from the Characters of their embedded Fossils. 239 



M. Nicolet in the molasse, though very slightly worn, are 

 easily distinguished by their solidity and petrified condition, 

 independently of their zoological characters, from the tertiary 

 shells which accompany them.* In the beds of ancient allu- 

 vial gravel there can be no difficulty : the secondary fossils 

 there found are not only petrified and worn, but are com- 

 monly attached to portions of their original matrix ; while the 

 mammiferous bones and fresh- water shells are both uninjured 

 and unaltered, except by the subtraction of part of their 

 animal matter. There can, therefore, be no doubt that these 

 bones and fresh-water shells are contemporary with the depo- 

 sition of the beds of gravel. f 



To the above arguments it might be added, that the re- 

 moval of fossils from one stratum to another must always be 

 a local and partial occurrence, and, therefore, that, when any 

 species is found to characterise a given formation over a large 

 area, we cannot err in regarding it as an inhabitant of the 

 sea which deposited the stratum. There is, however, no oc- 

 casion to enlarge on this topic ; enough has been said to 

 prove that the errors likely to arise from the removal of fossils 

 from one stratum to another, though not to be lost sight of 

 by geologists, are yet confined within very narrow limits. 



* " This introduction of secondary shells in the tertiary beds of Nor- 

 folk and Suffolk has been detected solely by an attention to lithological 

 characters ; and the evidence derived from this source is no longer avail- 

 able, when there is reason to suspect an admixture of organic remains 

 belonging exclusively to rocks of the supracretaceous series. The species 

 which are common to the chalk and red crag are very few, when compared 

 with those common to the red crag, and to the subjacent tertiary strata. 

 In the latter case, however, we have no means of ascertaining whether 

 those individual species which occur in separate formations existed 

 throughout distinct periods, or, like the fossils of the chalk, were, by the 

 natural process of degradation, removed from their original matrix, to be 

 again entombed with the races of a more recent epoch. Unless this diffi- 

 cult problem be solved, it is clear that the application of the per-centage 

 test may be attended with the most fallacious results." {Ibid., p. 541.) — 

 Edit. 



f Mr. W. Richardson concludes his interesting account of the mixture 

 of bones from the gravel and shells from the London clay, with existing 

 shells in the estuary of the Thames, with a speculation as to the geolo- 

 gical appearances of that estuary, and the errors into which they might 

 lead a future geologist. He says, " The sedimentary and tranquil cha- 

 racter of the formation, consisting of alternating bands of sand and clay, 

 and the total absence of extraneous materials, give assurance to his (the 

 supposed geologist's) conclusions ; and the synchronous existence of the 

 organic contents is instanced with unhesitating confidence. He meets 

 with no fact that can excite suspicion, or create distrust ; the intrusive 

 and extinct fossils are linked with the recent by the closest of all ties : 

 they lie in peaceful juxtaposition, and upon undisturbed beds of oysters." 

 This hypothetical statement is probably not quite accurate. If the for- 

 mation was of a " sedimentary and tranquil character," the bones of large 



