90 HANDBOOK OF FOSSSLS. 



the most important of all the formations, comprising as it does 

 the coal-bearing strata. It is subdivided into 



a. Coalmeastires, a series of sandstones and shales with which 

 are interstratified the seams of coal, varying in thickness from six 

 inches to as much in one instance as thirty feet. 



Coal is the carbonized remains of innumerable plants, chiefly 

 ferns and gigantic clubmosses, that grew in swamps bordering on 

 the sea-coast of the period. Each coal seam is underlain by a 

 bed of clay called "under-clay," containing the roots of the 



Elants that grew on it. Some of the best impressions of 

 ;rns, etc., are to be obtained in the shaley beds forming the 

 roof of the coal seam ; many good specimens, however, are to be 

 got by searching the refuse heap at the pit's mouth. Besides 

 plants, the remains of fish are abundant in some of the beds of 

 shale. And in Nova Scotia the bones of air-breathing reptiles 

 and land snails have been discovered. Cockroaches and other 

 insects were also denizens of the carboniferous forests. 

 The following are the principal coalfields : 



1. Northumberland and Durham coalfield. 



2. South Lancashire coalfield. 



3. Derbyshire coalfield. 



4. Leicestershire and Staffordshire coalfields, 



5. South Wales coalfield. 



6. Bristol and Somerset coalfields. 



b. Millstone grit or Farewell-rock* The former term explains 

 itself, the latter designation has been applied to it in the southern 

 districts, because when it is reached, then good-bye to all work- 

 able coal-seams. 



It consists of coarse sandstones, shales, and conglomerates with 

 a few small seams of coal. Fossils are not very common in 

 it. 



c. Yoredale Rocks, a series of flagstones, gritstones, limestones 

 and shales, with seams of coal, occurring in the northern counties. 

 It is underlain by 



d. Carboniferous or Mountain Limestone, which in places is 

 upwards of 1,000 feet thick, and full of fossils. The stems of 

 encrinites, or "stone-lilies," corals, brachiopods (e.g. Productus, 

 Orthis, etc.), and Mollusca, including some Cephalopods, like 

 Goniatites and the straight Nautilus (Orthoceras), with fish 

 teeth, etc. , go to compose this tough, bluish-grey limestone which 

 is largely quarried for marble mantlepieces, etc. 



e. The Tuedian group in the north, and Lower Limestone 

 Shale in the south, follow next, and consist of shales, sandstones, 



