•288 CAUSES OF rPHEAYAL AND SUBSIDENCE. cn.vr. xir. 



boulders \iY>on those poles, a counterpart of the boulder clay 

 •which overlies the forest bed on the Xorfolk cliffs. 



We have seen that all the plants and shells, marine and 

 fresh-water, of the forest bed, and associated fluvio-marine 

 strata of Norfolk, ai'e specifically identical with those of the 

 living European flora and fauna; so that if upon such a 

 stratum a deposit of the present period, whether fresh-water 

 or marine, should be thrown down, it might lie conformably 

 over it, and contain the same invertebrate fauna and flora. 

 The strata so superimposed would, in ordinary geological lan- 

 guage, be called contemporaneous, not only as belonging to 

 the same epoch, but as appertaining strictly to the same sul> 

 division of one and the same epoch; although they would in 

 fact have been separated by an interval of several hundred 

 thousand years. 



If, in the lower of the two formations, some of the mam- 

 malia of the genera elephant and rhinoceros were found to be 

 distinct in species from those of the same genera in the upper 

 or "recent" stratum, it might appear as though there had 

 been a sudden coming in of new forms, and a sudden dying 

 out of old ones; for there would not have been time in the 

 interval for any perceptible change in the invertebrate fauna, 

 by which alone we usually measure the lapse of time in the 

 older formations. 



"When we are contrasting the vertebrate contents of two 

 sets of superimposed strata of the cretaceous, oolitic, or any 

 other ancient formation in which the shells arc identical in 

 species, we ought never to lose sight of the possibility of 

 their having been separated by such inteiwals as b}' two or 

 three thousand centuries. That number of years may some- 

 times be of small moment in reference to the rate of fluctua- 

 tion of species in the lower animals, but very important when 

 the succession of forms in the highest classes of vertebrata is 

 concerned. 



