The American geologist J.P. Lesley’s career began with a degree from the University of Pennsylvania in 1838. After helping Henry D. Rogers’ work with the Geological Survey of Pennsylvania for three years, he enrolled in the Princeton Theological Seminary from which he graduated with a licence to preach in 1844. There followed a year at Martin-Luther University in Halle-Wittenberg, Germany and more work with Rogers, as well as two years with the American Tract Society. In 1847 he became pastor of a Congregational church in Milton, Massachusetts, but four years later, in 1851, he had a change of heart, becoming a Unitarian: he left the ministry and moved back to Philadelphia with his family to spend the rest of his life doing geological research in the United States and Canada. As a consulting geologist he also went to Europe in 1863 to examine the Bessemer Iron Works in Sheffield, England, and to the 1867 Paris World Fair as one of ten commissioners sent by the United States Senate. He taught geology at the University of Pennsylvania from 1872 to 1885 and as an emeritus professor the following year. He became state geologist of Pennsylvania in 1874.
Despite these responsibilities, he found time to write many reports, papers and books on geology, and on iron and coal in particular. In addition, he was secretary and librarian of the American Philosophical Society from 1858 to 1885 and was one of the original members of the National Academy of Science. In 1884 he became president of the American Association for the Advancement of Science. George Mercer Dawson wrote his obituary in 1903 for the Transactions of the American Philosophical Society.