240 



THE COAL MEASURES. 



gradation from the one set of deposites to the 

 other, and points to the curious fact that the pro- 

 cesses which brought together the materials of the 

 coal commenced before the previous movements 

 that caused the conglomerate had wholly ceased. 

 This very coarse aggregate has rarely more than 

 one or two coal-beds below it ; and, ascendipg a little 

 in the series, we find that its place is supplied by 

 thick beds of shale and masses of soft argillaceous 

 sandstones, some of w^hose layers have a sprink- 

 ling of pebbles, which give the aspect of conglom- 

 erates. These pebbles are smaller and more irreg- 

 ular than those composing the rock at the very 

 base of the series. The coal, and the slates imme- 

 diately in contact with the coal, lie interstratified 

 with these numerous coarse beds in an alternating 

 group of great thickness. 



Between the conglomerates, or even the coarser 

 sandstones and the beds of coal, argillaceous sand- 

 stones and blue shales are almost invariably inter- 

 posed. The predominant rock of the upper part of 

 the series is a compact blue sandstone, containing 

 much argillaceous matter and oxide of iron, which 

 cause the atmospheric agents to decompose it su- 

 perficially, and to impart a dingy brown colour and 

 a tendency to a conchoidal fracture, and to a sca- 

 ling off at the corners. 



" The shales, which are next in importance to the 

 argillaceous sandstones, are commonly of a dark 

 blue or bluish gray colour when freshly broken ; 

 but many of them, by exposure to the atmosphere 

 and to the vicissitudes of the seasons, assume a 

 brownish ochreous hue, and crumble rapidly to 

 pieces. Occasionally these shales contain highly 

 ferruginous bands, in some of which occur layers of 

 tolerably rich argillaceous iron ore. In the anthra- 

 cite coal measures, as a general rule, this ore does 

 not appear to exist in that abundance which it ex- 

 hibits in many portions of the bituminous coal se- 



