GEOLOGY. 283 



essential use in geological investigation, and even in the practical 

 applications of geology. The genius of William Smith first made 

 known, that by means of their fossils the rocks could be identified 

 and followed from district to district, and recognised even on 

 opposite sides of the island. This remarkable conclusion was 

 soon found to hold good among the rocks and fossils of other 

 countries, and gradually the fact came to be firmly established, 

 that in the long past history of our globe, there has been a gradual 

 progress of plant and animal life from low forms in early times up 

 to man in the present. Each" particular period has its own tribes 

 of organic forms. And as from the earliest ages, layers of sand, 

 mud, and lime have been formed, and have enclosed and preserved 

 the remains both of plants and animals, so in the deposits of each 

 successive period are found, with more or less completeness, the 

 peculiar and characteristic organic forms of that period. The 

 older deposits lie, of course, below those of later date. When, 

 therefore, the general order of succession has been established, the 

 geologist finds that apart from differences in colour, texture, and 

 composition, the various formations can be distinguished from 

 each other all over the world, by their distinctive fossils. Having 

 acquired this knowledge in a district where the deposits have been 

 little disturbed by subsequent underground movements, he is fur- 

 nished with a clue to the investigation of those tracts where they 

 have been so fractured and displaced that, but for some such key 

 as is supplied by the fossils, he would probably find it impossible 

 to arrange them in their original and proper order. It is only 

 when the rocks of a district have, in this way, had their chrono- 

 logical sequence determined, that they can properly be compared 

 with those of another region. From arrangements and compari- 

 sons of this kind, the geological history of the earth and its 

 inhabitants is in great measure compiled. 



But the paramount importance of fossil evidence is not less 

 apparent in the practical applications of geology. Wherever the 

 tracing of a particular deposit or vein depends upon a knowledge 



