PETROGRAPAICAL PROVINCE OF ESSEX COUNTY 473 
series, bot{h on geological grounds and petrographical, such as 
the transition forms between the essexite and diorites. They 
represent, however, a distinctly different magma; one not only 
more basic, but richer in Na,O and Al,O, and poorer in CaO, 
MgO, and FeO. They arealso notable for the fact that consider- 
ing their very small amount they are relatively more differen- 
tiated than the main magma. This is in accordance with obser- 
vations on nepheline-syenite regions elsewhere, which, it is well 
known, carry not only a very great number and variety of rare 
component minerals, but also show a comparatively large num- 
ber of rock varieties. _ 
Since it has been shown that in the granito-dioritic series 
Na,O tends to increase relatively to K,O as SiO, decreases, and 
at the same time as it increases inversely as SiO,, it follows that 
in the course of a differentiation of such a magma there should 
be an enrichment of Na,O at the basic end. The rocks of the 
foyaitic series may then be held to represent the further differ- 
entiation products of such a basic, soda-rich portion of the main 
magma, this further differentiation taking place in accordance 
with the tendency of magmas rich in soda to differentiate, while 
in the more acid portions the relations would remain more simple. 
This explanation is essentially that of Pirsson* to account for 
the phonolitic dikes of the Judith Mountains. 
Lack of space forbids the full discussion of the foyaitic dike 
rocks, comparing them with the main types as Brogger has done, 
but the evidence goes to show that the paisanite, sdlvsbergites, 
and tinguaites are probably to be regarded as diaschistic dikes, 
i. e., further differentiation products of the foyaitic magma, and 
not simply dike forms of this. This is analogous to the Chris- 
tiania region, where Brégger? has shown that the dikes of the 
grorudite-tinguaite series are diaschistic. 
The differentiation probably laccolithic.—We have now to examine 
the question as to where the differentiation of the Essex county 
magma took place. Are the rocks, as we see them, due to 
*PIRSSON: Eighteenth Ann. Rep. U. S. Geol. Surv., p. 573, 1898. 
? BROGGER: of. cit, p. 127 ff. 
