CLASSIFICATION OF STRATIFIED ROCKS. 197 



as we pass upward. Thus in the newer rocks, just where the other 

 method (comparison of fossil faunas with one another) begins to fail, 

 we may synchronize strata of different localities, by comparing their 

 shell fauna with the shell fauna of the present day, in the same locali- 

 ties. Those are said to be of the same age ivhich contain the same per- 

 centage of shells identical with those of the present day. To this we 

 may add that, if not species, at least many genera and families, especially 

 among vertebrates, are characteristic of each horizon even of the newest 

 rocks. 



Section 2. — Classification of Stratified Rocks. 



Geology is essentially a history. Stratified rocks are the leaves on 

 which this history is recorded. The fundamental idea of every clas- 

 sification is therefore relative age. The object to be attained in classi- 

 fication is, first, to arrange all rocks in chronological order, so that the 

 history may be read as it was written ; and then, second, to collect them 

 into larger and smaller groups, called systems, series, formations, cor- 

 responding to the great eras, periods, epochs, of the earth's history. 

 There are several different methods of determining the relative age of 

 rocks : 



1. Order of Superposition. — It is evident, from the manner in which 

 stratified rocks are formed — viz., by sedimentation — that their original 

 position indicates, with absolute certainty, their relative age, the lower 

 being older than the higher. If, therefore, the original position of any 

 series of strata be retained or not very greatly disturbed, and we have 

 a good section, the relative age of the strata which compose the series 

 may be easily determined. But the strata, as we have already seen, 

 have in many cases been crushed and contorted and folded in the most 

 intricate manner, sometimes even turned over; they have also been 

 broken and slipped, and large masses carried away by erosion, and often 

 so changed by heat and other agents, that their stratification is nearly 

 or quite obliterated. For these reasons it is often very difficult to 

 determine the relative position, and thus to construct an ideal sec- 

 tion of the strata of a series of rocks, even in a single locality. Nev- 

 ertheless, the method of superposition is conclusive, and takes prece- 

 dence of all others whenever it can be applied. In spite of all these 

 difficulties, if the whole geological series were present in any one lo- 

 cality, it would be comparatively easy to construct the geological chro- 

 nology. 



But a series of rocks in any one locality can not give us the whole 

 history of the earth. Since sedimentation only takes place at the bot- 

 tom of water, those places which were land-surfaces during any geo- 

 logical epoch received no deposit, and therefore the strata representing 

 that epoch must be wanting there. Now, as there have been frequent 



