The Belfast quadrangle is located west of Penobscot Bay in east-central Maine. The stratified rocks of the quadrangle have been regionally metamorphosed, multiply folded, and intruded by plutonic rocks. They are exposed in two major structural blocks separated by the St. George longitudinal fault. The oldest rocks in the Passagassawakeag block, the Passagassawakeag Formation, are Precambrian(?) in age and are exposed in the Liberty-Orrington anticline. These rocks consist primarily of quartz-plagioclase-biotite gneiss; minor amphibolite and gneiss with sillimanite and muscovite are present. A calc-silicate member and a fine-grained gneiss unit composed almost entirely of quartz, oligoclase, and microcline (the metavolcanic Mixer Pond Member) are mapped separately. Unconformably overlying the Passagassawakeag Formation northwest of the Liberty-Orrington anticline is the Hogback Schist of probable Cambrian or Ordovician age. Aluminous schist, micaceous quartz-feldspar gneiss, biotite gneiss, and calc-silicate gneiss are included in the Hogback Schist. Southeast of the Liberty-Orrington anticline, the Bucksport Formation of probable Late Silurian or Early Devonian age is interpreted to overlie unconformably the Passagassawakeag Formation. The Bucksport Formation is composed of calc-silicate gneiss, biotite gneiss, and minor biotite schist. It is overlain conformably by the Appleton Ridge Formation of probable Early Devonian age. The Appleton Ridge Formation consists of phyllite or staurolite schist cyclically interbedded with quartzite; at the top of the exposed sequence there is schist rich in garnet and cummingtonite (Muzzy Ridge Member). The stratigraphic section of the Passagassawakeag block is bounded on the southeast by the St. George fault. The oldest rocks exposed in the Penobscot block that lies southeast of the St. George fault are marble and other poorly exposed rocks found in the core of the Coleman antiform. They are correlated with the Rockport sequence of the Camden-Rockland area. Also exposed in the Coleman antiform are quartzite, quartz-mica schist, retrograded sillimanite gneiss, feldspar-quartz-biotite gneiss, and calc-silicate quartzite correlated with the upper part of the Battie Formation in the Camden area. The age of these rocks is believed to be Cambrian or Ordovician. The Penobscot Formation of Ordovician age overlies the Battie Formation and constitutes most of the Penobscot block. The Penobscot Formation consists dominantly of phyllite, andalusite schist, sillimanite gneiss and granofels, quartz-mica schist, and quartzite; most of these rocks are sulfidic. There are rare marble, calc-silicate gneiss, amphibolite, garnet-cummingtonite schist, and plagioclase-quartz granofels of volcanic origin. Metamorphosed volcanic rocks of mafic and felsic composition (Gushee Member) are largely restricted to the upper part of the formation. The Jam Brook Formation is exposed between the Penobscot Formation and the St. George fault but thins and terminates along strike to the northeast. The Jam Brook contains quartz-pebble metaconglomerate, quartzite, quartz-mica schist, andalusite schist, calc-silicate gneiss, and marble, in generalized order of appearance from east to west. There are changes of facies within the formation. The age of the Jam Brook is believed to be Cambrian or Ordovician. The Jam Brook probably correlates with the Rockport sequence, and its contact with the Penobscot Formation is inferred to be a thrust fault.