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Oblique fault

A fault that strikes oblique to, as opposed to parallel or perpendicular to, the dominant structure

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Offset

A zone of secondary mineralization intersecting a main strike from a perpendicular or other angle

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Oxygen

A clear, diatomic gaseous element. Symbol, O

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Oxide

The compound of an element with oxygen produces an oxide of that element

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Obsidian

A dark to black coloured highly siliceous volcanic glass

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Overturned

Where older rock beds have been moved over younger rock beds

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Orogeny

The period of geological time where the Earth’s crust folded sufficiently to create mountains