PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 467 
It will be seen that in what are obviously connected series of 
rocks there is a regular variation in one direction, an increase 
toward the basic end. The most striking instance is the paisanite- 
tinguaite series, in which the ratios are 1.23, 2.17, 2.47, 3.62, and 
5-97. Also in the pulaskite-foyaite-essexite series, where the 
Fatios-are 1,71, 2.00, 3.00, 2.98, and 3.64. The granite, quartz- 
syenites, diorites, and gabbros show the relation less well, 
though even here the ratio is markedly higher in the basic 
members. 
These observations leave no doubt of the fact that in Essex 
county Na,O increases relatively to K,O as SiO, decreases. 
This agrees with the Christiania region, where Brégger* shows 
that in the grorudite-tinguaite series the same holds good. In 
both cases the effect is due chiefly to increase in Na,O relatively 
to SiO,, K,O remaining comparatively stationary. The line 
giving the ratio of Na,O:SiO, will show this for Essex county. 
Such a variation is not universal, as in other regions exactly the 
contrary obtains,” the K,O increasing as SiO, decreases. 
It is evident, therefore, that Na,O and K,O differentiate 
with respect to each other, a fact to which Pirsson has already 
called attention.3 This is of theoretical interest since it indicates 
that, notwithstanding their general similarity, there isa difference 
between the two alkalis in their functions in igneous rocks, a 
subject which space does not permit of being treated here. 
FeO 
Bes O: 
the preceding, ranging from 0.37 to 16.0, but examination reveals 
certain ~tregularities:. It is very high in the granite, aplite, 
quartz-syenite, and porphyry, and in all the basic rocks. It will 
also be observed that it is higher in the dike rocks than in their 
corresponding plutonic forms. It is lower in the paisanite and 
sdlvsbergites, still lower in the pulaskites and foyaites, as well as 
The variations in this ratio are far greater than in 
™BROGGER: of. cit., Vol. III, p. 249, note 1, 1897. 
?PirssON: Bull. 139 U.S. Geol. Surv., p. 138, note 5, 1896. HARKER: Geol. 
Mag., Vol. IX, 203, 1892. 
3 PIRSSON: Bull. 139 U.S. Geol. Surv., p. 138, 1896. 
