COAL FORMED ON LAND. 
399 
coal, eight or ten inches thick, which has been found to ex¬ 
tend across the railway, or to the distance of at least ten 
yards. Just above the covering of the roots, yet beneath 
the coal-seam, so large a quantity of the Lepidostrohus vari~ 
abilis was discovered inclosed in nodules of hard clay, that 
more than a bushel was collected from the small openings 
around the base of soine of the trees (see figure of this genus, 
p. 424). The exterior trunk of each was marked by a coat¬ 
ing of friable coal, varying from one-quarter to three-quarters 
of an inch in thickness; but it crumbled away on removing 
the matrix. The dimensions of one of the trees is 15^ feet 
in circumference at the base, feet at the top, its height 
being eleven feet. All the trees have large spreading roots, 
solid and strong, sometimes branching, and traced to a dis¬ 
tance of several feet, and presumed to extend much farther. 
In a colliery near Newcastle a great number of Sigillarim 
occur in the rock as if they had retained -the position in 
which they grew. Not less than thirty, some of them four 
or five feet in diameter, were visible within an area of 50 
yards square, the interior being sandstone, and the bark hav¬ 
ing been converted into coal. Such vertical stems are famil¬ 
iar to our coal-miners, under the name of coal-pipes. They 
are much dreaded, for almost every year in the Bristol, New¬ 
castle, and other coal-fields, they are the cause of fatal acci¬ 
dents. Each cylindrical cast of a tree, formed of solid sand¬ 
stone, and increasing gradually in size towards the base, and 
being without branches, has its whole weight thrown down¬ 
ward, and receives no support from the coating of friable coal 
which has replaced the bark. As soon, therefore, as the co¬ 
hesion of this external layer is overcome, the heavy column 
falls suddenly in a perpendicular or oblique direction from 
the roof of the gallery whence coal has been extracted, wound¬ 
ing or killing the workman who stands below. It is strange 
to reflect how many thousands of these trees fell originally 
in their native forests in obedience to the law of gravity; 
and how the few which continued to stand erect, obeying, 
after myriads of ages, the same force, are cast down to im¬ 
molate their human victims. 
It has been remarked that if, instead of working in the 
dark, the miner was accustomed to remove the upper cover¬ 
ing of rock from each seam of coal, and to expose to the day 
the soils on which ancient forests grew, the evidence of their 
former growth would be obvious. Thus in South StaflTord- 
shire a seam of coal was laid bare in the year 1844, in what 
is called an open work at Parkfield colliery, near Wolver¬ 
hampton. In the space of about a quarter of an acre the 
