288 CAUSES OF UPHEAVAL AND SUBSIDENCE. CHAP. xiv. 



and boulders upon those poles, a counterpart of the boulder 

 clay which overlies the forest-bed on the Norfolk cliffs. 



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

 freshwater, of the forest bed, and associated fluvio-marine 

 strata of Norfolk, are 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 freshwater 

 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 

 language, be called contemporaneous, not only as belonging 

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

 subdivision 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 

 mammalia 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 are identical in 

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

 their having been separated by such intervals or by two or three 

 thousand centuries. That number of years may sometimes 

 be of small moment in reference to the rate of fluctuation of 

 species in the lower animals, but very important when the 

 succession of forms in the highest classes of vertebrata is 

 concerned. 



