On a two-year visit to Cornwall, he met fellow antiquarian and naturalist William Borlase through a mutual friendship with Sir John St. Aubyn 4th Baronet of Clowance (son of the 3rd Baronet of the same name, September 27th).  Borlase, the Rector of Ludgvan, enthused Pennant with a fascination in mineralogy and fossils and which formed the foundation of most his research and writings in the 1750s. An extremely wealthy man, Pennant learnt enough about geology to establish a lead mine in Derbyshire. 

Pennant and Borlase corresponded for over two decades on matters as widely ranging as mineralogy, standing stones and other antiquities, sea life and seabirds.  Pennant became known for his ‘British Zoology’ which is the first book to use the term Cornish Gannet but he used it to describe a Great Skua.