XXXIV PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



In one of the subdivisions of this " Etage D" of his Lower Siki- 

 rians, M. Barrande describes the occxirrence of isolated patches, as it 

 were, of fossihferous strata, the population of which consists not of 

 Lower Silurian fossils, but of organic remains characteristic of the 

 Upper Silurian. M. Barrande designates these assemblages by the aj)- 

 pellation of " colonies." This colonial fauna, becoming extinguished 

 after a short existence, does not reappear until after the extinction, by 

 trappean eruptions, of the normal fauna of the epoch, and the cessa- 

 tion of the formations amid which the colony is an intruder. In these 

 colonies, he states, there are as many as 63 species, of which 4 are 

 exclusively peculiar, 2 (viz. Trinucleus ornatus and BaJmanites 

 socialis) common to the colonies and the true fauna of the beds in 

 which they are intercalated, and 5/ common to the colonies and 

 third fauna, or that of the lowermost section of his Upper Silurians. 



This doctrine of colonies is original with M. Barrande and demands 

 our serious consideration. It is one that materially affects the value 

 of the evidence of organic remains as determining the age and 

 sequence of formations. The proposition that it involves asserts the 

 introduction of a group of species that experience has shown nor- 

 mally to belong to a later and distinct formation, not merely among 

 and mixed with the fauna of an earlier stage, but amid and separate 

 from that fauna. We can conceive, indeed we have ample proofs in 

 many instances, of the fact of the appearance of many species earlier 

 in one geographical region than another, and we can understand how 

 under temporary favouring circumstances any one or a number of 

 such species might be laterally diffused, so as for a time to become a 

 component part of the fauna of a neighbouring region, at an epoch 

 previous to that in which, after having for a time retired, they 

 returned to play a more conspicuous and characteristic part in a later 

 formation. But in any such instance they would be mingled with 

 the ordinary inhabitants of the region they colonized. Yet we can 

 scarcely conceive a colony, composed entirely of strangers and of 

 species known in beds of a later epoch, only in the exclusive associa- 

 tion presented by their being intercalated without admixture in the 

 midst of an earlier fauna. On the other hand, in a disturbed Silurian 

 country, where the strata lie at very high angles, and where there are 

 probably numerous convolutions, contortions, and rollings of the beds, 

 . there is a probability of the occurrence of overturns and truncated 

 crumplings, that until traced out would cause the appearance of strata 

 containing newer fossils lying under and amid those containing older 

 ones. Such deceptive appearances are not unfrequent in the Alps, 

 and some well-known cases occur in our o^\n country. With these 

 instances vivid in our memory, I feel warranted in objecting to a 

 theory which seems to me as dangerous as it is ingenious, and ask 

 first for the minute local details of the course of the Silurian beds in 

 Bohemia, before accepting a doctrine so repugnant to received belief. 



M. Barrande, it is true, endeavours to show most ingeniously that 

 the currents which determined the immigration of his colonies came 

 from the N.E., and that the fauna of his Upper Sihirians arrived by 

 the same direction ; whereas the fauna of the Lower Silurians of 



