§ 5. COALBROOK DALE. 665 



the nodules of which contain organic remains ; and softer 

 argillaceous beds, which being of a slaty structure, are 

 called shales. Two of the layers of clay abound in fresh- 

 water mussel-shells, of extinct species, and are termed 

 mussel-bind ; these bivalves very much resemble some of 

 the small species of Unio of the Wealden (p. 404). The 

 total thickness is 1310 yards, which includes thirty different 

 beds of coal, varying from six inches to eleven feet, and 

 making the amount of coal about twenty-six yards. In the 

 shales below the coal, there is a transition from marine 

 calcareous strata with animal remains, to fresh-water depo- 

 sits, with terrestrial vegetables : this may have originated 

 from occasional intrusions of freshes from a river. 



The series above enumerated is often repeated ; shales, 

 clays, and sandstones occurring under different beds of coal, 

 with a perfect similarity in the succession and thickness of 

 each. Interruptions to the continuity of the beds, from 

 cracks and fissures which have taken place since the origi- 

 nal deposition of the strata, are every where^ apparent (see 

 Lign. 152). Dikes or intrusions of extraneous mineral 

 matter are of frequent occurrence, separating the strata 

 by vertical walls, which are from a few inches to many 

 yards in thickness. These intrusive masses sometimes 

 consist of indurated clay, but more commonly of the ancient 

 volcanic rock termed basalt or trap. 



5. Coalbrook Dale. — In Shropshire the carboniferous 

 strata occupy several detached areas.* Around Shrews- 

 bury the coal-beds are associated with limestone of 

 fresh-water or estuary origin, peculiar to the coal-fields of 

 the central counties of England, and containing fresh-water 

 crustaceans (Cyprides), shells, (Cyclas, Planorbis, Unio), 

 and fishes. But the most important and productive car- 

 boniferous tract in Shropshire is Coalbrook Dale, which is 



* Consult Sir R, I. Murchison's admirable description of the Car- 

 boniferous System • " Silurian System." chap. vi. 



