112 M. Barrande on the Silurian System of Bohemia. 



ing only thin bands of quartzite. They all yield readily to 

 the action of the atmosphere, and the powerful erosion of the 

 surface has produced a valley of an elongated annular form, 

 corresponding to the outline of the mass. This valley is 

 bounded on the north-west side by the thick band of quartzites 

 forming the Drabow mountains, and on the other side of the 

 axis by the corresponding mass of quartzite in the Brdi- 

 Wald. From the middle of this valley the more recent cal- 

 careous groups rise up like an island to the height of 100 

 metres or more. 



In this, as well as in the inferior stages, limestone rocks 

 are almost entirely wanting, — a circumstance characteristic 

 of the lower division of the Silurians of Bohemia, and perhaps 

 explanatory of the diversity of its fauna from that of other 

 similar regions. But the slates above mentioned, near Beraun 

 and other localities, contain spheroidal masses, in some of 

 which a large proportion of carbonate of lime is found, whilst 

 in others calcareous matter is entirely wanting. 



Two other very remarkable deposits of calcareous sphe- 

 roids occur in the micaceous slates, about six kilometres 

 (four miles) to the south-west of Prague, one on each side of 

 the axis. The more northerly at Mottol, near the road from 

 Prague to Beraun, is accompanied by a mass of trap in 

 its entire length. The other lies near Grosskuchel on the 

 Moldau. Both contain fossils similar to those in the higher 

 division, and are embedded in a mass of slates with grapto- 

 lites, exactly like those forming its base. 



Though all the species, and even the geYieric types, with 

 the exception of Agnostus and Orthis, had disappeared at the 

 close of the former stage, there is still a certain order and 

 connection in the successive development of animal life in 

 this isolated region. The Trilobites continue to predominate, 

 both from the multiplicity of their forms and the number of 

 the individuals. The new genera and species are introduced, 

 not all at once, but at intervals, and as it were in a fixed 

 relation to the changes in the medium in which they lived, 

 as marked by the successive changes in the mineral character 

 of the deposits. Towards the close of the period, some repre- 

 sentatives of the family of Cytherinides also add to the rich- 



