The Ben Nevis igneous complex consists of a central mass of rhyodacites surrounded by four plutonic intrusions. In order of intrusion, these are: the Outer and Inner Quartz Diorites, which are complex rock types with varying chemical and mineralogical compositions; the Porphyritic Outer Granite; and the Inner Granite. The plutonic rocks crystallized under comparatively low pressure and thus contain no primary aluminou shornblende, its place being taken in all but the most acid rocks by primary pyroxene, which was commonly replaced by an alumina-poor hornblende at lower temperatures. The ferromagnesian minerals in the acid rocks are more magnesian than those in the more basic rocks. This is considered to be due to the partial pressure of oxygen remaining constant at an unusually high level as the magmas consolidated, causing the minerals to be increasingly enriched in magnesium as crystallization proceeded.
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