Mr. Logan on the Coal Seams of South Wales. 217 



mulation of the vegetable matter ; and, 2ndly, what thickness of 

 vegetable matter was necessary to form the stratum of coal nine 

 inches thick, over which the trees stand. IVIr. Schomburgk is of 

 opinion that a dicotyledonous tree which would require in temperate 

 climates one hundred years to attain a certain diameter, would arrive 

 at the same dimensions within the tropics in sixty or eighty years. 

 The largest of the fossil trees forming the immediate subject of the 

 paper is equal in circumference to an oak of 130 years growth in 

 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 resj^ect 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.S. 



Immediately below every regular seam of coal, in South Wales, 

 (and nearly 100 are known to exist) is constantly found abed of clay, 

 varying in thickness from six inches to more than ten feet, and called 

 the underclay, underchfF, 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- 

 naceous matter, and it then sometimes resists the effects of the 

 weather. Under a part of the lowest seams of coal l)etween 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 to 

 the top than the bottom ; and they are occasionally compressed, their 



