There are now three principal gas-bearing horizons in southwestern Ontario. In order of importance, they are: (1) the Guelph-Salina, (2) the Clinton-Medina-Cataract, and (3) the Trenton.
Very little is known of the Guelph as it is almost impossible to distinguish it from the upper and lower formations by ordinary methods. It forms an erosional contact with the overlying Salina which undoubtedly is the origin of the porosity of the Guelph.
The Trenton has been sparsely prospected. So far only two gas pools have been discovered; they are 175 miles apart, and neither is of major importance.