Common mafic rocks include basalt, diabase and gabbro. Rocks found include andesite, basalt, dacite, diabase, gabbro and rhyolite. There are intrusions of igneous rocks, such as gabbro, diabase, and diorite. Types of trap rock include basalt, peridotite, diabase, and gabbro. Subvolcanic rocks include diabase (also known as dolerite) and porphyry. There is very little coarser-grained gabbro and diabase. Igneous rocks are sparsely represented by intrusions of porphyry, diabase, and trap. The region also includes igneous intrusions of granite and diabase. Magma was injected between layers of sedimentary rock, forming diabase sills. Metavolcanic rock and diabase underlie areas around the granitic zone. This geological process created large, thin, subvertical sheets of impermeable basalt, or diabase dikes. The hill consists of basalt or its intrusive equivalent diabase. Devil Mountain is underlain by Nipissing diabase. Trap rock, i.e. basalt or diabase, has a variety of uses. Minerals were found in veins in both the diabase and metamorphosed sediments.