disappeared, in the case of South Wales, a thickness of 11,000 feet of material.

Of the many remarkable things in connection with coal-beds, not the least is the state of purity in which coal is found. On the floor of each forest there would be many a streamlet or even small river which would wend its way to meet the not very distant sea, and it is surprising at first that so little sediment found its way into the coal itself. But this was cleverly explained by Sir Charles Lyell, who noticed, on one of his visits to America, that the water of the Mississippi, around the rank growths of cypress which form the "cypress swamps" at the mouths of that river, was highly charged with sediment, but that, having passed through the close undergrowth of the swamps, it issued in almost a pure state, the sediment which it bore having been filtered out of it and precipitated. This very satisfactorily explained how in some places carbonaceous matter might be deposited in a perfectly pure state, whilst in others, where sandstone or shale was actually forming, it might be impregnated by coaly matter in such a way as to cause it to be stained black. In times of flood sediment would be brought in, even where pure coal had been forming, and then we should have a thin "parting" of sandstone or shale, which was formed when the flood was at its height. Or a slight sinking of the land might occur, in which case also the formation of coal would temporarily cease, and a parting of foreign matter would be formed, which, on further upheaval taking place, would again give way to another forest growth. Some of the thicker beds have been found presenting this aspect, such as the South Staffordshire ten-yard coal, which in some parts splits up into a dozen or so smaller beds, with partings of sediment between them.