288 
The N.Z. Journal of Science and Technology. [July 
A Quantitative Study of the Silica-saturation of Igneous Rocks, by 
J. Allan Thomson. 
Abstract. 
Variation curves for chemical constituents (mainly oxides) plotted 
against silica have been given by Harker for Pacific and Atlantic rocks, 
but trial plottings of a large number of analyses show that the points 
representing given rocks group themselves together into curved belts (a 
sort of Milky Way) and do not tend to lie on lines, as Harker’s diagrams 
might suggest. If these belts could be sufficiently defined by the plotting 
of all reliable analyses, each rock could be then classified as abnormally 
high, normal, or abnormally low for each chemical constituent compared 
to silica, and a valuable means of comparing rock-analyses would result. 
The combination of all such classifications for each important constituent, 
however, would yield more classes of analyses than there are superior 
analyses, and would be cumbersome. The attempt has therefore been made 
to obtain a small number of functions of the analyses combining relation¬ 
ships between various oxide molecules- such as exist in minerals. A norm 
somewhat similar to that of the American classification has been adopted, 
but in order to avoid the difficulty of attempting comparisons of such related 
molecules as orthoclase and leucite, albite and nepheline, hypersthene and 
olivine, in which the second mineral in each of the groups named is under¬ 
saturated in silica, it has been assumed in calculation that unlimited silica 
is available. The amount of silica actually used in satisfying the various 
bases on the assumption of complete silica-saturation is then compared with 
the amount shown to be actually present by the analysis, and a percentage 
figure is obtained expressing when positive the amount of free quartz 
present in the norm, and when negative the degree of undersaturation of 
silica. A further difference from the American norm is also the calcula¬ 
tion of excess A1 2 0 3 and Fe 2 0 3 to hydrous silicates Al 2 0 3 .Si0 2 .2H 2 0 
and Fe 2 0 3 .Si0 2 .2H 2 0, since alumina and ferric iron control at least an 
equivalent of silica in the alferric minerals after possible feldspars and 
metasilicates have been calculated. By this means a quantitative esti¬ 
mate of silica-saturation for any rock-analysis is obtained. This in turn 
is plotted against total silica, and a variation belt obtained as in the case 
of the oxides. Other functions used are the total feldspars (calculated on 
the assumption of unlimited silica), the total metasilicates, and the iron-ores 
and minor accessories. A trial plotting of the feldspars has not shown very 
definite grouping into a belt, and if a further series of analyses does not 
remedy this defect it will be necessary to seek for other functions of the 
analyses which do satisfactorily group themselves. 
About a thousand analyses have been calculated and plotted, but some 
years ago the research was laid aside in view of the anticipated early publi¬ 
cation of the second edition of Washington’s Superior Analyses of Igneous 
Rocks, which would render unnecessary the individual collation of analyses 
from original papers. This publication was, however, delayed by the war, 
and has not yet reached New Zealand. It is hoped to resume and com¬ 
plete the research by the aid of a Government research grant. Meanwhile 
it is of interest, in view of recent discussion of silica-saturation from the 
qualitative point of view, to point out that a quantitative study could 
easily be made. 
The Tectonic Conditions accompanying the Intrusion of Basic and 
Ultra-basic Igneous Rocks, by W. N. Benson. 
(This paper will appear in an English or American journal.) 
