The Monteregian Hills extend for 200 kilometres across the St. Lawrence Plain and a portion of the Appalachians. The hills are monadnocks of more resistant plutonic rocks which stand above the sedimentary rocks forming the flat plain of the Saint Lawrence Lowlands.
The Monteregian volcanoes erupted during the early Cretaceous and are evidence of the presence of a mantle-plume of magma which traces its path from west to east. This mantle-plume trace continues as sea-mounts on the Atlantic Ocean floor. The magma generated altered in composition from west to east. Rocks in the west are iron rich and silica poor. As the plume moved east, silica content increased and iron content decreased so that syenites were formed and granite was injected, probably because silica-rich rocks beneath the Appalachian mountains were melted and mobilised by the plume.
Two of the Monteregian Hills are of particular interest to the mineralogist and mineral collector, Oka Hills and Mont St. Hilaire.
The Oka Hills are the location of the St. Lawrence Columbium and Metals Corporation Mine. The mine started as an open pit in 1961, continued underground in 1965 and is now closed. The niobium was extracted by mining pyrochlore (Na,Ca)2Nb2O6(OH,F) in carbonate rich carbonatite rocks. A niobium rich variety of perovskite called latrappite is present as black crystals which have the properties of a natural semi-conductor. The ore had an average grade of 0.45% Nb2O per ton. The Oka Hills consists of outward dipping ring dykes and breccia pipes. The carbonate-rich rocks were formed from a gradually changing igneous source of CO2 bearing peridotite magma which altered through time as chemistry and temperature changed, to an alkali-rich syenite magma.
Mont Saint Hilaire has a similar volcanic history to the Oka Hills. The syenites at the Poudrette and Demix quarries is extracted for crushed stone. During the operation of these quarries cavities have been found containing a wonderful variety of minerals, some of which are being studied and may be prove to be new minerals. Spectacular orange serandite crystals 13 centimetres in length and well formed rhombs of siderite up to 25 centimetres across are two of the larger minerals lining vugs. Micro-minerals provide endless fascination for amateur mineral collectors who are the discoverers of some of the new material studied by professional mineralogists at the Royal Ontario Museum and National Museum in Ottawa. Over 277 mineral species are known from Mont Saint Hilaire.
Francon Quarry extracts limestone on Montreal Island in the city limits. A sill was intruded into the limestone layers from the same magma source as the Monteregian Hills. This sill is the source of over 60 exotic mineral species including weloganite which is named after the first director of the Geological Survey of Canada, Sir William E. Logan. The mineral's composition is Sr, Na, Zr(CO3)6*3H2O.
Famous Mineral Localities in Canada, 1989, Joel Grice, published by Fitzhenry and Whiteside for the National Museum of Natural Sciences, Ottawa.