ROCK CLASSIFICATION ON THREE CO-ORDINATES 223 
either an excess or a deficiency of silica in forming pyroxene. The 
biotite and hornblende are assumed to have the composition 
obtained by analysis of such minerals from monzonite (analysis ¢, 
Table XIV, and 6, Table XIII, Quant. Class. Ign. Rocks). 
As a preliminary, it is necessary to calculate the mineral compo- 
sition in terms of the theoretical feldspar molecules; these may be 
changed into the feldspars actually present as follows. The feld- 
spar of alkaligranite is either anorthoclase or perthitic albite with 
microcline or orthoclase. The second condition needs no further 
calculation; the first condition is obtained by simply combining the 
calculated orthoclase and albite. Andesine is the commonest 
plagioclase of quartz monzonite and monzonite; assigning to it the 
formula Ab,;An, the calculation results as follows: 
APPROXIMATE AVERAGE MINERAL COMPOSITION OF ALKALI- 
GRANITE, QUARTZ MONZONITE, AND MONZONITE 
I 2 3 
Alkaligranite eae Monzonite 
ONGDTTT 3 din tS Hoa OO ICS eee ae 24.7 Merny sak 
OTN OCLASC mnt eR ere is Sis Seek Ercueg os 24.7 ahs ieee 
ANIDBYIGS’,. 6. bao ete ke aor ole Rare ecole One ne 7 ae 
SOda-OnUNOCIASE sla. cas ccs es ts sens asa | 24.4 33.9 
EAMIGESINEP NDR ATot Pete a crates Sito cee =: dere | 6.2 34.0 
RGD ECKL C Hear ar tn hohe te See ch erate leita wate 8.5 Stag I aes 
HenMerOUSIGIOPSIGGs 24. ee+- 004s nee. Dei | a ee 
J) ROMGITOS 0S Sn 8 5 ee ee (ce 18.8 
ullsid esse eer 3.2 
IBNOUIOs 6 6S Sate aime Oe eee Onn 2.5 
VIRION GEILC Hee MeN tancd mira isis so kcnccrare a, Nes 1.8 4.3 
MIMMEMTCCH ss ook eases et sses 3 0. Poe eel 0.2 ae eae 
TERETE or caccorceetreinic 20 cata ae ee ae ee Brea 0.6 I.4 
AN DRIGIS Go. gierG. ote Ore Oneere OCD Oren eee 0.3 0.4 | eit 
CUBIGR) 3 3: Gata Roe te HCL ee ssa anenen 0.8 0.7 °.8 
100.00 T00.00 | 100.00 
