234 PHYSIOGRAPHY. [chap. 



Roman remains, and even Roman roads, have been found 

 beneath eight feet of peat. In Ireland, peat-bogs are so 

 abundant as to cover about one-tenth of the entire surface 

 of the country; and, in some cases, the peat may be as much 

 as forty feet in thickness. The peat is cut from the bog, in 

 brick-shaped blocks, by means of a pecuhar spade known 

 as a "slade," and, after being dried in stacks, is used as fuel. 

 In England, peat is not so important as in Ireland, but it is 

 to be found in many damp localities. It was mentioned, in 

 the last chapter (p. 212), that an old peaty soil extends 

 for miles along the estuary of the Thames, though hidden 

 beneath the surface. 



In the deeper, and therefore older, parts of a thick peat- 

 bog, where the decomposing matter is most compressed and 

 altered, it usually takes the form of a brownish-black, slightly 

 compact mass, in which the vegetable structure may be 

 almost obliterated : the material is in fact converted into a 

 substance not altogether unlike coal. Indeed, the resemblance 

 has led to the suggestion that beds of coal may, in some 

 cases, have been formed by the alteration of old peat-bogs. 

 Although there are certain objections to such a view, it is 

 nevertheless beyond question that coal owes its origin to 

 the alteration of vegetable matter. The evidence upon 

 which this conclusion is founded is derived, partly, from the 

 chemical and microscopic structure of the coal and, partly, 

 from the conditions under which the substance is found in 

 nature. 



Coal occurs in the shape of beds, or seaijis, of variable 

 thickness, associated with shales, sandstones, and other sedi- 

 mentary rocks. The succession of strata, or measures, cut 

 through in a colliery, is generally similar to that represented 

 in Fig. 65, but the series may include hundreds of separate 

 beds. The "roof" of the coal, or the rock immediately 



