164 '. WILLIAM J. MILLER 
(2) gabbro chiefly of the normal type but with irregular patches 
or masses of lighter colored rocks of syenitic or dioritic make-up, 
these patches blending with the normal gabbro; (3) the whole 
stock made up of lighter colored, more acidic rock; and (4) any one 
of the above types with blocks or inclusions of the country rock. 
These four types are all primary variations. Examples of the 
last three types will be given later. 
MEGASCOPIC FEATURES 
The gabbro and its derivatives present a truly remarkable 
number of facies or varieties clearly visible to the naked eye and 
these will now be briefly described. 
The coarseness of grain varies from the merest fraction of a 
millimeter to fully an inch (e.g., stock No. 4 of accompanying map). 
The fine grained portions are confined to the borders of the stocks 
or the few branching tongues, and were caused by the more rapid 
chilling of the rock in those positions. However, even the finest 
grained rocks are holocrystalline. As a rule the coarseness of 
grain increases toward the interior of the masses, though often 
medium to coarse grained rocks extend to the very contact. The 
typical or prevailing gabbro shows a medium grain; that is, the 
grains are from 1 to 5 mm. across. 
The texture varies from coarse to medium to fine grained 
granitoid, to medium to coarse grained diabasic (ophitic). The 
gabbro from stock No. 4 is an excellent example of diabase texture 
in which the feldspar laths attain a length of an inch or more. 
The typical gabbro always exhibits the diabasic texture. 
In color the gabbro and its derivatives range from nearly 
black through dark to light gray, the darker varieties, in many 
cases, showing a slight reddish tinge due to the presence of gar- 
nets. The gray rocks all belong to the more acidic—dioritic and 
syenitic—facies below described. In one case a greenish-gray 
color was noted. The very dark color of the typical gabbro is 
due to the fact that the feldspars are so charged with tiny black 
inclusions. 
In the typical gabbros the minerals commonly recognizable 
with the naked eye or hand lens are plagioclase, pyroxene, horn- 
