546 MANUAL OF NATURAL HISTORY. 



were formerly called primary rocks, and others with 

 the secondary formations. Some were separated, 

 under the name of " transition- rocks/' but they have 

 since been more accurately defined, and named 

 Palceozoic, from their containing the first remains 

 of organized existence. They comprise the Silu- 

 rian system ; the Devonian, or old Red-Sandstone ; 

 and the carboniferous strata ; to which Murchison 

 has added the Permian, or magnesian- limestone 

 system. 



I. THE SILURIAN GROUP. 



This group constitutes a series of marine deposits 

 of vast extent, composed of sandstones, limestones, 

 shales, grits, flagstones, and slates. It derives its 

 name from the Silures, who formerly inhabited 

 those districts of Britain where the strata occur 

 most conspicuously. The Silurian strata extend 

 from the heart of South Wales to that of England, 

 in Russia, in the Falkland Islands, and in North 

 America; they are also found in widely-extended 

 bed? abounding in the remains of fishes, mollusks, 

 crustaceans, and polyps. The deposit termed Cam- 

 brian, the earliest known fossiliferous formation, 

 occurs in North Wales, and is placed under the 

 Silurian strata ; it consists of slaty and gritty beds. 

 The Murchisonian, or Upper Silurian rocks, are 

 composed of gray and bluish limestones, coloured 

 micaceous shales and flagstones. The Lower Silu- 

 rian rocks are formed of impure shelly limestone, 

 mottled sandstones, and dark calcareous flags, and 



