one of the foremost Canadian geologists of the 19th century.

Logan was educated at the University of Edinburgh and began working for his uncle in London in 1818. From 1831 until 1838 he managed his uncle's coal and copper-smelting interests in Swansea, Glamorganshire, and in this capacity he prepared geologic maps of the Welsh coalfields. Logan's observations of the close association of underlying clay layers and fossil tree roots with local coal beds provided decisive evidence for the theory that coal beds are formed in place.

In 1842, when the Geological Survey of Canada was formed, Logan was made its director, and he served in this capacity until 1869. His chief work for the Geological Survey was his monumental Report on the Geology of Canada (1863), a compilation of 20 years of research. Another of Logan's important achievements was his recognition that the Paleozoic (from 570 million to 245 million years ago) rocks of northeastern North America were divided by a prominent zone of thrust faulting running along the valley of the St. Lawrence River and then trending south along the Hudson River valley and southwest across Pennsylvania. This line is known as Logan's Line. The Paleozoic strata west of Logan's Line are relatively undisturbed, while those lying east of the line have been greatly deformed. Logan was knighted in 1856.