COMPOSITION or KOCKS. 9 
In elementary form. 
100. 000 
In the elementary column the iron reported in iron pyrites is 
included, but hygroscopic water is thrown out. The elements not 
included in the calculation represent minor corrections, to be applied 
whenever the necessity for doing so may arise. For estimates of 
their probable amounts Vogt’s papers may be consulted.* The per- 
centages assigned to C, Zr, Cl, F, Ni, Cr, and V are nothing more 
than very rough approximations. 
By a similar statistical process I have tried to ascertain something 
with regard to the relative abundance of the more important rock- 
forming minerals. Nearly 700 analyses of igneous rocks were studied, 
and the foregoing table of averages was also taken into account. 
For apatite, and for the titanium minerals titanite, ilmenite, and 
rutile the calculation was simple, but the other figures in the follow- 
ing table are approximative only: 
Apatite......22.......2 222 eee eee ee eee eee rete teen eee 0.6 
Titanium minerals.............----- 22-22 -e+ eee eee eee eee eee 1.5 
QUaTLe: cScnccmacccc tty asus ose bed eee dae Be ee ese tee desley eee 12:0 
Feldspars.......----- dem Eiht Aeee Me canGniwnd syGlde se MEaeeeaeikse 59. 5 
Bidtites scsexcdesges oe eee teeeeceeaaeds + nese seem eeeesaes 3.8 
Hornblende and pyroxene.........------+--+-+2+ 25222 eee eee eee 16.8 
94.2 
The less frequent minerals make up the remaining 5.8 per cent. 
The computation, although it is by no means conclusive, is not with- 
out some significance. It is interesting to note that the average 
@Zeitschr. prakt. Geologie, 1898, pp. 225, 314, 377, 413; and 1899, pp. 10, 274. 
