Approximately one-half of the Reading Prong consists of Precambrian salic intrusive igneous rocks, most of which are now metamorphosed. The intrusive rocks of the western Reading Prong and of the southeastern massif consist predominantly of gneissoid hornblende granite or hornblende granite gneiss. The rocks are relatively poor in clinopyroxene and mesoperthite content and relatively quartz-rich. In the northwestern massif, hornblende and clinopyroxene granite is common, but hornblende- or clinopyroxene-bearing syenitic and monzonitic rocks and their variants are equally abundant. These intrusive rocks are relatively enriched in clinopyroxene and mesoperthite and rather poor in quartz.
Reconstruction of the geology of the Reading Prong suggests that the western Reading Prong and the southeastern massif were on strike prior to Paleozoic time. This reconstruction indicates the existence of two parallel intrusive rock belts during Precambrian time — a northwestern syenitic belt and a southeastern granitic belt.
It is concluded that the syenitic and monzonitic rocks were generated mainly by very deep-level partial melting of quartzo-feldspathic and anorthositic rocks in a high-temperature, relatively anhydrous environment. A somewhat steeper geothermal gradient in the northwestern Reading Prong might account for the regional differences in intrusive rock distribution.