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this climate, or about 100 for a climate equal in temperature to that 
of the tropics. Allowing therefore that some time elapsed after the 
commencement of vegetation on the surface of the then dry land 
before the trees began to grow, Mr. Bowman infers, that 100 years 
must be the minimum of time which would be required for the 
production of the vegetable matter out of which the nine inches of 
coal were produced. With respect to the depth of the stratum of 
vegetable matter from which it was formed, Mr. Bowman takes for 
his data, the thickness of the bed of coal, nine inches; the distance 
between the top of the seam and the bottom of the trunk under the 
arch formed by the roots, fifteen inches; and for the distance to the 
surface of the ground, four inches, or in all twenty-eight inches ; 
whereby he infers that the thickness of the solid coal is equal to about 
one-third that of the vegetable matter out of which it was produced. 
A paper was lastly read, ‘‘ On the character of the beds of clay ly- 
ing immediately below the coal seams of South Wales ; and on the 
occurrence of coal-boulders in the Pennant grit of that district ;” by 
William Edmond Logan, Esq., F.G.5. 
Immediately below every regular seam of coal, in South Wales, 
(and nearly 100 are known to exist) is constantly found a bed of clay, 
varying in thickness from six inches to more than ten feet, and called 
the underclay, undercliff, understone, pouncer, or bottom stone. It 
is so well known to the collier, that he considers it an essential ac- 
companiment of the coal; and only where it ceases, does he give up 
his expectation of finding coal. Seams which have thinned out in 
one portion of a work, have been recovered in another by following 
this bed. 
The underclay is always more or less argillaceous, but it is never 
without a considerable admixture of sand; and in most cases it yields 
a very good fire-clay; which, though generally tough when freshly 
cut, yet crumbles on exposure into a mass of a grey colour. Oc- 
casionally it is quite black, in consequence of the presence of carbo- 
maceous matter, and it then sometimes resists the effects of the 
weather. Under a part of the lowest seams of coal between Swan- 
sea and the Bury river, it is a hard, durable, finely grained, siliceous 
stone. It is however by containing innumerable specimens of Stig- 
maria ficoides, that these beds are most strongly marked, other 
portions of the coal measures presenting the same mineral composi- 
tion. ‘The stems of the Stigmaria, which are usually of considerable 
length, always lie parallel to the plane of the bed, and nearer te 
the top than the bottom; and they are occasionally compressed, their 
diameter varying from two to six inches. ‘Their long slender pro- 
cesses, covered with a pellicle of carbonaceous matter, form an en- 
tangled mass, and traverse the beds in every direction, vertically, 
horizontally, and obliquely ; but Mr. Logan has never been able to 
trace them to their termination, though he has followed single pro- 
cesses for considerable distances. Portions of the stem of the Stig- 
maria are found in other parts of the coal measures, but it is only 
in the underclay that the fibrous processes are attached to the stem 
