162 The American Geologist. March, 1891 
afforded three specimens taken in the same sequence as before. 
The first, from the contact, shows a pronounced porphyrite 
structure consisting of a fine ground mass of plagioclase, augite 
and magnetite in which are imbedded lath-shaped crystals of rather 
cloudy plagioclase of an earlier generation and polysornatic aggre- 
gates of pyroxene, which in many cases is partially altered to a 
serpentinous, greenish-yellow substance either on the periphery, 
having a fresh core, or in patches and shreds through the section. 
In the second specimen, taken at six feet from the contact, the 
structure is ophitic and in marked contrast to that of the last. 
The plagioclase is fresh, the augite is in scattered grains and in 
polysornatic aggregates and is more or less altered to hornblende. 
Quartz is present and is intergrown with the feldspar after the 
manner of pegmatite. Magnetite occurs in skeletal forms and 
apatite in slender needles. In the specimens taken from the mid- 
dle of the dyke the general structure is granular rather than 
ophitic although the latter structure is observable. Augite is seen 
in occasional large polysornatic grains with a good deal of filmy 
or shreddy perimorphic hornblende and some chlorite. Hornblende 
also occurs in independent masses. Quartz is very abundant and 
is nearly all intergrown with feldspar in pegmatitic structure. 
Magnetite and apatite are present, the former in irregular scat- 
tered grains and the latter in slender needles. A partial chemical 
examination of the specimens from this dyke gave the following 
fio-ures for the percentage of silica and the specific gravity : 
Contact. 
Middle. 
Si0 2 
Sp. g. 
49.26 
3.077 
51.04 
3.007 
Near the mouth of Shoe Bay, on the north side is another dyke 
similar to the last. It is about seventy feet wide. Thin sec- 
tions of three specimens taken from the same parts of the dyke 
as before, present the same general features as in the dyke last 
described. The rock at the contact is a porphyrite with the 
usual plagioclase crystals and augite aggregates imbedded in a fine- 
grained base. The latter is remarkable for the uniformly even 
distribution of the magnetite grains. The porphyritic augite has 
no crystallographic boundaries and its alteration is for the most 
