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that the millstone grit also finds no place in our series. In England 

 and Ireland the development is more complete, and the lines of 

 division more strongly marked. The mountain limestone is an 

 important formation, and widely persistent in both countries ; and in 

 England the millstone grit, overlying the limestone, in most cases 

 fully developed. In Ireland it is less universal, and its thickness is 

 much less. In Scotland, on the other hand, the millstone grit has 

 not been identified ; and as to the mountain limestone, we may 

 almost say that it is, as it were, split up into numerous bands ; and 

 strata of shale, coal, ironstone, and sandstone, intercalated among 

 the separated members. The mineral type differs widely ; but the 

 organic remains are identical throughout the two series. They are 

 thus of one age, but formed under different conditions of the ter- 

 restrial surface which adjoined. An ancient sea channel filled the 

 space already described (Art. 14) as forming the great synclinal 

 trough in which lie our coal strata. Warm and humid lands 

 stretched far and wide to the N.W. and S.E. ; they nourished a 

 luxuriant vegetation of tropical or sub-tropical species, and were 

 drained by large rivers emptying into this common receptacle. 

 Like the tropical rivers of our own era, these were subject to sudden 

 and violent floods, which swept away the exuberant vegetation and 

 spread it widely over the bottom of that ancient frith. Sand and mud, 

 mixed with peaty matter, produced by decomposition in sheltered 

 situations, were the river sediment, in an incalculable disproportion 

 to the trees and plants carried down. The calcareous matter, sus- 

 pended in small proportion in these turbid waters, formed, by 

 chemical segregation, subscrystalline beds of limestone of trifling 

 thickness and limited horizontal extent. Under peculiar circum- 

 stances existing in particular spots, the purity of the water and a 

 lengthened period of repose favoured the growth of a few corals, and 

 the rearing of pigmy coral banks from the bosom of that primaeval 

 sea. In the region of the ancient Earth which we now call Ireland, 

 the conditions were widely different : the whole interior, to two-thirds 

 of the present area, was occupied by the waters of an inland sea or 

 gulf, as large as the Aral Lake. Around its shores on all sides rose 

 high ridges of granitic rocks and silurian slates, breached in a few 

 places by narrow straits, uniting it to the ocean outside ; and its 

 surface was varied by a few islands of elongated form, less elevated 

 than the bordering land, but formed of rocks of the same old types. 

 The pure and warm waters of this archipelago were tenanted by 

 myriads of rock-building polypi, whose ceaseless toil through long 

 ages reared up from its still depths numerous reefs of coral ; while 

 the high temperature of the waters favoured the prolific growth 



