62 



ROCKS FOEMED THEOUGH IGNEOUS AGENCIES 



Mieroeline .... 56.8 Biotite 10.2 



Ortboclase 2.1 Muscovite 0.1 



Plagioclase .... 1.3 Titanite 0.6 



Quartz 28.5 Total 99.6 



Chemical Composition. — A general idea of the varying char- 

 acter of the granites may be gained from the following analyses ; 



Chemical Composition of Granite 



Kinds and Localities 



Biotite granite, near Dublin 



Ireland 



Biotite granite, Silesia 

 Hornblende granite, Salt 



Lake, Utah .... 

 Gneissoid biotite granite 



District of Columbia . 

 Hornblende mica granite 



Syene, Egypt .... 



SiO, 



73.0 

 73.13 



71.78 



69.33 



68.18 



ALO 



2V/3 



13.64 

 12.49 



14.75 



14.33 



16.20 



PeO 

 FeaOa 



2.44 



2.58 

 1.941 



3.60 

 4.10 



CaO 



1.84 

 2.40 



2.36 

 3.21 

 1.75 



MgO 



2.11 

 0.27 



0.71 

 2.44 

 0.48 



K^O 



4.21 

 4.13 



4.89 

 2.67 



6.48 



NagO 



3.53 

 2.61 



0.12 



2.70 

 2.88 



Although the mineral apatite is so universally a constituent 

 of granitic rocks, yet it occurs in such small quantities as to 

 he quite overlooked in the ordinary methods of analysis. Such 

 tests as have been made show that the amount of phosphoric 

 acid (PaOg) contained by rocks of this class rarely exceeds 

 0.2% and may fall as low as 0.05%. Small as is the amount, 

 it is nevertheless probable that it was from just such minute 

 quantities in granites and the more basic eruptives, that was 

 derived the main supply of phosphates existing in soils. 



Structure. — The granites are holocrystalline granular rocks. 

 As a rule none of the essential constituents show good crystal 

 outlines, though the f eldspathic minerals are often quite perfectly 

 formed. The quartz has always been the last mineral to so- 

 lidify, and hence occurs only as irregular granules occupying the 

 interspaces. It is remarkable from its carrying innumerable 

 cavities filled with liquid and gaseous carbonic acid or with 

 saline matter. So minute are these cavities that it has been esti- 

 mated by Sorby that from one to ten thousand millions could 

 be contained in a single cubic inch of space. The microscopic 



^ Yielded also 1.09 % manganese oxide. 



