§ 10. IHE SILURIAN SYSTEM. 765* 



the placoid fishes, species of the genera Onchus and 

 Ctenacanthus have been discovered ; but this order is but 

 feebly represented in the Devonian epoch, while the ganoid 

 is largely developed ; the Cephalaspides and the Dypterians 



appear exclusively in these strata.* 



10. The Silurian System. — By a reference to the 

 synoptical arrangement of the formations (ante, p. 204 — 

 206), it will be seen, that the interval between the Devo- 

 nian system, and the uppermost of the hypogene rocks, 

 the Mica-schist^ is occupied by an immense thickness of 

 slate rocks, limestones, sandstones, and argillaceous strata. 

 These deposits were formerly grouped together under the 

 name of Transition rocks ; a designation applied by the 

 celebrated Werner, upon the supposition that they were 

 formed when the world was in a state of transition 

 from a chaotic to a habitable condition : they were also 

 termed Graurcache\\ from the hardened conglomerate 

 character of many of the strata ; but the whole series is 

 now divided into two natural groups. The uppermost is 

 designated the Silurian System,} by Sir R. Murchison, 

 whose indefatigable researches have determined the true 

 position, relation, and character of these deposits : and the 

 lowermost, consisting principally of slate rocks, has been 

 named the Cumbrian' System, § by Professor Sedgwick, 

 whose successful labours in this difficult field of geological 

 investigation, have rendered clear and intelligible pheno- 



* See M. Agassiz's Genealogical Table in Eeeherches but les Poissons 

 Fossiles, torn. i. p. 170. 



f From the German grau, grey, and wadte, a name employed by 

 the German miners to denote hardened conglomerates. 



+ Silurian— & term derived from SUures, the ancient Briton? who 

 inhabited those parts of our Island where these strata are most dis- 

 tinctly developed. 



§ Cumbrian, from the Lake district of Cumberland, which is prin- 

 cipally composed of slate rocks. In the Map, p. 461, the Silurian and 

 Cumbrian systems are denoted by the same number (8) and colour. 



