86 GEOLOGY. 



strikingly exhibited there. It consists of greywacke, 

 greywacke slates, shales, and limestones. Anthracite 

 is formed in this system. Minerals are very numerous 

 in Transitive rocks. Fuci, corals, and shells, are the 

 principal organic remains found in this group. 



The Silurian group, so named from a tribe of ancient 

 Britons who inhabited that part of Wales where these 

 formations are found, consists of flags, sandstones, and 

 limestones, some of which are many hundreds of feet in 

 thickness. 



The Transitive rocks are sometimes called the Lower 

 Secondary they are a marine formation. 



Numerous organic remains have been found in the 

 Silurian system, particularly of radiated animals, but 

 they are found to differ from any known species now in 

 existence. The Radiata* are the lowest of the animal 

 world, and apparently the first that were formed ; and 

 as we proceed we find that almost every successive for- 

 mation produces animals of a more perfect nature. 



A peculiar kind of radiated animal, called the Kncri- 

 nite, t is found in this system. It is composed of a 

 column with many joints, supporting a body of the 

 shape of a cup, which contains its stomach. From this 

 cup proceed arms or branches furnished with fingers 

 for the purpose of seizing its prey, and conveying it to 

 its stomach . Petrified portions of this body are often 

 found in marble, which is thence called encrinitic 

 marble. 



SECONDARY SERIES. 



Names. Subdivisions. 



Old Red Sandstone, orDe- J Conglomerates and 

 vonian group \ Limestones. 



Carboniferous or Moun- j Alternating Sandstones 

 tain Limestone | and Limestones. 



* The Radiata are so called from their organs radiating as it were 

 from a common centre. The star-fish, the echinus, coral, sponge, 

 and all those which are called Zoophytes, belong to this branch. 



f From xpiw, a lily, as it resembles that flower. 



