10 PROCEEDINGS COTTESWOLD CLUB IQOI 
Under each seam of coal is a bed of clay—the “underclay,” 
as itis termed. This discovery was made by the late Sir 
William Logan‘ in 1840, and was a most important fact in 
support of the growth-zw-sz¢w theory of the coal-forming 
vegetation. This view found further support from the 
fact that in this underclay were found peculiar vegetable 
fossils which proved to be roots. They are known as 
Stigmarta, and are the roots of the well-known Lefzdo- 
dendrovd types of the Carboniferous flora. Among these are 
Szeillaria, Lepidodendron, Ulodendron, and _ probably 
others yet to be determined. Now some of the Lepi- 
dodendroids are known to be of considerable size ; and this ~ 
gave rise to the theory that coal seams were the remains 
of submerged forests. Personally I have never been able 
to accept this theory. My difficulty has been this: It 
would take about eight feet of solid vegetable matter to 
make one foot of bituminous coal; therefore, for a seam 
four feet thick there would be required thirty-two feet of 
solid vegetable matter. Now how coulda submerged forest 
give this, and produce seams of coal of fairly regular thick- 
ness over considerable areas—also a coal with, say, only 5 
per cent. of ash? There is evidence of detrital material 
in the submerging waters, as instanced in the shales and 
sandstones associated with Coal Measures. Therefore, 
the coal-forming vegetation must have been a compact 
mass which prevented the detrital material from pene- 
trating. Such a compact mass we have in the bogs of the 
present day. 
With regard to the “underclays” and the occurrence 
of St7gmarta, 1 have more than once seen these beds 
without signs of S¢zgmaria, and in others the fossils are 
not numerous. Still, doubtless, they are usually to be - 
discovered. It may therefore be said :—Given that Sézg- 
maria is the root of Lepidodendroid vegetation, it is 
1 Quart. Journ. Geol. Soc., Vol. xxxix., p. 275. 
