The possible velocity transitions, if the uppermost mantle beneath Canada conforms to the pyrolite–peridotite model, have been explored. It is found that partial melting of pyrolite or peridotite with a small amount of water would occur at depths consistent with known velocity transitions, in particular with the low velocity zones for S waves that have been interpreted as occurring beneath various regions of Canada. In addition, a spinel–garnet transition would occur for a pyrolite–peridotite mantle at a depth of 50–60 km beneath the shield, and somewhat deeper to the south and east of that region. These depths correspond to velocity transitions which have been detected in the mantle. Reasonable variations in composition could lead to variations of up to 30 km in these depths. The outflow of basalt in the Archean beneath the Lake-of-the Woods area, through its effect on mantle composition (and thus on the depth to the spinel–garnet transition) may explain the depression there in a shallow mantle refractor mapped by B. Gurbuz.