These roots were Stigmaria, and the stuff into which they penetrated was an underclay.
"Scientific American Supplement, No. 447, July 26, 1884" by Various
Those roots penetrated into the under clay of the coal; and those roots were Stigmarias.
"Town Geology" by Charles Kingsley
In this richly ornamented Stigmaria the characteristic areolae present the ordinary aspect.
"The Testimony of the Rocks" by Hugh Miller
Binney, Mr., on stigmaria and sigillaria, 315.
"A Manual of Elementary Geology" by Charles Lyell
Their roots were formerly described as quite a distinct vegetable form (under the name of Stigmaria).
"The History of Creation, Vol. II (of 2)" by Ernst Haeckel