108 Chronicles of Science. [Jan., 



Mr. Salter the rocks of the same localities promise to yield still 

 further additions to the English Primordial fauna. The chief im- 

 portance of these fossils is, however, their bearing on the larger 

 question of the distinctness of the Lingula-flags (or Primordial Zone 

 as it is called), as a formation, from the great Silurian " System." This 

 is one of the many questions to which field-geologists give one answer, 

 and palaeontologists another. But while palaeontologists have always 

 a definite principle to guide them, geologists are often obliged to resort 

 to a theory of probabilities of their own manufacture,* or to use pre- 

 conceived ideas in the place of facts which nature refuses to furnish. 

 Palaeontologically, if the Primordial fauna differs much more in degree 

 from the Llandeilo than the latter does from the Caradoc, and the 

 Caradoc from the Lower Llandovery, and so on ; or if the Primordial 

 fauna differs from the Llandeilo in facies, or kind, while the faunae of 

 the other stages have the same general facies in common ; then the 

 Primordial Zone is distinct from the Silurian " System," and vice versa. 

 Mr. Salter shows that the Primordial and Llandeilo faunae are essen- 

 tially distinct, and although some shells and a cystidean are of genera 

 common to both formations, yet the entire distinctness of the trilo- 

 bitic fauna overbalances this fact, the Crustacea being the surest 

 indices of the age of Palaeozoic rocks, and, we may add, the molluscan 

 genera having generally a very wide range. Mr. Salter thus gives his 

 conclusion respecting the distinctness of the Primordial and Silurian 

 formations : " The intervention of the whole of the Tremadoc rocks, 

 containing a remarkable assemblage of fossils distinct from both, easily 

 explains the meaning of this wide difference in the fossils [alluded to 

 above], and indicates that the epochs of the Llandeilo and Lingula- 

 flags were separated by an enormous period of time." 



3. The Mineralization of Corals. — The third of Dr. Duncan's im- 

 portant memoirs " On the Fossil Corals of the West Indian Islands," 

 is devoted to a mineralogical description of the specimens he has 

 already treated of palaeontologically. The mode of fossilization of 

 various organisms has long been a subject with great attractions, but 

 of greater difficulty ; and few authors who have written on the subject, 

 have hitherto endeavoured to explain the phenomena by means of 

 arguments and theories founded on observed facts ; f but most of them 

 have had recourse to supposititious attractions, repulsions, and mutual 

 movements of all kinds, between equally hypothetical atoms and 

 molecules belonging to the large group of the " infinitely little." 



Dr. Duncan, however, has patiently and laboriously noted the 

 composition, texture, colour, &c, of every specimen he has already 



* Since writing this sentence we have noticed the following passage in Mr. 

 Prestwich's last paper on Drift Deposits containing Flint Implements : — " In look- 

 ing back at the subjects we have discussed, we are forcibly reminded of our depend- 

 ence on the value of probabilities. On various points Geology has not at present, 

 and probably never will have, any other means of inference. All that can be done 

 to give weight to our argument is to multiply probabilities, and by attending to the 

 general concordance to reduce to the minimum the chances of error." — ' Phil. 

 Trans.,' 1864, p. 298. 



t Petzholdt, Leopold Von Buch, and Bischof are among those who have treated 

 this subject philosophically. 



