Horace Bolingbroke Woodward, 1911 Encyclopædia Britannica, Volume 16… (1911)
“ He then pointed out that each coal-seam rests on an under-clay with rootlets of Stigmaria, and he expressed his opinion that the under-clay was the old soil in which grew the plants from which the coal was formed. To confirm this observation he visited America in 1841 and examined the coal-fields of Pennsylvania and Nova Scotia, where he found the under-clay almost invariably present beneath the seams of coal. ”
