CLAYS OF NEW YORK 537 



ing the insoluble residue (r) from the total analysis, thus ob- 

 taining: 



Per cent 



SiOa 17.83 



AlaOg 12.21 



FeA 4.40 



CaO 36 



MgO : 1.02 



K,0 



Ignition 3 . 70 



If we take the average composition of mica (including muscovite 

 and biotite) as: SiOg, 50^; ^s^g, 32^; alkalis, 10^; and other 

 fluxes, 8^, then we have: 



Per cent 



1.2x3.20= 3.84 AlA 



1.2x5= 6.00 SiOs 



1.2X1= 1.20 alkalis 



50 magnesia 



1.2x0.8= ^ -^ . 



.46 iron 



Substracting column 4 from 3 gives us clay substance and ferric 

 oxid: 



Per cent 



SiOa 11.83 



AI2O3 8.37 



FeA 3.94 



CaO .36 



MgO -.52 



Ignition 3 . 70 



Total 28 . 72 



By this operation the clay has been resolved into: 



Per rent 



Quartz 47 . 23 



Feldspar '. 12 . 80 



Mica 11 . 84 



Ferric oxide 3.94 



Clay substance 24.78 



