58 
HAROLD’S DISCUSSIONS. 
a million years. Adding to that the time it would 
take to deposit the other strata between the coal- 
measures and above them and belonging to this age, 
we should get from two to three millions of years for 
the total period. The geologist is so accustomed to 
think of long periods of time that he scarcely takes 
notice of a thousand years, but always talks of mil¬ 
lions. 
It must, however, be remembered that on account 
of the climate and the kinds of plants, the growth of 
vegetation in that age was much more rapid, and de¬ 
cayed matter accumulated much faster than it does 
now; these facts would proportionately shorten the 
time required. 
ANOTHER USEFUL CIVILIZING MINERAL. 
Frequently in connection with coal we find beds 
of iron, varying in thickness from a fraction of an 
inch to many feet. This iron almost invariably rests 
upon a bed of clay. It is not ferric oxid like that in 
the Archaean rocks, but is found combined with car¬ 
bonate of lime, forming what is known as ferrous 
carbonate. When clay is mixed with it, as is often 
the case, it is called clay ironstone. 
Such iron beds are found in connection with all 
coal-beds, not only those of the Carboniferous age, 
but those of all other ages, for it must not be sup¬ 
posed that coal was formed only during the Carbon¬ 
iferous period. 
As we always find iron in close proximity to coal, 
we conclude that plants must have had something to 
