The Poudrette and Demix quarries are found in what is called a pluton or intrusion. A pluton is formed when the magma, or melted rock, hardens under the surface of the earth. The Mont St. Hilaire pluton now rises some 350 meters above the much less weather resistant Ordovician limestone of the St. Lawrence lowlands. Odovician limestone is a common ocean sediment that has hardened into rock and dates back to between 505 to 438 million years. This location has become a world famous collection site because it yields at least 250 different mineral species.
Most sites only offer half a dozen or so minerals.
It is, however, these quarry operations that exposed these fabulous minerals. Minerals are formed when magma begins to cool and solidify, and the longer the cooling process, the larger the minerals. As mentioned, Mont St. Hilaire was formed from magma cooling underground which therefore occurred at a very slow rate and resulted in very well formed minerals in great abundance. Some of the minerals for which Mont St. Hilaire is famous are serandite, catapleiite, leucophanite, carletonite, leifite and analcime. A very special feature of this location is the fact that the solidified magma created a rock type known as an alkali syenite.