 After McGill, he joined the trading company of John Frothingham and William Workman in Montreal, then in 1865 accepted an apprenticeship under Sir William Edmond Logan, director of the Geological Survey of Canada, becoming a full member a year later.For fifteen years he worked on the geology of Ontario and Pontiac County, Quebec, paying close attention to the Survey's aim of assisting economic development by mining.In 1866 Vennor's work was connected with the finding of gold in Precambrian rocks at a mine in Madoc Township, Hastings County, correctly predicting that the resulting gold rush would not last long.