788 HENRY S. WASHINGTON 
While the general geology of the region lies outside the 
scope of the present paper, yet it will be not amiss to give a 
sketch of its chief features. The igneous rocks of Essex county 
extend along the coast from Lynn to the New Hampshire line, 
and inland for a distance of about five to fifteen miles, covering 
an area of about 342 square miles. To the south igneous 
rocks of similar character, and probably genetically related, are 
met with in the Boston Basin, but this paper does not include 
them. In Essex county they form a long ridge ‘‘extending in 
a northeasterly direction until cut off by the sea. To the north- 
west and to the southeast of this extended axis we have. rocks 
of a less crystalline character verging upward into distinctly 
sedimentary deposits.”* The igneous rocks have broken through 
and are later than pre-Cambrian and Lower Cambrian strata, 
blocks of which, often highly metamorphosed, are met with at 
many points within the igneous area.* The igneous rocks are 
chiefly granites, quartz-syenites, syenites, nepheline-syenites, 
essexites, diorites and gabbros, cut by numerous dikes, and 
with later flows of rhyolite. According to Sears3 these are all 
pre-Carboniferous. A geological map of Essex county, pre- 
pared by Mr. Sears, and published by the Essex Institute in 
1893, has lately (1897) been revised by him‘ and will be referred 
to frequently. 
PETROGRAPHY. 
In the description of the rocks the ideas as to classification 
generally prevailing in this country will be followed, the age 
distinction being ignored, and the rocks grouped according to 
structure and chemical and mineralogical composition. The 
separation of the dike rocks is largely for convenience and 
involves no assumptions as to the question of the ‘‘ Ganggesteine.”’ 
A number of chemical analyses will be given, and the descrip- 
<N. S. SHALER. oth Ann. Rep. U.S. G. Surv., p. 542, 1889. 
2]. H. Sears, Bull. Essex Inst., Vol. XXII, 1890. 
3 SEARS, Bull. Essex Inst. Vol. XXVII, p. 112, 1895. 
4 Bull. Essex Inst., Vol. XX VI, 1894, and 242d, Vol. XX VII, 1895. 
