Its important to be aware of the pyrite problem on the south shore of montreal. In scientific circles, awareness of the problems caused by the swelling of rocks containing pyrite dates quite far back. The first cases were reported around 1930 in the United States. In Canada, a very explicit text on pyritic clays in the Ottawa region was published in 1975 by the National Research Council in the Construction Digest. In the Montreal region, cases of swelling were reported as early as 1985.  Before buying an older house on the south shore of montreal call your excavation professional Pyrite Pro.