FOSSILS, e'j 



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 fossils. 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 Era, and each system represents a lapse of time called a 

 Period, These may be tabulated as follows : — 



