The gabbro is composed of unstable minerals (plagioclase, pyroxene, olivine and amphibole) and easily succumbs to granular disintegration. The interlocking texture and more stable mineral assemblage of the hornfels (quartz, K-feldpar, plagioclase, mica, cordierite and andalusite), makes it more resistant than both the plutonic rock that created it and the slate from which it was derived.
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Jul14 08
