349 
1909-10.] The Glenboig Fireclay. 
soil, and the alkalies are usually present in the form of silicates, which 
decompose slowly. Hence the total removal of alkalies from a soil by 
vegetation would be a very lengthy process. Moreover, alkalies are not 
the only, or indeed the most common of fluxes, for iron is usually the most 
potent. Further, some good fireclays are not associated with coal seams, 
while well-washed river muds may be poorer in alkali than an underclay, 
the alkalies having been washed out of the material. Thus the river muds 
from the Rhine near Bonn, analysed by BischofF,* contain only *89 per cent, 
of potash and *39 per cent, of soda; or after deducting the water and 
organic material the percentages of potash and soda are B02 and 045 
respectively. Another analysis by BischofF of river mud from the Rhine 
collected above Lake Constance contained potash • 55 per cent, and soda 
*54 per cent., or after deducting the water and carbonates the percentage 
of these two constituents only rose to - 91 per cent, of potash and '90 per 
cent, of soda. 
Nevertheless, as plants certainly collect potash in their leaves, which 
are readily carried away by the wind, and also in their fleshy parts which 
could be blown away after drying, the poverty in potash of the clays below 
coal seams must, no doubt, be to some extent due to the action of the forests 
that once grew over them. Riesf is, however, no doubt correct in his 
warning that solution has sometimes been the efFective agent in the elimina- 
tion of the alkalies, and their absorption by the plant roots is not the 
universal explanation of the conversion of what would have been a common 
clay into fireclay. 
That many of the common British fireclays were once the actual subsoils 
of Carboniferous forests is shown by the abundance of fossil roots found in 
them. Thus the late Professor A. H. Green J remarks of the Yorkshire 
fireclay “they always contain the fossil called Stigmaria, which is now 
known to be a root ; and long black ribbon-shaped filaments, which are the 
rootlets given ofF by the larger Stigmaria root, ramify through them in 
every direction. Many instances have been observed where fossilised 
trunks of trees, still standing erect in the position in which they grew, and 
attached to their roots, rise out of an underclay. There can be no doubt, 
then, that the underclays are old vegetable soils, and that, unlike all the 
Carboniferous rocks we have hitherto noticed, they were formed not under 
water but on dry land. They mark, in fact, a succession of periods during 
* F. Bischoff, Elements of Chemical and Physical Geology , vol. i., 1854, p. 123. 
t H. Ries, Clays, 1906, p. 179. 
I A. H. Green and others, “ The Geology of the Yorkshire Coalfield,” Mem. Geol. Surv ., 
1878, p. 19. 
