The center part of  the Carnmenellis (one of the places where exposed masses of the Cornubian batholith granite rock can be seen) outcrop is a medium-grained granite with many larger megacrysts, while the main outcrop consist of coarse-grained megacrystic granite with small megacrysts.
The term 'Carnmenellis Granite' refers to the plateau of high ground in this area, one of five granite plutons in Cornwall that make up part of the Cornubian batholith (see also Geology of Cornwall). The Carnmenellis pluton and the smaller intrusion of Carn Brea appear to be part of a single intrusive body. The central part of the Carnmenellis outcrop is a medium-grained granite with few megacrysts. The bulk of the main outcrop and the Carn Brea and Carn Marth masses consist of coarse-grained megacrystic granite with small megacrysts. 
