The igneous and metasedimentary dikes and volcanic vents of the Mount Washington area were studied in detail incidental to mapping the bedrock geology. The igneous dikes are typical of those in other areas in New Hampshire, but metasedimentary dikes have not been described elsewhere in the State. A few volcanic vents have been reported from other localities.
One hundred twenty-two dikes were observed. They constitute only from 0.05 per cent to 0.1 per cent of the exposed bedrock. The largest dike is 60 feet wide and is exposed discontinuously for 2½ miles and probably extends for at least 7¾ miles. The average width of dikes is 5 feet.
The attitude of only 99 dikes was determined with sufficient precision to plot them on an equal-area diagram. Most dikes strike northeasterly and dip steeply; a second set is horizontal; a third set strikes N.10°E. and dips steeply; and a fourth set strikes N.30°W. and is also steep. The fractures occupied by the four sets apparently originated at various times under different conditions of stress.
Tension fractures during north-south doming may be responsible for the N.10°E. set; the horizontal set presumably occupies shear fractures; compression as well as doming may explain the northeasterly set; and the N.30°W. set may occupy fractures formed during northwest-southeast compression of the area.
Sixty thin sections were studied, 8 from the volcanic vents. Twenty-five of the dikes are metadiabases; 7 camptonites; 6 kersantites; 10 actinolite-biotite granulites; and there is 1 syenite porphyry, 1 rhyolite, 1 quartz diorite, and 1 amphibolite.
The metasedimentary dikes were intruded into the schists of the Devonian Littleton formation after the climax of the late Devonian folding and metamorphism. They were intruded as impure dolomites, later recrystallized to actinolite-biotite granulites. The metadiabases, kersantites, camptonites, and volcanic vents were intruded as part of the Mississippian White Mountain magma series.