GENESIS OF THE ALKALINE ROCKS 1EAAA 
crystallization.’ Nor here again does the hypothesis explain the 
comparative homogeneity of the shonkinite itself. 
Origin of quarts diabase.—A difficulty is found also in the nature 
of that common sill rock, quartz diabase. Table I gives the 
average composition of twelve typical quartz diabases from various 
parts of the world (col. 1) and the average analysis of 108 fresh 
rocks of basaltic composition (col. 2), each average being reduced 
to 100 per cent. 
TABLE I 
If 2 
SIO ent ANC mee Ea 52.34 49.00 
Moi ©) eee aR kek Rade A 1.82 1.36 
VANE Osos Barats seach ienatins se Lae 13.70 15.70 
TCH O) es Gen crse cane res ave 5.05 5-38 
EO a i Sica. mettle 8.78 6.37 
IVE Oe aie aise mene nenri 0.23 0.31 
IVI ©) ee a nar en Sn Ul 4.72 6.17 
CaO eRe art wate a 8.03 8.95 
INE TO) Boe eiseicths titra ce WEA 2.60 Bait 
KO Bena eager ca ueetis Se i 1 TS 2 
IAS O)igseseninet icy eure sree stage 1.56 1.62 
PP Oras ate les tact See malbe: Heseayet wars crt 0.45 
100.00 100.00 
Typical basalt, like normal diabase, contains no quartz. The 
quartz diabases mentioned carry 4 per cent to about 20 per cent 
of that mineral, and varying proportions of biotite, from zero to 
perhaps to per cent. According to Bowen (p. 46) the quartz and 
biotite have crystallized because the residual liquid, from which 
pyroxene, calcic plagioclase, and possibly olivine had settled out, 
became “‘enriched in alkaline feldspar molecules and water to give 
a high concentration and consequent separation of most of those 
molecules which are formed by the breakdown of the alkaline 
feldspar molecules (biotite or quartz or both). But the high 
content of lime and magnesia in quartz diabase shows the amount 
of sunken pyroxene, olivine, and plagioclase to be very small. In 
fact, Collins has illustrated quantitatively the fact that much 
quartz diabase is essentially normal diabase bearing interstitial 
Cf. N. L. Bowen, Amer. Jour. Sci., XXXIX (10915), 178 ff. 
