PETROGRAPHICAL PROVINCE OF ESSEX COUNTY .465 
and nordmarkite, the tinguaite, sdlvsbergite, and paisanite. In 
the basic rocks, on the other hand, the opposite seems to hold 
goo d, that they are more basic than usual. 
In the next place they are rich in both alkalis, and Na,O 
is constantly greater than K,O. On the whole, however, Na,O 
does not predominate to such an extent as to stamp the region 
as essentially one of soda rocks, but is sufficiently predominant 
to determine the character of the types, as shown in the min- 
eralogical composition. 
In general the rocks are rich in iron oxides, FeO being espe- 
cially, and in some cases abnormally, high. It may be men- 
tioned that TiO, is high, comparatively speaking, and that BaO 
seems to be absent. 
The province as a whole then, may be characterized as one 
of rocks which are more acid or more basic than normal, high 
in alkalis, with Na,O predominating over K,O, high in iron 
oxides, especially FeO, rather high in Al,O,, and low in MgO 
and CaO. 
Mineralogical characters.—We find the main chemical features 
well expressed in the general mineralogical composition. Cor- 
responding to the high SiO, and alkalis, the prevailing feld- 
spars are albite and orthoclase, with quartz-syenites abundant. 
The albite molecule is very abundant, giving rise to the charac- 
teristic microperthites and the albitic syenites of the litchfieldite 
and pulaskite types. At the same time the soda-hornblendes 
and soda-pyroxenes are common. The low CaO forbids the 
formation of much lime-soda feldspar, which we find to be rare, 
and even in some of the basic rocks the plagioclase is more 
albitic than usual. 
The paucity in MgO is of great influence. Olivine is rare 
even in the basic rocks. A striking peculiarity in this connec- 
tion is the replacement of the deficient MgO by FeO. This 
gives rise to the presence of varieties of minerals normally mag- 
nesian, but which are here largely ferrous instead. This is 
exemplified by the abundance of the MgO-free biotites, lepi- 
domelane and cryophyllite, as well as the occurrence of the 
