90 DOCTRINE OF EVOLUTION 



formation and age. But by far the most impressive 

 demonstration of the basic principle of geology em- 

 ployed for the determination of the relative ages of 

 rocks is the mighty Canon of the Colorado. As the 

 traveler stands on the winding rim of this vast chasm, 

 his eye ranges across 13 miles of space to the opposite 

 walls, which stretch for scores of miles to the right and 

 left ; upon this serried face he will see zone after zone 

 of yellow and red and gray rock arranged with mathe- 

 matical precision and level in the same order as on the 

 steep slopes beneath him. Plain common sense tells 

 him that the great sheets of rock stretched continuously 

 at one time between the now separate walls, and that 

 the various strata of sandstone and limestone were 

 deposited in successive ages from below upwards in the 

 order of their exposure. When now he extends his 

 explorations to another state like Utah or Wyoming, he 

 may find some but not all of the series exhibited in the 

 Grand Caiion, overlaid or underlaid by other strata 

 which in their turn can be assigned to definite places in 

 the sequence. By the same method, the geologist 

 correlates and arranges the rocks not only of different 

 parts of the same state, or of neighboring states, but 

 even those of widely separated parts of North America 

 and of different continents. But he learns that he must 

 refrain from over-hasty conclusions, for he soon finds 

 that the sedimentary rocks have not been constructed 

 at the same rate in different places during one and the 

 same epoch, and that rocks formed even at one period 

 are not always identical in nature. But his guiding prin- 

 ciple is sensible and reasonable, and by employing it with 

 due caution he provides the palaeontologist with the req- 



