CARBONIFEROUS PERIOD. 525 



few beds of shale. In other places, however, it is more or 

 less broken up into a series of different beds of limestone, 

 alternating with sandstones, grits, and shales, and sometimes 

 containing beds of coal. In North America it is never purely 

 calcareous, but consists mainly, or entirely, of sandstones and 

 shales, sometimes with thin beds of coal, or deposits of clay 

 iron-ore. Westward, however, it becomes more highly cal- 

 careous. 



II. THE MILLSTONE GRIT. The highest beds of the Car- 

 boniferous limestone are succeeded, usually conformably but 

 sometimes unconformably, by a series of sandy and gritty beds 

 which have been termed the Millstone Grit. In its most 

 typical form the Millstone Grit consists of a series of hard 

 quartzose sandstones, the component grains of which are some- 

 times so large as to be more properly called small pebbles, 

 when the rock becomes a fine conglomerate. In other cases 

 regular conglomerates are present, and there are sometimes 

 shales, limestones, and thin beds of coal. The thickness of 

 the Millstone Grit varies from 1000 to 1700 feet as a rule; 

 but sometimes its thickness is very greatly diminished. Fos- 

 sils are scarce, and offer no peculiarity. 



III. THE COAL-MEASURES. The Coal-measures proper 

 succeed the Millstone Grit conformably, and consist of a great 

 series of shale, sandstone, grit, and coal, attaining a total 

 thickness, when well developed, of from 7000 to 15,000 feet. 

 Except in Scotland, where workable coal-seams occur below 

 the horizon of the Millstone Grit, it is mostly from the true 

 Coal-measures that coal is obtained ; the largest and most 

 productive coal-fields of the world occurring in Britain, North 

 America, and Belgium. In their mineral nature, the Coal- 

 measures, all over the world, exhibit a wonderful general uni- 

 formity of composition. They consist, namely, of dark, often 

 nearly black, earthy and laminated shales, yellow, brown, and 

 purple sandstones, sometimes spotted, but very rarely red in 

 colour, along with occasional beds of limestone and clay iron- 

 ore, and beds of coal of varying thickness. These alternating 

 beds may follow one another in any order, and may be re- 

 peated over and over again, the total thickness sometimes 

 reaching the enormous amount of 14,000 feet, or nearly three 

 miles. In the South Wales coal-field the series consists as 

 usual of sandstones, shales, and coals, alternating with one 

 another, and indicating a slow, but probably intermittent, 

 depression of the area which they now occupy. In this coal- 

 field there are about 80 distinct beds of coal, each of which 

 represents an ancient land-surface. Each of these beds re- 



