THE TEST OF OUGANIC REMAINS. 



Yet the improbability of this assumption may be readily 

 shown from Sir R. Murchison s own facts. If, as he points 

 out, the crustacean fossils of the uppermost Silurian rocks 

 in Lanarkshire arc, &quot; with one doubtful exception,&quot; &quot; all 

 distinct from any of the forms on the same horizon in Eng 

 land ; &quot; how can it be fairly presumed that the forms exist 

 ing on the other side of the Earth during the Silurian 



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period, were nearly allied to those existing here ? Not 

 only, indeed, do Sir R. Murchison s conclusions tacitly as- 

 Burne this doctrine of universal distribution, but he distinctly 

 enunciates it. &quot;The mere presence of a graptolite,&quot; he 

 says, &quot; will at once decide that the enclosing rock is Silu 

 rian ; &quot; and he says this, notwithstanding repeated warnings 

 against such generalizations. During the progress of Geolo 

 gy, it has over and over again happened that a particular 

 fossil, long considered characteristic of a particular forma 

 tion, has been afterwards discovered in other formations. 

 Until some twelve years ago, Goniatites had not been found 

 lower than the Devonian rocks; but now, in Bohemia, they 

 have been found in rocks classed as Silurian. Quite re 

 cently, the Orthoceras, previously supposed to be a type 

 exclusively palaeozoic, has been detected along with meso- 

 zoic Ammonites and Belemnites. Yet hosts of such experi 

 ences fail to extinguish the assumption, that the age of a 

 stratum may be determined by the occurrence in it of a 

 single fossil form. 



Nay, this assumption survives evidence of even a still 

 more destructive kind. Referring to the Silurian system 

 in Western Ireland, Sir R. Murchison says, &quot; in the beds 

 near Maam, Professor Nicol and myself collected remains, 

 some of which would be considered Lower, and others 

 Upper, Silurian ; &quot; and he then names sundry fossils which.} 

 in England, belong to the summit of the Ludlow rocks, or 

 highest Silurian strata ; &quot; some, which, elsewhere are known 



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only in rocks of Llandovery age,&quot; that is, of middle Silu- 



