EVIDENCE IN STRATIGRAPHICAL GEOLOGY. 43 



Pal^eontological Evidence in Stratigraphical Geology. 



As regards the division of the entire series of stratified deposits 

 into the above enumerated primary " systems," the value of palaeon- 

 tological evidence has never been disputed. In any given country, 

 it would be possible, undoubtedly, to determine the order and rela- 

 tive succession of the great formations, to some extent at any rate, 

 by a mere appeal to the mineral character and order of superposition 

 of the rocks themselves ; but it is perfectly clear that this method of 

 procedure would necessarily break down totally the moment we came 

 to try and determine which were the corresponding formations in 

 some far-distant region. By the stratigraphical evidence alone we 

 could determine the relative position and age, for example, of the 

 Silurian, Devonian, and Carboniferous systems in Britain ; but it 

 would be an entire impossibility to identify these same systems, 

 say in North America, except by means of the fossils which they 

 contain. So far, then, as this goes, no question has ever been 

 raised as to the value and powers of Palaeontology ; but when we 

 come to consider the minor rock-groups included in these systems, 

 we find much difference of opinion as to the extent to which the 

 evidence of the fossils is available in determining stratigraphical hori- 

 zons. Part of this difference of opinion is due to imperfect acquaint- 

 ance on the part of stratigraphical geologists with the methods of 

 palaeontological inquiry, and needs no discussion here ; but part is 

 well founded, and either arises from actual defects in the modes of 

 research employed by palaeontologists, or is due to the fact that the 

 conditions under which different systems, or different portions of the 

 same system, have been deposited have not been identical, and that 

 conclusions which might be well founded in one case might be found 

 to break down in another apparently similar case. To both of these 

 points a brief consideration may be given. 



As regards imperfections in the methods of palaeontological re- 

 search, by far the most important arises from the fact that too 

 much weight has been attached by observers, especially in the earlier 

 periods of the science, to the age of the rocks in which any given 

 fossil occurred. So long as the opinion was current that fossils occur- 

 ring in different formations were necessarily different, it followed of 

 necessity that the smallest and most trivial varietal, or even indi- 

 vidual, peculiarities of form or structure were considered as sufficient 

 to establish specific distinction. At present, however, palaeontolo- 

 gists are tolerably agreed that the mere fact of a difference of physi- 

 cal position, and consequently of age, ought not to be taken into 

 account in considering the true affinities and systematic position of 

 a fossil. At the same time it is, for many reasons, most important 

 that palaeontologists should have a general personal acquaintance 



