THE CARBONIFEROUS PERIOD. 159 



appear to be of great moment whether these strata be referred 

 to the former or to the latter series. Perhaps the most satis- 

 factory course is to regard the Coomhola Grits and Carbon- 

 iferous Slates as " passage-beds " between the Devonian and 

 Carboniferous ; but any view that may be taken as to the 

 position of these beds, really leaves unaffected the integrity 

 of the Devonian series as a distinct life-system, which, on the 

 whole, is more closely allied to the Silurian than to the Car- 

 boniferous. In North America, lastly, the Sub-Carboniferous 

 series is never purely calcareous, though in the interior of the 

 continent it becomes mainly so. In other regions, however, 

 it consists principally of shales and sandstones, with subor- 

 dinate beds of limestone, and sometimes with thin beds of 

 coal or deposits of clay-ironstone. 



II. The Millstone Grit. The highest beds of the Carbon- 

 iferous Limestone series are succeeded, generally with perfect 

 conformity, by a series of arenaceous beds, usually known as 

 the Millstone Grit. As typically developed in Britain, this 

 group consists of hard quartzose sandstones, often so large- 

 grained and coarse in texture as to properly constitute fine 

 conglomerates. In other cases there are regular conglomer- 

 ates, sometimes with shales, limestones, and thin beds of coal 

 the thickness of the whole series,' when well developed, varying 

 from 1000 to 5000 feet. In North America, the Millstone 

 Grit rarely reaches 1000 feet in thickness; and, like its Brit- 

 ish equivalent, consists of coarse sandstones and grits, some- 

 times with regular conglomerates. Whilst the Carboniferous 

 Limestone was undoubtedly deposited in a tranquil ocean 

 of considerable depth, the coarse mechanical sediments of 

 the Millstone Grit indicate the progressive shallowing of 

 the Carboniferous seas, and the consequent supervention 

 of shore-conditions. 



III. The Coal-measttres. The Coal-measures properly so 

 called rest conformably upon the Millstone Grit, and usually 

 consist of a vast series of sandstones, shales, grits, and coals, 

 sometimes with beds of limestone, attaining in some regions a 

 total thickness of from 7000 to nearly 14,000 feet. Beds of 

 workable coal are by no means unknown in some areas in the 

 inferior group of the Sub-Carboniferous; but the general state- 

 ment is true, that coal is mostly obtained from the true Coal- 

 measures the largest known, and at present most produc- 

 tive coal-fields of the world being in Great Britain, North 

 America, and Belgium. Wherever they are found, with 

 limited exceptions, the Coal - measures present a singular 

 general uniformity of mineral composition. They consist, 



