ADIRONDACK BASIC INTRUSIVES 165 
blende, garnet, biotite, and ilmenite, while in addition to these 
orthoclase and quartz may often be seen in the more acidic phases. 
Except for the pretty common presence of highly gneissoid to 
even schistose amphibolite borders, the stocks of typical gabbro 
are practically devoid of gneissoid structure. Some of the lighter 
colored, more acidic phases; however, show fairly well developed 
foliation. 
It is important to note that many of the above described 
variations may be found within a single stock as, e.g., No.1. The 
following statements from Smyth’s description! of a similar western 
Adirondack gabbro fittingly apply here: 
These (primary) changes in character take place very suddenly, and the 
different phases are most irregularly distributed, seeming to conform to no 
apres. sia“ These primary variations in the rock suffice to give considerable 
diversity to different portions within a limited area, but this diversity is 
greatly intensified by certain secondary modifications of structure and com- 
position. As a result of the combined effect of primary and secondary varia- 
tions, it would be easy to collect, within an area of a few square rods, a half 
dozen or more specimens whose appearance even in thin-section would scarcely 
suggest that they had any connection with one another. 
Cushing says? of the Adirondack gabbros in general that they 
show much variation, both primary and secondary, from place 
to place. Both of these investigators proceed to discuss the 
secondary variations and their causes but, so far as the writer 
is aware, little or no attention has been given to the causes of the 
primary variations, which is the chief object of this paper. 
MICROSCOPIC FEATURES 
Mineralogical composition.—The following table will serve to 
show the great range in mineralogical composition of the gabbro 
and its derivatives. The figures refer to percentages by volume 
and are meant to be close approximations only. 
Perhaps the most striking feature brought out by this table 
is the range of rock types, through many intermediate phases, 
from a very basic olivine norite to quartz-hornblende syenite. 
Thus, No. 1 is an olivine norite; Nos. 2, 3, 4, 6, 7,.and g are 
t Amer. Jour. Sci. (April, 1896), pp. 273-74- 
2N.Y. State Mus. Bull., 95, p. 328. 
