Metasedimentary rocks of probable Silurian to Devonian age underlie about three-fourths of the Clinton quadrangle. Metamorphosed marine mudstone, siltstone, fine-grained graywacke, and limy siltstone form two thick sequences. Primary structures preserved within the rocks include compositional layering, laminations, tabular and lenticular beds, low-angle cross-stratification, and graded beds. The thicker sequence (4,570 m to 6,100 m) is divided into four units (in ascending order): (1) white to light-gray, very fine grained quartzite interlayered with minor thin beds of gray phyllite; (2) alternating very thin laminae of dark-gray to greenish-gray phyllite and light-brown ankeritic metasiltstone; (3) dark-gray carbonaceous slate and phyllite and minor interbedded metagraywacke; and (4) dark-gray phyllite with abundant chiastolite porphyroblasts and many beds of light-gray metagraywacke. Intrusive igneous rocks form the base of this sequence, and a fault at the top separates it from the other sequence. The second, thinner sequence (1,830 m) consists chiefly of thin-bedded biotitic feldspathic quartzite, greenish-to purplish-gray calc-silicate rock, and thin layers of biotite schist. These rocks are probably older than the rocks of the first sequence. The rocks of the second sequence previously were included in the Oakdale Quartzite, those of the first sequence in the Oakdale Quartzite and the Worcester Formation of central Massachusetts. They may be broadly correlative with parts of the Gonic, Rindgemere, Towow, Berwick and Eliot Formations and possibly with part of the Kittery Quartzite of southern New Hampshire and southwestern Maine.