28 INTRODUCTION. 



The basis of geological chronology, the sequence of the 

 stratified rock formations of the globe, rests entirely upon the 

 nature of the contained fossils. As a matter of course it is a 

 comparatively simple thing to make out the true succession of 

 the beds in any given locality, and to understand that ordina- 

 rily the lowest are the oldest and that the uppermost are the 

 most recent. But the case is not so simple when the investi- 

 gation is extended, when a comparison is made with similar 

 exposures in distant places, for there are usually physical diffi- 

 culties in the way in attempting to trace the separate layers or 

 groups of strata through all intermediate points. When the 

 sections are near together, the continuity of the different lay- 

 ers may be inferred from the lithological characters. But 

 inasmuch as these constantly change, correlation by this 

 method becomes the more uncertain according to the distance 

 from the original locality. Finally, it becomes impossible to 

 say whether or not the rocks of one place are older or younger 

 than those of another, whether one lies above or below the 

 other. For there are great beds of limestone, shales and sand- 

 stone identical in all lithological characters with other rocks, 

 but separated by thousands of feet of strata representing enor- 

 mous periods of time. Should the geological structure be 

 such that two of these similar layers were nearly on a level, 

 they might easily be taken for the same stratum, if the peculiari- 

 ties of the rock components alone were relied upon. Now, it 

 is the great service which fossils perform in acting as media of 

 correlation in widely separated outcrops of rock, to show 

 whether the beds of one region were contemporaneous with 

 those of the other; to indicate which is the younger of the 

 two deposits, and how much, in units of geological time. Every- 

 where on the globe, observation has shown that the general 

 succession of organisms has been the same from the dawn of 

 life to the present time. Thus does the conception of the genetic 

 relationships of organic beings awaken a keen interest in the 

 extinct forms of life, and furnish the key in deciphering the 

 great book of Nature. 



