207 
in the sheeted diabases and porphyries which comprise the 
bulk of the volcanic series. On the geological map the essexite- 
porphyrite is not shown separately but included in the volcanic 
series. 
The rock in question is porphyritic in appearance, charac- 
terized by very numerous and large plagioclase-phenocrysts 
imbedded in a scant ground-mass of very fine grain and of 
greenish-black colour. The phenocrysts are of a shape giving 
broadly tabular or rounded sections and sometimes they are 
broken. In the variety of which an analysis is given below they 
seldom exceed 1 centimeter in length whereas otherwise they 
often reach a length of 10 centimeters. They are greenish of 
colour, the cleavage planes are often dull. 
Upon microscopic examination the rock proves greatly al- 
tered by contact-metamorphism and the original structure of 
the ground-mass is quite effaced. The felspar-phenocrysts 
where they are fresh, are twinned according to the albite-law, 
sometimes also according to the Carlsbad-law. They are labra- 
dorites of about the composition Ab,An,. The smaller phe- 
nocrysts have often a coating of orthoclase. The crystals are 
partially, and many of them throughout, converted to a colour- 
less, nonhomogeneous aggregate of extremely fine structure. 
The main constituent of this aggregate has about the same re- 
fractive index and interference-tints as scapolite and it is mixed 
with minute grains of a somewhat more highly refracting mi- 
neral, probably epidote. 
The ground-mass has a structure indicating intense con- 
tact-metamorphism. Its original structure was probably less 
fine-grained than the actual structure. It consists of small 
irregular grains and scales of colourless minerals finely dotted 
with innumerable small black grains. The minerals are: plagio- 
clase, orthoclase, augite, iron-ore, and apatite. Plagioclase is 
the predominant constituent, the sections are sometimes elong- 
ated and may attain a length of 0°2 millimeters. It is sometimes 
