32 THE OPEN BOOK OF NATURE 
are composed mainly of carbonate of lime ; they, too, 
make a contribution to the limestones, and the same 
may be said of those most interesting creatures of 
microscopic proportions, known as Foraminifera, 
whose tiny shells of carbonate of lime accumulate in 
countless millions on the sea-beds. Some plants— 
seaweeds—also add to the deposits of lime. That 
a bed of limestone consists of the skeletons of 
myriads of creatures of past ages compacted till 
they have become solid rock is a marvellous fact, 
and one that almost staggers the imagination. 
Think of the almost infinite time it must have taken 
for them to accumulate. Some organically-formed 
limestones are thousands of feet thick. Chalk is a 
variety of limestone, and it is composed chiefly of 
the microscopic shells of the wonderful foraminifera. 
When plants occupying marshy places decay, they 
produce certain acids which act chemically upon 
salts of iron contained in stones and rocks under 
water. ‘The ironis dissolved and removed, but later 
may be laid down in a new form known as Limonite, 
or bog-iron ore. 
Sponges, as you know, are animals ; their skeletons 
are called siliceous. Silica is found in the form of 
flint. Other tiny animals, called ‘“ Radiolarians,”’ 
have flinty skeletons, and so have the very tiny 
plants called “ Diatoms.” All three have played 
their part in the formation of Siliceous Deposits, 
which are found in the forms of Flint or Chert. 
Then those exceedingly important rocks known as 
Carbonaceous are of organic origin. They include 
Peat, Lignite, Ordinary Coal, and Anthracite. What 
thanks we owe to Nature for such useful substances ! 
