166 
time, insomuch that if they are lighted in the evening the fire will 
keep alive during the whole night*.” 
Coal is found at various depths in the earth, and interposed between 
strata of different kinds. The veins of coal also vary considerably 
in the thickness of the vein ; sometimes being many yards thick, and 
other times being merely a seam, little more than an inch in thickness. 
The strata, which are generally interposed between the gravel, 
clay, sand, and mould, at the surface, and the coal itself ; or which 
appear to be most generally connected with coal, have commonly a 
considerable degree of hardness ; but they differ from each otLr 
very much in this respect, as well as in their thickness and their 
other properties. Do not fear that, by endeavouring to furnish you 
with some information respecting these several strata, I shall engross 
your time with the consideration of matters irrelevant to the grand 
object of our inquiry ; on the contrary, be assured that it will derive 
from this investigation a considerable degree of illustration. 
Sand-stone is among the uppermost of the solid strata. It is of 
a granulated substance, of various degrees of hardness and coherence; 
and is formed by very small grains of various silicious stones, and 
frequently of mica, imbedded in a calcareous, argillaceous, or even a 
silicious cement. In the latter instance it possesses a great degree 
of hardness ; the hardness is also considerably increased, in every 
species, by the admixture of iron which has undergone but a slight 
degree of oxydation. 
Rubble-stone is a species of sand-stone, containing also a bluish 
slaty substance. 
Lime -STONE also frequently occurs among the superior solid 
strata, and like the former is also found among the inferior strata. 
It exists in very considerable varieties of hardness and coherence, 
breaking with various kinds of fracture, according to the sub- 
* Marcus Paulus Venetus. Purchas’s Pilgrimage, vol. iii. p. 88. 
