Agate of Mont-Lyall (Gaspésie)
In the Devonian period, 350 million years ago, there was a volcanic eruption at Mount Lyall and lava erupted to the surface. Some lava flows contain gas bubbles (lithophysis), sometimes very large, which form volcanic rocks called a geode.
The mineralogy of the geodes of Mount Lyall consists of feldspar, quartz and chalcedony.
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