The Mt. Washington area of New Hampshire contains the highest peaks in the northern Appalachian Mountains. The metamorphic rocks belong to four stratigraphic units: the Ordovician (?) Ammonoosuc volcanics, chiefly fine-grained biotite gneiss; the Ordovician (?) Partridge formation, largely gneiss derived from shale; the Silurian Fitch formation, consisting of lime-silicate granulite and schist; the Devonian Littleton formation, mostly quartzite and schist.
The oldest intrusive rocks are quartz monzonites and granites belonging to the Devonian (?) Oliverian and New Hampshire series. Granite, syenite, tuff, and breccia belong to the Mississippian (?) White Mountains magma series.
The chemical composition of some of the minerals differs with the formation. Pyroxene and amphibole in the Fitch formation are magnesia-rich and iron-poor, whereas in the Ammonoosuc volcanics the reverse is true. Oligoclase characterizes the Partridge formation, Littleton formation, and Bickford granite; andesine is typical in the Ammonoosuc volcanics; and bytownite is common in the Fitch formation. Biotite and muscovite are relatively uniform, except for magnesia-rich biotite in the Fitch formation.
The Mt. Washington area is on the southeast flank of a large dome, the center of which is occupied by the intrusive Oliverian magma series. The folds in the Presidential Range trend north and northeast. The major folds are en echelon, and upon them are superimposed many minor folds. Schistosity, due to platy minerals, parallels the bedding. Fracture cleavage is essentially parallel to the axial planes of the minor folds. The Pine Mountain fault is the largest of several normal faults.
Metamorphism is high-grade (katazonal). Original shales are now andalusite schist, coarse rough pseudo-andalusite schist, fine grained pseudo-andalusite schist, and staurolite schist. Contrasting metamorphic history explains the various types. Metamorphism was syntectonic, and the coarser schists show three major stages in the paragenesis. Many rocks, particularly paraschists, suffered no significant chemical change. Potash was introduced into some of the coarser schists. Many of the gneisses are derived from shale by metamorphic differentiation, and less than 1 per cent of soda, lime, and potash has been introduced. Four or five per cent of soda, lime, and potash has been added to shale to form the lighter colored gneisses.