FOSSILS. 67 
to the internal energy of the earth, be dissipated, as no 
doubt they will in the far future, the dry land would 
disappear for ever below the sea. 
The products of destruction, in the form of mud, sand, 
or silt, are deposited as strata, and in these are found the 
organic remains we term /ossz/s. The commonest form of 
fossil owes its existence to the power of organisms to 
construct skeletons for their mechanical support in life. 
These as we have seen are either calcareous, siliceous or 
chitinous. They are shed in aquatic organisms into the 
mud or sand and covered up by fresh deposits, or in the 
case of land animals they may be carried out to sea or into 
lakes by floods and other accidents. 
In many cases, the skeletons only remain sufficiently 
long for a cast of their shape to be taken, the fossil really 
consisting of mineral matter but of precisely the same 
shape as the original skeleton. Another way in which 
fossils may be produced is by impressions. Soft sand 
takes an exact impression of any body from a footmark to 
a scratch, and in many instances these impressions have 
been produced by the soft and perishable parts of an 
organism. If mud or some fresh deposit differing from 
the sand be then deposited in the impression a permanent 
memorial of the organism is preserved in the rocks. 
Skeletons and other remains of more recent date may 
be found deposited in caves, peat-bogs and elsewhere, little 
altered from their normal condition. 
The strata of rocks can be arranged or classified by 
careful study into a series corresponding with their succession 
in time. They are thus divided into five primary groups, 
called :— 
I. Primordial. III. Secondary. 
II. Primary. IV. Tertiary. 
V. Quaternary. 
These five groups are further subdivided into a number 
of Systems. Each group evidently corresponds to a certain 
lapse of time, during which it was produced, which is called 
an Lyra, and each system represents a lapse of time called a 
Period, These may be tabulated as follows :— 
