Middle-grade and high-grade metamorphic rocks, derived from sediments and volcanics, which range from Middle Ordovician to Lower Devonian, comprise half the rocks in the Mascoma quadrangle. The Orfordville formation (probably Middle Ordovician) is a new formation. Biotite gneiss of the Oliverian and New Hampshire magma series comprise the rest of the rocks.
The outstanding structural features are domes of metamorphic rocks with cores of the Oliverian magma series. Evidence indicates these bodies are laccoliths. A primary foliation is believed to have developed in these igneous rocks parallel to the contacts and a schistosity produced parallel to the bedding in the roof rocks. Bedding, schistosity, and foliation are thus all parallel. Field and laboratory studies indicate that considerable microcline later replaced some of these igneous and metamorphic rocks, producing rocks with the composition of granite.
The Mt. Clough pluton of Bethlehem gneiss with a marked primary foliation paralleling its contacts is a later major feature. It was intruded probably in the late Devonian as a huge sheet essentially parallel to the regional structure. Field evidence indicates that it forced its way over two of the Oliverian domes while buried beneath several thousand feet of rock.
The area sheds considerable light on metamorphic problems. Chlorite (amesite) is stable in the presence of muscovite in the garnet zone. Kyanite forms in the lower part of the middle-grade zone. Disequilibrium rocks are believed to be common. Rotated porphyroblasts indicate a post-metamorphism deformation. Two distinct periods of metamorphism are evinced by the fact that the schistosity formed by laccolithic intrusion has in places been highly folded and a second schistosity superimposed.