THE PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 801 
The close resemblance of this analysis to those of the granite 
enclosure and the akerite is evident, the main difference being 
in the silica, while, like the akerite, it is richer in CaO and MgO 
than the enclosure. It also closely resembles the analysis of 
nordmarkite from the Christiania region, although rather more 
acid and with less alkalies. Since it corresponds to these in 
mineral composition the name of nordmarkite is justified, espe- 
cially as the akerites are a more basic group, while the nord- 
markites are essentially more acid in their characters and 
affinities. It may be noted en passant that here also, as in so 
many cases elsewhere, we meet with biotite and hornblende 
occurring in the more acid, while pyroxene occurs in the more 
basic, members of the same series. 
Nepheline-syenite. — The rocks belonging here, first noticed 
by Streeter, have been described by several petrographers, of 
whom may be mentioned Kimball,? Wadsworth,? Sears3 and 
Rosenbusch.+. They are not abundant in the region and are con- 
fined to an area along the coast about eight miles long and much 
less wide, extending from Salem Neck to Gale’s Point, Man- 
chester, and including the islands in and near Salem Harbor. 
These rocks, as is usual, are prone to variation so that there 
is much variety among the specimens, even from the small area 
of Salem Neck. Structurally they may be divided into two 
groups, a granitic and a trachytic, which correspond to the 
ditroite and foyaite of Brégger.s One or two representatives of 
Brégger’s laurdalite-structure were found, but not in typical 
development. While the two main structural types shade into 
one another the foyaitic seems to predominate, or at least there 
is a strong tendency to foyaitic development. 
The ditroites from Salem Neck, Great Haste Island and 
Mackerel Cove are not very coarse-grained rocks, of dull gray 
x KIMBALL, Am. Jour. Sci. (2), Vol. XXIX, p. 67, 1860. 
2 WApDsworTH, Geol. Mag., 1885, p. 209. 
3 SEARS, Bull. Essex Inst., Vol. XXIII, 1891. 
4 RoSENBUSCH, Mikr. Phys., Vol. II, p. 185, 1896. 
5 BROGGER, Zeit. Kryst., Vol. XVI, p. 39, 1890, and also ERUPTGEST, der Krist. 
geb., Vol. III, p. 165, 1898. 
