The Franconia quadrangle of New Hampshire supplements the geology of the Littleton and Moosilauke quadrangles in such a way that a very representative picture of the geology of western and central New Hampshire may be obtained by a knowledge of these three quadrangles.
Three groups of rocks occupy the Franconia quadrangle. The oldest group contains highly deformed metamorphic rocks of Ordovician (?) and Devonian age. The second group consists of plutonic rocks of the New Hampshire magma series. This is a subalkaline series, in part syntectonic, and is probably late Devonian. The chief dark mineral is biotite, and the rocks are more or less foliated. The third group is the White Mountain magma series, which is alkaline and probably Mississippian. Such minerals as hedenbergite, fayalite, hastingsite, fluorite, and allanite are characteristic, and biotite is subordinate. The rocks of this group are massive.
The structure reflects the genetic differences of the three groups. The metamorphic rocks are thrown into closed, isoclinal folds which trend northeasterly. The New Hampshire magma series occurs as great sheets and lenses, which have forcefully injected the older metamorphic rocks. The White Mountain magma series is represented by both extrusive and intrusive members, the latter occurring in ring-dikes and stocks. The space occupied by the ring-dikes was attained by piece-meal stoping along essentially vertical arcuate fracture zones.