SO-CALLED PORPHYRITIC GNEISS OF NEW HAMPSHIRE 791 
but which has been recently used more than once. He states 
that ‘the granite contains veins similar to those caused by 
shrinkage on cooling in granites of admittedly eruptive origin.’’? 
Emerson makes the principle more definite. Speaking of the 
largest granitic intrusion in Massachusetts west of the Connecti- 
cut River, he says: ‘The great mass is cut everywhere by a 
very great number of dikes of a coarse muscovite-granite, which 
seem to represent later intrusions of the central portions of the 
mass into shrinkage cracks in the already cooled peripheral por- 
tions, and thus to represent more truly its original composi- 
tion.’’? This seems to be the best interpretation of those coarse 
dikes cutting the porphyritic granite composed of large individ- 
uals of the same minerals that make up that rock. They occur 
everywhere, though there is a concentration of them along the 
boundaries. With them are often associated the pegmatite 
veins of variable mineralogical constitution of aqueo-igneous 
origin and apparently without direct connection with the under- 
lying magma.? On Gun Mountain, on Bear Hill, on the road 
following the valley of Rixford brook, and in the eastern area 
near New Hampton Centre, dikelike bodies of the former kind 
transect the porphyritic granite. Such localities suggest that in 
this respect also the main granitic mass is eruptive. 
The age of the porphyritic granite—We have seen that the 
porphyritic granite intrusions were posterior to the stress-period 
during which the chief metamorphism of the New Hampshire 
rocks was brought about. The position of the axis of flexure 
determined largely the shape of the different important areas. 
Each one is batholitic in its nature. Since the eruptions ceased, 
no considerable deformation has occurred. In this, all the areas 
are alike, and from other facts, too, they were without much 
doubt essentially contemporaneous. 
Now, the geologically highest fossiliferous zone within the 
*Geol. Mag., 1887, p. 216. 
? Bull. Geol. Soc. Am., 1890, p. 559. Cf. HARKER and Marr, Q. J. Geol. Soc., 
1891, p. 284. 
3Cf. BARLOw, Am. Geol., VI, 1890, p. 29. 
