92 
COAL MEASURES. 
Crustacea are abundant in the shale, and ironstone 
nodules, and the remains of insects sometimes oc- 
cur." 
On the Continent of Europe, especially in Central 
France, the coal deposites rest on granite and other 
primary rocks, without the intervention of lime- 
stones or sandstones. In Poland, the lower beds of 
the coal measures pass insensibly into the transi- 
tion rocks upon which they rest. Coal is worked in 
China and in Van Diemen's Land, where it is asso- 
ciated with the same rocks as in Europe. It is fully 
ascertained that the coal measures sometimes over- 
lie the millstone grit and shale, and sometimes al- 
ternate with the carboniferous limestone ; hence 
their formation may be of any date between the new and 
the old red sandstone. Indeed, coal has been found 
in both these rocks, but not in beds of sufficient 
thickness to pay the expense of exploration. 
Coal occurs in regular strata, coextensive with 
the other rocks of the formation, though each coal 
district has its peculiar series of strata, the arrange- 
ment of which differ from every other. Each coal- 
field also contains generally many beds of coal, 
separated from each other by strata, often numer- 
ous, of other rocks. These beds vary from an inch 
to many feet in thickness. One bed in the coal- 
field at Liege is 60 feet thick ; another, in Scotland, 
56. A thickness of six or eight feet is the most con- 
venient for working; and, if less than 18 inches, it 
will not defray the expense. In the Leinster coal 
district Mr. Griffith describes 119 distinct strata, 
consisting of slate, clay, sandstone, and coal, in 
which the coal occurs 17 times, in layers of from 
one to four feet thick. The total thickness of the 
Derbyshire coal-strata is 1310 yards, in which there 
are 30 different beds of coal, varying in thickness 
from six inches to 11 feet, making 26 yards of coal. 
Making every allowance for excess, Mr. Bake well 
