156 
The American Geologist. 
March, 1891 
either in distinctly isolated individuals or aggregated together in 
irregularly radiating clusters ; (2) Augite, in rounded or bleb-like, 
colorless polysornatic masses. The plagioclase is occasionally in- 
cluded in the augite. The microscopic aspect of this portion of 
the dyke is shown in fig. 1 . 
In the specimen taken only four feet from the last, i. e. , four 
feet from the dyke wall, the structure of the rock presents a 
marked contrast to that just described. There is no ground 
mass, and the section presents the character of a typical ophitic 
diabase as shown in fig. 2. 
Fig. 2. 
Stop Island dyke.— Section of dyke-rock four feet from dyke wall. 
X3S. a. Augite; f. plagioclase: h. hornblende; m. magnetite. Illus- 
trating ophitic or diabase structure. 
In this portion of the dyke the dominant minerals are polyso- 
matic augite and green hornblende. The latter mineral is in part 
derived from the augite, while part of it presents no evidence of 
such derivation and may be original. These two minerals occur 
in large interlocking masses in which are embedded beautifully 
sharp idiomorphic crystals of plagioclase and irregular grains of 
magnetite. It seems probable from a careful inspection of the 
