The dolomites of the lower part of the Salina formation and the upper part of the Guelph-Lockport formations, Silurian in age, constitute the most important and prolific gas-productive zone in southwestern Ontario, the total production to date amounting to more than 275 billion cubic feet. This zone has also yielded 1,500,000 barrels of oil. Commercial production began in the Salina-Guelph zone in the year 1889, and by 1906 the two most important Salina-Guelph fields in the province had been discovered. Subsequent exploration has been very sporadic, and although eight relatively moderate-sized pools, and several smaller pools have been discovered, 10,000 square miles, or 80 per cent, of the area underlain by these formations remain relatively untested.

Folding and faulting appear to be the dominant factors controlling accumulation, but reefs, variations in porosity and incipient fracturing seem to be almost equally important.

The author describes the geological conditions affecting accumulation and the recently renewed efforts to find additional pools in this zone, and briefly discusses the prospects for future discoveries.