358 HENRY S. WASHINGTON 
cooling, the presence of mineralizers, and so forth, upon the order 
of affinity are all important elements in the problem. The list of 
these possibly disturbing factors is a long one, and the established 
facts of chemistry show that affinities which hold good, or reactions 
which take place, under certain conditions may be much modified 
or entirely altered under others. 
Bearing these possibilities in mind, we may proceed to the com- 
parison between the actual rocks and our assumed norms. This may 
be done by plotting the rocks on the same diagram as our theoretical 
areas, and observing the correspondences and divergences, for which 
we may again turn to Plate II (p. 270). 
The rocks chosen for comparison embrace practically all those 
known to me which carry modal leucite, whether or not it is present 
in the norm, and those which show normative but not modal leucite. 
Some seriously altered rocks are omitted, and only rocks of which 
superior, trustworthy analyses exist, and of which the norms can be 
properly calculated, have been selected. With these are also plotted 
certain rocks which do not carry leucite either in the norm or in the 
mode, but which are high in potash, as well as a few which are not 
specially high in potash, but which belong to the same comagmatic 
regions as leucitic rocks, and are genetically connected with them. 
The list of the non-leucitic rocks could be increased very greatly, 
as they form the vast majority of all known rocks; but the plotting of 
an indefinitely large number would not aid the present discussion, 
so that only those few are chosen which fall near the leucitic areas 
or which seem to bear on the problem. 
The analyses are taken from Roth’s Tabellen, from my collection 
of rock analyses published between 1884 and 1900, from the recent 
paper on the rocks of the Roman Comagmatic Region, and from 
numerous analyses published since 1900, of which a collection is 
being made preparatory to the publication of a supplement to the 
first. A list of those plotted will be round in the Appendix. 
Nearly all of these analyses are plotted on Plate II, the abscissa 
being the percentage of silica and the ordinate that of potash, both 
expressed molecularly. The following conventional signs are adopted: 
Black indicates a persalane rock, green a dosalane, and red a salfe- 
mane or dofemane rock. A circle denotes that leucite is present in 
