Central Bohemia, by Joachim Barrande? 317 



succession of generic and specific forms in the tribe of tri- 

 lobites, we find everywhere three grand physical masses 

 similar one to the other, and superimposed in the same 

 order. These masses or groups are characterised by as 

 many general faunas — that is to say, faunas whose extent 

 embraces the whole Silurian world, and which present among 

 themselves a striking harmony, in respect both of their zoo- 

 logical composition and the uniform order of their succession, 

 wherever their presence has been determined. We distin- 

 guish these three Silurian faunas by the names of Primor- 

 dial fauna, second fauna, third fauna. The first two 

 divide unequally the geological height of the lower Silurian 

 formation, whilst in the third we comprise, provisionally, all 

 the beings buried in the superior division." 



M. Barrande has represented the facts bearing on this 

 subject, so far as the trilobites are concerned, in a very clear 

 and striking manner, in Plates 50 and 51. His primordial 

 fauna is by far the most distinct; no species, and in Bohemia 

 only one genus, Agnostus (in Sweden also another, Amphion), 

 of trilobites, passing from it into the higher beds. But we 

 agree with him that this is no sufficient ground for separa- 

 ting the beds containing this fauna from the other Silurian 

 rocks as a distinct formation. Still less can we, for a similar 

 reason, separate the second and third faunas, or the lower 

 and upper Silurian groups, connected as they are, not only 

 by many genera, but even by several species, including, 

 as we must do, those in his " colonies." On looking at the 

 plate, the true physical cause of the great break in the zoo- 

 logical series is at once apparent. The primordial fauna is 

 cut off by an enormous eruption of igneous masses (por- 

 phyries, &c.) which destroyed all organic beings in the limit- 

 ed basin of Bohemia. These are followed by conglomerates 

 marking shallow seas, and a bottom on which trilobites could 

 not live ; and it is only when the appropriate sea-bottom of 

 schists and quartzites returns that they again reappear in 

 great abundance, but of course, after such a long interval, in 

 new types and forms. His .second division is closed in like 

 manner by eruptions of trap ; but these we would conjecture, 

 from the nature of the connected rocks, both less extensive, 

 and effecting less physical change on the sea-bottom, and, 



VOL. LVI. NO. CXII.— APKIL 1854. Y 



