78 DISCOVERY CH. 



were formed. The only trustworthy test of geological 

 age is that afforded by fossil remains, and this principle, 

 established by William Smith, is now used by all 

 geologists as a guide to the order in which deposits 

 were laid down in any part of the earth. The oldest 

 rocks in Great Britain are those in the west ; and as 

 we pass eastward, we traverse beds of rock formed in 

 successive epochs of the earth's history until we come 

 to the relatively recent formations of Essex and other 

 parts of the east coast. Relics of elementary forms of 

 life are found in the older rocks and are succeeded in 

 regular order by remains of fishes, amphibians, reptiles, 

 birds and mammals, until at last deposits are reached 

 containing human remains and evidence of man's 

 handiwork. 



Though the biological significance of this development 

 of life from the simple coral to the complex structure of 

 man was not realised by Smith, it was he who gave the 

 first complete proof that there was a regular succession 

 of forms of life in the beds of the earth's crust. " To 

 effect his object," said Sir Andrew Ramsay, " Smith 

 traced the English formations from end to end of the 

 country with unwearied devotion, and at length, in 

 1815, produced his great geological map of England. 

 He struggled long, almost unrecognised in his labours, 

 but when they were well-nigh at an end, men began by 

 degrees to realise that a master was among them." 



The insight which a naturalist can acquire into the 

 forms of life of the past, by studies of the records of 

 the rocks, is well illustrated by an episode in the life 

 of Louis Agassiz. At a meeting of geologists in England, 

 Agassiz was once asked what kind of fish would be 

 found in fossil form in a particular stratum of the 

 earth's crust. He thought for a moment and then 



