BARRANDE ON THE SILURIAN COLONIES. 37 



of great importance ; and I intend to treat of them fully in a treatise 

 *' on the succession and relation of contemporaneous Silurian faunas 

 in different parts of the world." I am still, however, in want of im- 

 portant materials requisite for the accomplishment of the work. 



[T. R. J.] 



On the "Colonies" in the Silurian Formation 0/ Bohemia. 

 By M. J. Barrande. 



[Leonhard u. Bronn's N. Jahrb. f. Min. u. s. w. 1852, 3 H. p. 306.] 



M. Barrande's ''Colonies* " consist of the local appearance of his 

 Third Silurian Fauna (Group E, belonging to the tipper Division) 

 within the region of the Second Fauna (Group D, belonging to the 

 Lower Division). The Colonies are of small extent, being confined, 

 both vertically and horizontally, in very narrow limits. The strata 

 of which they consist are intercalated in Group D, with a strike and 

 dip conformable to the rest of the group f. 



To show how closely these Colonies, intercalated in the Lower Si- 

 lurian, are connected with the fauna of Group E (Upper Silurian), 

 it is sufficient to point out that of the sixty-three species as yet found 

 in them, thirty-seven again occur in the Third Fauna, although other- 

 wise these two Silurian divisions (Upper and Lower) have scarcely a 

 species in common. 



M. Barrande explains these local appearances, of one fauna amidst 

 a second, by the supposition of an immigration of organisms from 

 another region into Bohemia, where they remained a short time, ^. e. 

 as long as the conditions that favoured their immigration continued 

 favourable for their existence there. The character of the strata of 

 these Colonies is also peculiar. During the continuance of the Colo- 

 nies the deposits consisted of Graptolite-schists with calcareous no- 

 dules, identical v/ith those subsequently forming Group E at the base 

 of the Upper Division, and strongly contrasting with the quartzites 

 peculiar to Group D. 



After the sudden and total extinction of the Second Fauna (Group D) 

 by the trap-eruptions which covered the whole surface of this Silurian 

 basin, the fauna of the Colonies, with their peculiar graptolite-schists 

 and calcareous nodules, once more appeared, and constitutes M. Bar- 

 rande's Third Fauna, characterizing the lower limestone-group, E, 

 that forms the basement-beds of his Upper Silurian Division. 



Among other important considerations to which these phamomena 

 must necessarily give rise, M. Barrande points out that, by aid of the 

 Colonies, we have evidence of the existence (probably in the north- 

 east of Europe) of a fauna which, had it not supplied Colonies to the 

 Bohemian district previously to its subsequent development in the 

 Upper Silurian rocks, could have been regarded only as of later date 

 to Group D, and not contemporaneous with it. He refers also to the 

 great length of time during which the contemporaneity of these two 



* See also Bulletin Soc. Geol, France, deux. ser. tome viii. p. 152 et seg. 

 Janv. 1851. t See Ideal Section, he. cit. pi. 3. 



