Gregory.] TESCHENITES. 185 
Microscopic de^criptio?i. — The microscope shows the feldspars shorter 
and narrower than those in the main dike mass, the micas much more 
abundant, and the augite, now largely represented by chlorite, in 
grains or small granular crystal sections, and not in long prism sec- 
tions. Their color and pleochroism, too, are absent. The apatite is in 
exceedingly line needles, and calcite is more common as an alteration 
product. The iron is still abundant, but in smaller, irregular grains 
and crystals. 
The granular representative from lot 106 is in the nature of a transi- 
tion from the rock just mentioned to the crystalline type. The min- 
erals revealed by the microscope are as before — iron, apatite, mica, 
feldspar, and analcite. Iron is in small, black and brown, irregular or 
polyhedral grains. Biotite is the most abundant dark mineral and 
occurs in good-sized sections, which often show alteration to the j r el- 
low-red mineral common in all these rocks. The augite is almost col- 
orless and occurs as small crystal sections and grains, always of smaller 
size than the mica, and never becoming a markedly noticeable compo- 
nent. The feldspars are oligoclase in laths shorter and narrower than 
the other types, and the analcite is not a prominent constituent. In 
structure this granular variety resembles the groundmass in tine- 
grained panidiomorphie kersantite. 
CONTACT FACIES. 
Macroscopic description. — A few feet from the sedimentaries on the 
south the teschenite of the dike along the west side of lot 99 assumes 
a particular variation due to contact with the slates. The rock here 
assumes a deep-black color, is exceedingly dense and tough, and is dis- 
tinctly porphyritic by the presence of pink feldspar laths and an occa- 
sional augite crystal in the cryptocrystalline groundmass. At the 
contact the hand specimen shows numerous angular fragments of slates 
embedded in the igneous rock and tongues of the teschenite, reaching 
into the hardened slates, but no smooth contact walls were observed. 
Weathered surfaces show cavities left by decaying feldspar phenocrysts, 
which give the rock a porous aspect. 
Microscopic description. — Microscopic examination reveals a black 
cryptocrystalline groundmass in which are embedded phenocrysts of 
feldspar and augite. The augite is represented only by its outline, 
which, however, is perfect, and with its bright filling of calcite stands 
in marked contrast to the black groundmass. The feldspars also have 
clear crystal outlines, rarely with notched ends, and have nearly all 
gone over to calcite. 
The groundmass is composed of a black dust with particles arranged 
in blotches or rude spherulitic form so as to leave yellow areas between 
them. Under the highest powers the radial structure is more pro- 
