Petrographical Differentiation of certain dykes. 157 
relative attitude of the constituents that a portion of the augite is 
of earlier generation than the rest, and earlier than such portion of 
the hornblende as may be original. The rounded contours of the 
fresher masses of polj-soinatic augite suggest analogy with the 
rounded porphyritic masses shown in fig. 1. In addition to the 
idiomorphic plagioclase there is a subordinate proportion of plag- 
ioclase which shows no crystallographic boundaries. Occasionally 
a small grain of quartz may be detected. At fifteen feet from the 
dyke walls the structure is still ophitic. Augite is the dominant 
mineral and appears to be of two generations, (1) large, irregu- 
larly bounded polysomatic masses and idiomorphic crystals, 
(2) allotriomorphic, interstitial between the idiomorphic plagio- 
clase. Quartz is sparingly present, and magnetite is in large, 
irregularly scattered grains, some of it allotriomorphicall}- devel- 
oped about the idiomorphic augite. 
In the middle of the dyke the structure is entirely different 
from either that of a porphyrite or of an ophitic diabase. It is 
the type of structure characteristic of granite, gabbro, or diorite. 
All the important constituent minerals interfere with one another, 
and the only idiomorphic crystals are those of aceessoiy minerals 
such as apatite. The aspect of a section of this part of the dyke 
is shown in the drawing, fig. 3. Quartz is abundant, and the 
augite appears to be entirely replaced b\ r hornblende, so that the 
rock would be classed with the quartz-gabbros or quartz-diorites 
according as the hornblende is secondary or original. Considered 
simply as a hand specimen it is best termed, probably, a uralitic 
quartz-gabbro. 
Mineralogical variation. — The most important mineralogical 
variation observable in the series of specimens taken across the 
dyke is the passage from a quartzless rock at the dyke wall to a 
quartzose one in the middle of the dyke. No quartz can be de- 
tected at the side of the dyke. At four feet from the side quartz 
may be observed in occasional grains, forming an exceedingly 
small proportion of the constituents ; at fifteen feet it is somewhat 
more abundant, and in the middle of the dyke quartz is a promi- 
nent constituent of the rock. Another important change in the 
mineralogical composition of the rock is the encroachment of 
hornblende upon the augite as one passes from the dyke walls, 
and the final complete replacement of the augite in the middle of 
