THE CARBONIFEROUS PERIOD. 1 59 



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 MillstoJie 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, 



