the weight has, in the course of time, compressed the tree into simply
the thickness of the double bark, that is, of the two opposite sides of
the envelope which covered it when living.

_Sigillarae_ grew to a very great height without branching, some
specimens having measured from 60 to 70 feet long. In accordance with
their outside markings, certain types are known as _syringodendron_,
_favularia_, and _clathraria_. _Diploxylon_ is a term applied to an
interior stem referable to this family.

[Illustration: FIG. 16.--_Stigmaria ficoides_. Coal-shale.]

But the most interesting point about the _sigillariae_ is the root. This
was for a long time regarded as an entirely distinct individual, and the
older geologists explained it in their writings as a species of succulent
aquatic plant, giving it the name of _stigmaria_. They realized the fact
that it was almost universally found in those beds which occur
immediately beneath the coal seams, but for a long time it did not strike
them that it might possibly be the root of a tree. In an old edition of
Lyell's "Elements of Geology," utterly unlike existing editions in
quality, quantity, or comprehensiveness, after describing it as an
extinct species of water-plant, the author hazarded the conjecture that
it might ultimately be found to have a connection with some other
well-known plant or tree. It was noticed that above the coal, in the
roof, stigmariae were absent, and that the stems of trees which occurred
there, had become flattened by the weight of the overlying strata. The
stigmariae on the other hand, abounded in the _underclay_, as it is
called, and were not in any way compressed but retained what appeared to
be their natural shape and position. Hence to explain their appearance,
it was thought that they were water-plants, ramifying the mud in every