ANORTHOSITE-GABBRO IN NORTHERN NEW YORK 41 
The hornblendite dikes are commonly from one-half an inch 
to one inch wide, and they are seldom traceable individually for 
more than a few yards or rods. They cut the anorthosite-gabbro 
in all directions, not uncommonly at high angles across its foliation. 
Contacts against the country rock are always sharp. The dike 
material is practically pure, common hornblende which, in the hand 
specimen, is greenish black where fresh, and dull green where 
weathered. The dikes are rather uniformly moderately coarse 
grained, the crystals usually varying in length from one-fifth of an 
inch to one inch. There is no suggestion of finer grain along the 
borders. Granulation is absent. 
Many of the dikes occupy curving cracks in the anorthosite- 
gabbro as described below. In some such cases hornblendite 
dikes lie in rather sharp contact against dikes of nearly pure 
plagioclase anorthosite. On the southern face of the little hill 
at locality 1c, several hornblendite dikes cut sharply across some 
branching, white, plagioclase-scapolite dikes, the latter in some 
‘cases being faulted where the hornblendite dikes cross. 
Both the hornblendite dikes and the white dikes rich in plagio- 
clase and scapolite are quite certainly late differentiates of the 
cooling anorthosite-gabbro magma at some depth below the 
present rock surface. They are in a real sense complementary 
dikes, the differentiated white-dike material having been forced 
upward into the nearly or quite consolidated, but still hot, 
anorthosite-gabbro, while the differentiated hornblendite was later 
forced upward into cracks in the relatively much cooler body of 
anorthosite-gabbro. This is in harmony with the lack of sharp 
contacts of the white dikes, and the sharpness of contacts of the 
hornblendite dikes. 
In the discussion of this paper at the Boston meeting of the 
Geological Society, N. L. Bowen suggested that the fissures filled 
with hornblende might better be called veins instead of dikes. 
But, because they show no vein structure and are in almost 
every way like true dikes, the writer considers them to be dikes, 
probably in the category of pegmatites because of their coarseness 
of grain and lack of chilled borders. It is accordingly reasonable 
to believe that the hornblende was either in solution in a pegmatite 
