464 HENRY S. WASHINGTON 
It must be noted that throughout this paper the terms acid 
and basic refer only to the relative amount of SiO,, no con- 
notation of the amounts of the other oxides being implied. 
Use of the term petrographical province-—The idea which 
underlies the general use of this term is that of a region of 
igneous rocks which possess in common certain characters, 
structural, mineralogical, or chemical, and in which the charac- 
ters may vary cOntinuously from one end to the other of the 
series of rocks represented. The term is usually applied to 
large areas embracing several centers of igneous activity, which, 
by their similarity in character, may be presumed to be related. 
Its application in the title of this paper is somewhat restricted, 
and therefore open to criticism, but seems justified on the 
grounds of convenience, the evident relationship of the rocks, 
and the fact that this region may serve as the type of the 
still larger New England one. 
In Table I are given my analyses of the 
Chemical characters. 
rocks of Essex county, with one by Dr. Eakle, and in Table II 
the molecular amounts of the various oxides. It is to be borne 
in mind that all references to the relative amounts of the oxides 
are to their molecular amounts, and not to their percentages as 
- obtained in the analyses. 
The range in composition is very great, the rocks varying 
from basic gabbro with 44 to acid granite with 78 per cent. of 
SiO,. Al,O, varies considerably and is notably higher toward 
the basic end. The total amounts of iron oxides are rather 
high, Fe,O, being low and varying little, while FeO is higher, 
especially so in the basic rocks. MgO and CaO behave alike, 
being low in the more acid rocks, and suddenly much higher in 
the basic. The alkalies areabundant, Na,O more so than K,O, 
but, on the whole, do not vary as much as the other con- 
stituents. 
The rocks as a whole are rather acid, 7. e., they contain more 
SiO, than most similar types elsewhere. The granite is 
decidedly an acid one, the foyaites more acid than most nephe- 
line-syenites, and the same is true of the pulaskites, the akerite 
