



Granite^ 



Quartz Silica 



Felspar -< 





Mica 



I Silicon 

 I Oxygen 

 ( Silicon 

 1 Oxygen 

 j Aluminium 

 I Oxygen 

 j Potassium 

 I Oxygen 

 f Silicon 

 t Oxygen 

 f Magnesium 

 \ Oxygen 

 J Potassium 

 I Oxygen 

 J Calcium 

 I Oxygen 



\Peroxide of Iron {oxygen 



Silica 



Alumina 



Potash 



Silica 



Magnesia 

 \ Potash 

 Lime 



Divisions. 



Orthoclase 

 Felspar 



Plagioclase 

 Felspar 



Crystanographic 

 Properties. 



Oblique system 



Doubly oblique 



system 



Varieties. 



Orthoclase 

 Sanidin 



,Albite 



Oligoclase J 

 I Andesite "1 



Auorthite 



VLabradorite) 



Chemical 

 Character. 



Potash ] 



Soda 



- acidic 



Lime [ basic 



In Granite, Gneiss, Syenite 

 and True Volcanic Eocks 

 only 



In Granite with Orthoclase 



and in many Diorites 

 In Granite with Orthoclase 

 In Diabase and Diorite 

 In Traehytie Eock 

 In old Lavas 



In Augitic Eocks (Dolerite 

 Gabbro and Diallage) 



