"Potassium-argon age measurements made on samples of rock from Charnwood Forest and surrounding districts indicate that the porphyroids and syenites probably date from late pre-Cambrian times. The area was affected by both the Caledonian and Hercynian orogenies. A number of the intrusive rocks of the Forest have been shown to date from Caledonian times. A dolerite from the Leicestershire coalfield gave a Permian age."