THE IMPERFECTION OF THE GEOLOGICAL RECORD 283 



sition, we may reasonably suspect that the process of deposition 

 has been more or less interrupted. Nor will the closest inspection 

 of a formation give us any idea of the length of time which its 

 deposition may have consumed. Many instances could be given 

 of beds, only a few feet in thickness, representing formations 

 which are 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 even 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 accumula- 

 tion. In other cases we have the plainest evidence in great fossil- 

 ized trees, still standing upright as they grew, of many long in- 

 tervals of time and changes of level during the process of depo- 

 sition, which would not have been suspected, had not the trees 

 been preserved: thus Sir C. Lyell and Dr. Dawson found car- 

 boniferous beds 1,400 feet thick in Nova Scotia, with ancient 

 root-bearing strata, one above the other, at no less than eighty- 

 six different levels. Hence, when the same species occurs at the 

 bottom, middle, and top of a formation, the probability is that it 

 has not lived on the same spot during the whole period of depo- 

 sition, but has disappeared and reappeared, perhaps many times, 

 during the same geological period. Consequently if it were to 

 undergo a considerable amount of modification during the depo- 

 sition of any one geological formation, a section would not in- 

 clude all the fine intermediate gradations which must, on our 

 theory, have existed, but abrupt, though perhaps slight, changes 

 of form. 



It is all-important to remember that naturalists have no golden 

 rule by which to distinguish species and varieties; they grant 

 some little variability to each species, but when they meet with a 

 somewhat greater amount of difference between any two forms, 

 they rank both as species, unless they are enabled to connect 

 them together by the closest intermediate gradations; and this, 

 from the reasons just assigned, we can seldom hope to effect in 

 any one geological section. Supposing B and C to be two species, 

 and a third. A, to be found in an older and underlying bed; even 

 if A were strictly intermediate between B and C, it would simply 

 be ranked as a third and distinct species, unless at the same time 

 it could be closely connected by intermediate varieties with either 

 one or both forms. Nor should it be forgotten, as before explained. 



