42 PRINCIPLES OF PALAEONTOLOGY. 



In spite of the above limitations and fallacies, there can be 

 no doubt as to the enormous value of palaeontology in enab- 

 ling us to work out the historical succession of the sedimentary 

 rocks. It may even be said that in any case where there 

 should appear to be a clear and decisive discordance between 

 the physical and the palseontological evidence as to the age 

 of a given series of beds, it is the former that is to be distrusted 

 rather than the latter. The records of geological science con- 

 tain not a few cases in which apparently clear physical evidence 

 of superposition has been demonstrated to have been wrongly 

 interpreted; but the evidence of palseontology, when in any way 

 sufficient, has rarely been upset by subsequent investigations. 

 Should we find strata containing plants of the Coal-measures 

 apparently resting upon other strata with Ammonites and Belem- 

 nites, we may be sure that the physical evidence is delusive ; and 

 though the above is an extreme case, the presumption in all 

 such instances is rather that the physical succession has been 

 misunderstood or misconstrued, than that there has been a sub- 

 version of the recognized succession of life-forms. 



We have seen, then, that as the collective result of observa- 

 tions made upon the superposition of rocks in different localities, 

 from their mineral characters, and from their included fossils, 

 geologists have been able to divide the entire stratified series 

 into a number of different divisions or formations, each charac- 

 terized by a general uniformity of mineral composition, and by 

 a special and peculiar assemblage of organic forms. Each of 

 these primary groups is in turn divided into a series of smaller 

 divisions, characterized and distinguished in the same way. It is 

 not pretended for a moment that all these primary rock-groups 

 can anywhere be seen surmounting one another regularly. * 

 There is no region upon the earth where all the stratified forma- 

 tions can be seen together ; and, even when most of them occur 

 in the same country, they can nowhere be seen all succeeding 

 each other in their regular and uninterrupted succession. The 

 reason of this is obvious. There are many places to take a 



* As we have every reason to believe that dry land and sea have ex- 

 isted, at any rate from the commencement of the Laurentian period to 

 the present day, it is quite obvious that no one of the great formations 

 can ever, under any circumstances, have extended over the entire globe. 

 In other words, no one of the formations can ever have had a greater 

 geographical extent than that of the seas of the period in which the for- 

 mation was deposited. Nor is there any reason for thinking that the pro- 

 portion of dry land to ocean has ever been materially different to what it 

 is at present, however greatly the areas of sea and land may have changed 

 as regards their place. It follows from the above, that there is no suf- 

 ficient basis for the view that the crust of the earth is composed of a 

 succession of concentric layers, like the coats of an onion, each layer 

 representing one formation. 



