PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 471 
An examination of Mr. Sears’ map, together with a considera- 
tion of my own observations, permits me to estimate, 7 @ very 
rough way, the relative percentage areas given in the following 
table, in which are also given the relative volumes, reduced to 
percentages, calculated from these data. This estimate, it must 
be observed, includes only the main area of igneous rocks, 
excluding the area of sedimentaries and gneisses, etc., which 
covers the western part of the county, as well as the quartz-dio- 
rite area near Newburyport, since the northerly extension and 
connections of this are unknown to me. 
Rock Area, % Volume, % 
Granite - - - 30 Cyne 
Quartz-syenites - - 24 26.4 
Diorites - - - 28 33.4 
Foyaites - - - 2 0.6 
Essexite - - - I O:2 
Gabbro- - - - . I 2 
Acid Dikes - - - I 0.2 
Basic Dikes' - - =e 3 122 
Rhyolite?—- - - 10 0.6 
It will be seen that the granites and quartz-syenites constitute 
nearly two thirds of the total, the diorites one third, while all 
the others make up only three percent. This result is of special 
interest since this region is generally regarded by petrographers 
as essentially one of nepheline-syenites. They occur, it is true, 
but form only a small, though important, part of the complex. 
tShaler (of. czz., p. 583) estimates the area of the dikes of Cape Ann at 5 to Io 
per cent. As dikes are apparently less abundant elsewhere I have reduced this con- 
siderably, especially as my observations lead me to think it too high. 
2As these are flow rocks their depth will be small compared to their area, and I 
have therefore estimated their volume at only a tenth of that calculated on the basis 
of the other rocks. 
31 have recently received from Mr. Sears a large and representative collection of 
the diorites of-the main western Ipswich-Danvers area. Although time is lacking for 
proper microscopical and chemical study, they are evidently quite basic, and to all 
appearance approximate very closely to those already described, chemically and 
mineralogically. 
