The Percy quadrangle in northern New Hampshire embraces an area of rugged topography maturely developed by stream action and subsequently modified by continental glaciation. The region is drained chiefly by the Upper Ammonoosuc River which flows generally westward into the Connecticut River.
Strongly folded and metamorphosed micaceous quartzites and schists of the Albee formation and somewhat younger amphibolites of the Ammonoosuc volcanics constitute the oldest (upper Ordovician?) rocks of the region. These have been intruded by silicic plutonics representing four distinct magma series: (1) Lost Nation group of the Highlandcroft magma series (upper Ordovician?), (2) Oliverian magma series (middle Devonian?), (3) New Hampshire magma series (upper Devonian?), and (4) White Mountain magma series (Mississippian?).
Ring dikes and stocks, composed of syenite and granite of the White Mountain magma series, form features of particular interest in the quadrangle and are believed to have been emplaced by ring-fracture stoping or cauldron subsidence. In some cases the process appears to have involved wholesale intrusion of magma along clean-cut, ring-shaped or arcuate fractures; in other instances arcuate shatter zones apparently developed and these were intruded by multitudinous small dikes which gradually replaced the country rock.