Plutonic rocks of four magma series occur in the Mt. Washington quadrangle, northern New Hampshire, but those of the Oliverian magma series are most abundant. In this quadrangle this late Devonian (?) series intrudes the Ordovician (?) Ammonoosuc volcanics and consists of six mappable units: hornblende-quartz monzonite, fine-grained gray quartz monzonite, porphyritic biotite-quartz monzonite, biotite-quartz monzonite, coarse syenite, and coarse granite. Although most of these types occur in the Oliverian magma series elsewhere in New Hampshire, the coarse syenite and hornblende-quartz monzonite appear to be unique.
Potash feldspar in these rocks is generally microcline and uncommonly orthoclase; perthitic intergrowths are rare. Oligodase is the characteristic plagioclase, except in the hornblende-quartz monzonite, in which it is andesine. Biotite, the most abundant dark mineral, has a considerable range in chemical composition assuming that the γ index is a reliable indicator. The iron-magnesia ratio of the biotite is considerably higher in the hornblende-free rocks than in those containing hornblende. The amphibole in this magma series is common hornblende, and the muscovite contains some iron and magnesia.
The igneous rocks form the core of the Jefferson dome, which, essentially concordant, is more than 40 miles long and 9 miles wide. In most respects this dome resembles those in which the Oliverian magma series is found elsewhere in New Hampshire. It differs principally in its larger dimensions and composite character. The various petrographic units were intruded separately as large lenses, each developing planar flow structures that are shown by oriented minerals and xenoliths. The texture is commonly granoblastic.