MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 333 
a) Originating from K,0, Na,O, and CaO we calculate the quantity 
KAISi,Os (Or), NaAISi,Os (Ab), and CaALSi,Os (An), and when this is done 
the sum of feldspar is deducted from the analytically determined sum of SiO,, 
Al,0;, K.0, Na2O, and CaO. The difference is quartz. This method contains 
a very great source of error, as an error in the determination of K,0, Na.O, 
or CaO in the calculation of the amount of feldspar will be doubled respectively 
6, 8.5, or 5 times. To this may be added the inaccuracy in the SiO, 
determination. 
b) Originating from the proportion K.O:Na.0:CaO we calculate the 
percentage of SiO. in the feldspar (ex. 65.28 per cent in No. 1) and then we 
calculate the proportion between quartz (mz) and feldspar (1—7), ex. for 
No. 1.:”. 100+ (1-).65.28=74.04. A relative error in the determination of 
K.O, Na.O, or CaO will in this manner be eliminated. But an error in the 
determination of SiO, will be doubled nearly three times. ! 
We have a control of the calculation, in method (a), in the calculated 
percentage of Al,O; in the feldspar, compared with the percentage of Al.O, 
found in the analysis. As an example, No. 5 shows 1.06 per cent too much 
A1,O; in the calculation, consequently also too much feldspar, and so too 
little quartz. The calculated 21.58 per cent quartz must consequently be 
increased. On the other hand, No. 9 shows 1.12 per cent too little Al.O, 
in the calculation, consequently too little feldspar or too much quartz. 
With combined consideration of both methods of calculation, and of the 
sum of the analysis minus ignition and Fe.0O;, we have under C written the 
probable percentage of the quartz. The rest is feldspar. 
TABLE II 
A B c 
No. Sea ceee Poe 
Qu Or Ab An Al.O; Qu Qu 
itt.6 qlee 25.60 55-51 OS 1.64 =O. 34 DG 22 WRoc! 
2 Sve ete eae 24.34 53-50 20.52 1.04 +o.16 24.90 |24.6 
26 leseueinete 24.54 2.78 20.78 it WG —=©, 02) 24.25 124.4 
Soo aie 21.58 S472 25.78 1.04 ++-1.06 23.52 |23.5(+?) 
Osa dee 20.55 59.84 18.57 0.65 +0.28 19.28 |20.3(+?) 
Tc Gee 20.43 58.12 9.84 T.59 —0.25 27.53 |28.5(+?) 
Ss beeen 27 .O1 53-38 SP CoH) alll see eA R —0.53 MB 57 Bos 
Os aes DN), BD Bee LAN Ty tienen sia —1.12 23.67 |25.5 
TO), 2 Steen 22.04 55.04 IQ .94 0.95 —1.18 20.79 |21.9 (?) 
iis 6 ee 22.70 460.07 ZO WANTING |e au —0.07 19.19 |21.0 (?) 
The specimen No. 6 is an erratic block (with 0.30 per cent Fe,O; and 
0.28 per cent ignition). Here a minimal, hardly perceptible, decomposition 
is not excluded, so the determination 20.3 per cent quartz (or according to 
Bygdén 20.81 per cent) is probably too low. For No. 11 I have no supply 
of the original literature. The analysis shows 0.26 per cent Fe.O;, 0.21 FeO, 
and 0.30 ignition, and to the calculated result is added an interrogation point. 
‘ 
