In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.
Bald Hill, northwest of Monroe, consists primarily of andesite flows. These blue-green porphyritic rocks weather quite readily to a light grey material with a reddish hue which can easily be disintegrated by hand. At several localities on Bald Hill vertical dikes are exposed which contain andesite xenoliths in a fine grained matrix.
The concept of mineral is distinct from rock, which is any bulk solid geologic material that is relatively homogeneous at a large enough scale.
The concept of mineral is distinct from rock, which is any bulk solid geologic material that is relatively homogeneous at a large enough scale.