350 Proceedings of the Royal Society of Edinburgh. [Sess. 
which the area where they occur became in some way raised above water 
and converted into dry land. 
“ The underclays, as their name implies, usually form the floor of a seam 
of coal ; but underclays do occur without any coal over them. There is 
usually, however, if coal be absent, a seam of highly carbonaceous black 
shale above an underclay.” 
2. The Glenboig Fireclay. 
When I visited the Glenboig fireclay mines I expected to see Stigmaria 
in the fireclay in its usual abundance. To my surprise I could find none, and 
on inquiry of Mr A. H. Dunnachie, the general manager, and Mr Macintyre, 
the mine manager, they were unable to show me any tree roots in the 
clay or to refer to any certain evidence of their existence. This was 
the more striking as Stigmaria rootlets occurred in the overlying sand- 
stones, and good specimens from this horizon are preserved in the mine 
laboratory. I have made repeated search for Stigmaria or other rootlets 
in the Glenboig fireclay, especially as I was told by Mr Hinxman that he 
has not been able to find any.* 
The microscope shows the presence of some decomposed vegetable 
material in the clay, but no roots or rootlets in situ in it. The plant 
fragments shown in microscopic sections probably resulted from rotten 
vegetation floating in water. Neither have I seen any remains of other 
organisms in the Glenboig fireclay. 
The absence of roots from the Glenboig fireclay, therefore, at once 
suggested that this clay had a different mode of origin from the typical 
English Carboniferous underclays ; and the Glenboig fireclay is of especial 
economic interest, as I understand the fire-bricks from it are unequalled in 
quality. 
The exact position of the Glenboig fireclay seam has been demonstrated 
by the recent careful survey of the Glenboig district by Mr Hinxman.f 
The fireclay mined at Glenboig is the lower fireclay of the Millstone 
Grit series. Its exact horizon is shown by its occurrence from two to three 
fathoms above a band of limestone which is said to be locally known as “ the 
Roman Cement,” though this name is not known at the mine. This Roman 
Cement lies, according to Mr Hinxman, from 40 to 50 feet above the 
Castlecary limestone, so that the fireclay is near the base of the Millstone 
* Professor Boyd Dawkins, on the other hand, has identified some markings as casts 
of rootlets ; the evidence for this identification seems to me, however, inadequate. 
t Summary of Progress of the Geological Survey for 1905 (pp. 119-121) and for 1907 
(pp. 102-103). 
