lO 



THE CHAIN OF LIFE. 



lacustrine. Thus we obtain the means of dividing the rocks 

 of the earth into groups of different ages, known as " Forma- 

 tions," and marking particular periods of geological time. 

 By tracing these formations from one district or region to 

 another, we learn the further truth that the succession is not 

 merely local, but that, though liable to variation in detail, 

 its larger subdivisions hold so extensively that they may be 

 regarded as world-wide in their distribution. 



Putting together the facts thus obtained, we can frame a 

 tabular arrangement of the earth's strata, as in the table pre- 

 fixed to this chapter ; and when we add the further discovery, 

 very early made by geologists, that the successive formations 





Fig. 8. — Unconformable superpositi m of Devonian conglomerate on Silurian slates, at 

 St. Abb's Head, Berwickshire. — After Lyell. 



differ from each other in their fossil remains, we have the means 

 of recognising any particular formation by its fossils, even when 

 the stratigraphical evidence may be obscure or wanting. Thus 

 our knowledge of Epochs of Life, and indeed of the whole 

 geological history of the earth, is based on the superposition 

 of beds in the earth's crust, and on the diversity of fossil re- 

 mains in the successive beds so superimposed on each other ; 

 and it is on these grounds that we are enabled to construct a 

 Table of Geological Formations representing the whole series 

 of beds as far as known, with the characteristic groups of fossils 

 of each period. Here I might close these preliminary con- 

 siderations, but there are a few accessory questions, important 

 to our clear comprehension of the subject, which may profitably 

 occupy our attention for a short time. _ 



