16 
MUSEUM BULLETIN NO. 30 . 
Relations to the Gabbro. That the aplites and hornblendites 
are genetically connected with the gabbro, and are pegmatitic 
vein deposits formed as the last exhalations of the cooling 
magma, is clearly shown by their internal and external relations. 
They cut all varieties of rqcks found in the stocks, including the 
later dykes, and in the case of the aplites frequently fill well- 
defined fissures; hence they must not only be younger than all 
the other rocks, but must have been intruded after the complete 
solidification of all. Their mineralogy, especially that of the 
later, aplitic veins, is that of a pegmatite, i.e., a rock deposited 
from a magma highly charged with water and other volatile con- 
stituents. In occurrence they are entirely confined to the gabbro 
bodies, and rarely extend beyond their edges for even a few feet ; 
so that they must have originated within it and probably at some 
depth. 
Changes of Composition During Hornblendization and Apli- 
tization. To determine the^ effects of the homblendizing and 
aplitizing solutions on the gabbro, and at the same time arrive 
at the composition of the solutions which produced them, chemi- 
cal analyses were made by M. F. Connor of a typical olivine gab- 
bro, a hornblendite, and an aplite. The results of the analyses, 
given below, were then plotted on the straight-line diagram 
described by Mead 1 in which the points on the curves are deter- 
mined by dividing the percentage of each oxide in the fresh rock 
— in this case the gabbro — by its percentage in the altered rock 
—the hornblendite and aplite. respectively. The results (Figure 
1 ) of the hornblendic alteration are shown by the solid line, the 
aplitic alteration by the broken line. They are discussed in detail 
in another part of the bulletin. 
The straight line diagram (Figure 1) is a convenient means 
of expressing graphically the character of the changes that have 
taken place during a roek alteration of any kind. The diagram 
is made up of a number of horizontal lines, which are subdivided 
by vertical lines according to any convenient method. The dia- 
i Mead, W. J., Some geologic short cuts; Econ. Geol. 7 , 1912, p. 136. 
