PETROGRAPHICAL PROVINCE OF ESSEX COUNTY . 469 
it corresponds ina general way with the grouping already used. 
In the acid rocks, from granite to akerite, the ratio is low and 
fairly constant, varying only from .088 to .121. In the foyaitic 
rocks from glaucophane-sélvsbergite to analcite-tinguaite, it is 
much higher and also fairly constant, from .156 to .228. Inthe 
basic rocks the ratio is again low, the essexite alone showing the 
high ratio of .166, analogous to that of the foyaites, as was to be 
expected. The ratio of the hornblende-gabbro (.112) is also 
in accord with its transitional character between the diorites and 
essexite. 
A large number of other ratios have been examined, but 
without any very significant results. The only one worth 
mentions. that:of All| Oo =.He,O.,-CaO--Na,O--K,0.: Inthe 
more acid rocks, from pulaskite up, this approximates closely 
to unity, but below this there are some other wide variations, the 
sesquioxides being deficient, except in the foyaites. 
The rock series. —Without going further into details, we may 
divide the rocks of Essex county into the following series as 
defined by Broégger.* 
The first may be called the granito-dioritic, and embraces the 
granites, quartz-syenites, quartz-diorites, diorites (which are 
partly monzonitic), and gabbro. These are characterized miner- 
alogically by the presence of microperthite (albite and ortho- 
clase) in the more acid members and plagioclase with some 
alkali-feldspar in the basic, and by iron-micas (in the more acid) 
and green and brown hornblendes and pyroxenes. Chemically 
they show comparatively low ratios of Na,O to K,O and Na, O 
ait. Oto oO. and high ratios, of, MeO to, Fe,O7,. 
Torthiswis, relatedia.'sertes of dike rocks, including”, the 
aplites and microgranites, quartz-syenite-porphyry, and, at the 
basic end, probably a part of the diabases. These dike rocks 
possess chemical and mineralogical characters similar to those 
of the preceding series. 
The next prominent series is the /foyaztc, embracing the 
pulaskite, litchfieldite, and essexite, and characterized by the 
*BROGGER: Eruptivgest. d. Kristianiageb. Vol. I, p. 169, 1894. 
