296 IMPERFECTION OF THE Chap. IX. 



generally have been due to geographical changes re- 

 quiring much time. Nor will the closest inspection of 

 a formation give any idea of the time which its depo- 

 sition has consumed. Many instances could be given of 

 beds only a few feet in thickness, representing forma- 

 tions, elsewhere thousands of feet in thickness, and 

 which must have required an enormous period for their 

 accumulation ; yet no one ignorant of this fact would 

 have suspected the vast lapse of time represented by 

 the thinner formation. Many cases could be given of 

 the lower beds of a formation having been upraised, 

 denuded, submerged, and then re-covered by the upper 

 beds of the same formation, — facts, showing what wide, 

 yet easily overlooked, intervals have occurred in its accu- 

 mulation. In other cases we have the plainest evidence 

 in great fossilised trees, still standing upright as they 

 grew, of many long intervals of time and changes of 

 level during the process of deposition, winch would 

 never even have been suspected, had not the trees 

 chanced to have been preserved: thus, Messrs. Lyell 

 and Dawson found carboniferous beds 1400 feet thick 

 in Nova Scotia, with ancient root-bearing strata, one 

 above the other, at no less than sixty-eight different 

 levels. Hence, when the same species occur at the 

 bottom, middle, and top of a formation, the probability 

 is that they have not lived on the same spot during the 

 whole period of deposition, but have disappeared and 

 reappeared, perhaps many times, during the same geo- 

 logical period. So that if such species were to undergo 

 a considerable amount of modification during any one 

 geological period, a section would not probably include 

 all the fine intermediate gradations which must on my 

 theory have existed between them, but abrupt, though 

 perhaps very slight, changes of form. 



It is all-important to remember that naturalists have 



